Done Deal is a 1998 Catalina 380 Sailed on SF Bay

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What follows here are essentially my notes on battery options for my Catalina 380, based on some internet shopping.  I run two 4D batteries in parallel for a house bank, and one group 24 as a starter battery, both were traditional flooded acid design.  My goal is to replace both with glass-mat batteries as appropriate for the two uses.  

My conclusion was to choose to install two Lifeline 4D's for the house bank, and a Lifeline group 24 starter battery.  Link to installed.


955 Todd Avenue
Azusa, CA 91702

Tel. 626-969-6886
Fax 626-969-8566

Picture: Lifeline batteries

The following information was found at

Note the six volt offering that I highlighted below is a standard golf cart size case.  This line is produced by Concorde which produces AGM batteries for the US Airforce.

Marine Batteries


Sort by

Part Number


Overall Dimensions

Unit Wt
lbs (kg)

Cold Cranking Amps


Minutes of Discharge

in (mm)

in (mm)

in (mm)

68º F

32º F

0º F

20 Hr

25 Amps

15 Amps

8 Amps



20.76 (527)

10.89 (277)

9.65 (245)

162 (73.6)










20.76 (527)

10.89 (277)

9.65 (245)

162 (73.6)










20.76 (527)

8.70 (221)

9.44 (240)

135 (61.2)










20.76 (527)

8.70 (221)

9.76 (248)

135 (61.2)










10.27 (261)

7.12 (181)

10.24 (260)

66 (30.0)










12.90 (328)

6.75 (172)

9.27 (236)

69 (31.4)










12.01 (305)

6.60 (168)

9.25 (235)

65 (29.5)










11.13 (283)

6.77 (172)

9.25 (235)

56 (25.5)










7.71 (196)

5.18 (132)

6.89 (175)

24 (10.9)








Pricing for Lifeline Batteries


 The following chart is included for comparison of the statistics above for a AGM to a standard design (flooded lead acid product).  Note the ___    and ___  highlights in each chart for comparisons of 4D and 8D statistics.



C.C.A. Res. Cap. Max. Overall Dimensions
BCI Battery at Minutes (in inches & mm.)
Part # Part # @ 0' F @ 25Amps. Length Width Height
______ ______ ______ __________ _________ _________ _________
24M EV24 520 135 11 1/4 286 6 3/4 171 9 3/4 248
27M EV27 550 160 12 3/4 324 6 3/4 171 9 3/4 248
31M EV31 620 200 12 7/8 328 6 3/4 171 9 7/8 251
4D EV4D 1240 400 20 5/8 524 8 11/16 221 9 1/2 241
8D EV8D 1200 450 20 5/8 524 11 279 9 1/2 241


6 Trojan Golf Cart Batteries

I found the configuration of 6 volt batteries below at yachtworld in some marketing photos at:

it shows Lilia out of San Diego featuring 

Many extras for extended cruising. Significant modifications to battery capacity to 600
total amps via 6
Trojan T105 golf cart batteries!
Siemens 75 watt solar panels (4) with Trace controller
Separate starting battery w/AmplePower E-lim
Auto battery charger
AmplePower E-monitor
Standard 70 amp alternator
Extra 105 amp alternator



This is the battery box with the 4 T-125s. Shot from standing just in front of the aft cabin door. You can see on the far starboard side where the installer notched the glass for the battery terminal and cap. Owner can get that cap off and get water in if needed.  Courtesy of Jim Norman #14.

NOTE:  the chart below show the statistics on the T105 Trojan product featured in the photos above.

Trojan Battery

Golf and Heavy Duty Deep Cycle Batteries

Trojan GOLF CART (flooded lead acid)


TROJAN C.C.A. Res. 20 Max. Overall Dimensions
BCI Part at Cap. Hour (in inches & mm.)
Num. Num. @ 0'F 25Amps. Rate Length Width Height
_______ _______ _______ _______ _____ __________ __________ __________
TG108 - 420 108 220 10 3/8 7 1/8 11 3/16
T105 - 447 115 225 10 3/8 7 1/8 11 3/16
T125 - 488 132 235 10 3/8 7 1/8 11 3/16
T-145 - 530 145 244 10 3/8 7 1/8 11 3/16


Products Featured at

Product Photo

 The selection below was offered at

These are all 12 volt AMG.



Component and System Pricing for
Power-Tech AGM Deep-Cycle Batteries


Item #:


Ship Weight:

Selling Price
(US $):

System Price
(US $):


245 amp hrs. size 20.75” X 11” X 10.”





200 amp hrs, size 20.75” X 8.5” X 10”





105 amp hrs, size12.94” X 6.75” X 9.3”





95 amp hrs, size 12.75 X 6.75” X 9.9”





80 amp hrs. size 10.9’X 6.75”X 9.9”






An interesting AGM battery I found online is the Optima, which doesn't offer a 6 volt, but has a deep cycle 12 volt in a golf cart footprint size (smaller), and considerably shorter seeDDeep CycleD34M Marine in chart below

Optima Marine Battery Models: Deep Cycle & Starter Batteries

Click here for Optima Batteries

OPTIMA: Proud to be made in the USA

Optima Marine Battery Models
Back: TF1800 Troll Fury, D31M
Front: D34M, 34M

OPTIMA: The Ultimate Marine Battery

Inside the unique shell of the OPTIMA deep cycle marine battery is a radically superior internal design. The structure of conventional marine batteries thick lead plates suspended in pools of acid requires those lead plates to provide their own structural support. To attempt to accomplish this, the lead is diluted with inert stiffening alloys, robbing them of performance. And despite this performance compromise, the conventional marine battery design still remains fragile.

The unique engineering concept of the OPTIMA deep cycle marine battery uses long thin layers of higher purity lead alternated with absorbent glass fiber mesh. This technique is called "AGM" for "Absorbed Glass Mat." Taking the high efficiency AGM technology a step further, in the OPTIMA deep cycle marine battery these laminated elements are wound into tight spiral cells and pressed into the "six pack" configuration that makes them unique. Battery life is typically twice that of the conventional marine battery. The OPTIMA marine battery's AGM Spiralcell technology is the most significant advance in battery design in years.

Optima Marine Battery Specs and Prices
Model No.
Model No.
Amp Hours*
CCA @ 
0° F
CA @ 
32° F
J240 Life Cycle
Deep Cycles
Physical Size
L" x W" x H"
 Net Lbs.
Total Price
(Includes Shipping **)
 D34M Marine
Deep Cycle
10 x 6.8 x 7.8
Top Post
+ Top Stud
Order Below
The D34M has both std. SAE top posts and 5/16" stainless steel threaded "wing nut" hookup posts.
Trolling Set
See Below
See Below
Top Post
+ Top Stud
Order Below
The TF-1800 Troll Fury consists of two D34M batteries in a removeable outer case. Hookup cables (included) allow connection of the two batteries either in parallel for extended reserve capacity OR in series for 24 volt operation. Like other Optima batteries, it can be charged with any battery charger running at 8 amps or higher. More specs in description above under "Selecting..." Click here for more info. on the TF1800 Troll Fury
D31M Marine Battery
NA (New)
Top Post
+ Top Stud 
Order Below
The D31M is a new model. It has both std. SAE top posts and 5/16" threaded stainless top studs. Size includes studs and posts. 
 34M Marine Starter Battery
10 x 6.8 x 7.8
Top Post
+Top Stud
Order Below
The 34M is a starter battery only. It is not a deep cycle battery. It has standard top posts PLUS stainless steel threaded stud posts. Size includes posts.

Text and image copyright 1st Optima Batteries, used by permission.

An couple of interesting articles on batteries below:




Entire books have been written on the subject of batteries, however, we will be discussing only the types of batteries currently used in modern RVs. And to narrow it down even further, this page will only be covering Flooded, Lead-Acid Batteries in detail. It is our opinion that the "gel-cell" type of batteries have proven inadequate in deep cycle environments and we don't recommend them.

There is a new type of sealed, lead-acid battery called an Absorbed Glass Mat (AGM) that is proving itself to be worthy of consideration. I've been testing these new batteries the last few years and have a report at the bottom of this page.

Now on to the discussion of Flooded, Lead-Acid Batteries.

How Batteries Work

What Affects the Longevity of Batteries?

What is a Deep Cycle?

Types of Batteries

The Operating Nature of Batteries

What is an "Equalization" Charge?

Absorbed Glass Mat Batteries Discussion

There is a "new" battery out there that is proving to be a valid option to the traditional Flooded Lead-Acid Deep Cycle Batteries. They cost about twice as much but may be worth it given their special benefits. I have been testing these batteries over the past few years and find myself appreciating them more and more. They are sold under the name of "Lifeline" for mobile applications and are made by Concourde (the same company that supplies the batteries for our Military Aircraft).

They are still lead-acid batteries but are sealed instead of vented. The electrolyte is held captive in a fibrous glass mat that can't be spilled and therefore can be shipped without hazardous material restrictions. This glass mat also provides pockets that assist in the recombination of hydrogen and oxygen gasses (that are generated during charging) back into water.

I found their charge acceptance to be greater than flooded batteries and it requires significantly less time to recharge these AGMs. This translates into higher efficiency which means a shorter generator run time during those times when you find it necessary to recharge quickly. They also hold up better and at a higher voltage when heavy loads are powered by them. Like when you fire up the microwave through the inverter.

These batteries have a very thick positive plates and belong in the true Deep Cycle class. They don't outgas (unless severly overcharged) and because of this don't corrode terminals and don't need to be watered The savings in maintenance alone will be worth the extra cost to some.

Charging Parameters

Technical Articles Written in Magazines

There are a couple of "glowing" articles written in Yacht magazines that would be worth hunting down if you want to know more. Two of them are listed below:

CRUISING WORLD June 1997 The Battery Revolution "In our test, AGM technology scores high over flooded and gel cells..."

SAIL February 1999 The Perfect Battery? "Absorbed Glass Mat (AGM) batteries boast a long list of impressive specifications..."


Boat Battery Basics

Tired of Replacing Batteries Every Two Years?
A better Understanding of Batteries Will Help Resolve Your Problems

by David Pascoe


One of the most common problems I run into on surveys is dead or severely depleted batteries. The usual reason why boat batteries are dead is due to having the wrong type, size or quality to meet the vessel's demands. Truly good batteries are expensive and there are few boat builders that provide good batteries with new vessels; usually the quality is minimal, the amount of power inadequate, and the type ill-suited.

The subject of batteries can be rather complex, but I'm sure most boat owners would rather not know too much about the details of how batteries work (or fail to work). The discussion of batteries can be divided into two major topics, battery construction and application, and charging. This essay deals mainly with battery construction and application, and will help you gain a better understanding of what type is best for your application, as well as what is needed to maintain them for longest service life and reliability. Application means the type of boat you have, how it is used, and the kinds of equipment on it.

Until the recent advent of electronic chargers we had big problems with ferroresonant chargers overcharging and damaging batteries. Now, to the best of my knowledge, all electronic chargers provide the basic 3 stage charging with electronic sensing that prevents overcharging. Therefore, if you have an old charger and are having premature battery failure problems, you'd best replace the unit. Symptoms of overcharging are hot batteries and unusual fluid loss.

System Designs

Batteries lie at the heart of all pleasurecraft DC electrical systems but there is wide variation on how DC systems are set up, meaning what purpose is assigned to each bank. The vast majority of all boats have relatively simple 12 volt systems consisting of banks of one, two or four batteries connected in parallel. Larger yachts may have 24 or 32 volt systems. In the standard, or I should say typical marine system, each bank is used for starting one engine, but is also wired to a battery selector switch. The selector switch may have positions marked 1, 2 or ALL. Other switches are marked ON/OFF, in which case bank source cannot be changed. In most cases the selector switch controls which bank runs the house system. In older boats, engine starting may be controlled by the switch. The ABYC standard requires all boats to have a master shut off switch, but not a selector switch.

Battery parallel switches join two batteries together in parallel (doubles amperage, not voltage), even if both are low, will often start an engine that won't start on one bank alone. This facilitates the starting circuit alone and will have no effect on the house system.

Boats which have a selector switch are usually set up in such a way that the source for the house system can be selected via the switch. In many, if not most, later model boats the house system is permanently wired to both banks. With older and particularly larger boats, there is likely to be one bank dedicated as the house bank.

Generators should have a separate starting battery so that if the main banks go dead, the generator can still be started. This is not always the case.

* * * * * *

Unfortunately, batteries are made in so many configurations and types that there are no quick and easy answers for those that desire quick answers. This essay is the culmination of several weeks worth of research into battery fundamentals, standards and testing. It is not a technical treatise of scientific exactitude because were I do so, this essay would end up dozens of pages long and I'm sure you have no interest in reading that.

This essay is intended to be of most benefit to those of you who suffer from the problems of premature battery failure and all-to-frequent replacement. If you're ever wondered why there is so much conflicting information about batteries, it is because not many people bother to take the time to learn, including many people that sell batteries. Amongst experts, there is wide agreement about performance of various types because actual performance is easily proven.

Contrary to popular misconception, the so-called "maintenance-free" battery is anything but. The only difference between the this type and those not so designated is that you don't have to top off  the electrolyte (add water) when it evaporates,  but batteries still need to be maintained in other ways as they will not function properly when ignored. Sealed batteries are not really sealed because all wet cell batteries have to be vented in order to discharge the build up of pressure during charging. Thus, even maintenance-free batteries can loose fluid, especially as a result of over charging. The primary difference is that one cannot add water to a "sealed" battery, though some will leak if laid over.

These batteries are also not maintenance free because they will naturally discharge themselves over time at a rate of anywhere from 1% to 15% per month, depending on type. These batteries should not be left uncharged month after month, but should be maintenance charged on a regular basis. Total discharge will destroy a battery so that it will never take a full charge again.

Most inboard powered boats are fitted with shore power systems and battery chargers to keep the batteries charged. Up until recently all battery chargers were the ferroresonant type capable of "trickle" charging, that is, supplying a very low charge rate sufficient to keep the batteries up to snuff. The problem with those older chargers was that they had a bad tendency to overcharge and boil all the electrolyte away which damages and eventually ruins the battery. Overcharging is deadly to gel cells.

The introduction of electronic, 3-stage chargers in recent years has been a vast improvement in battery maintenance because these chargers are able to sense when the battery cannot take any more charge and then shut off.

Installation Requirements

Batteries should be installed in a dry location and at a sufficient height above the bilge that a hull flooding incident will not immediately submerge the batteries and short them out. Batteries mounted close to the bottom of the hull run this risk.

Batteries generate hydrogen gas while charging; hydrogen gas is highly corrosive to most metals and particularly rubber products. Thus, hoses, wiring, fuel and oil lines should never be located ABOVE batteries as this gas is lighter than air and will rise.

Regardless of type, it is highly recommended that batteries be mounted in rugged, covered plastic boxes specially designed for this purpose. This is to contain the inevitable sulfuric acid leaks, this acid being very damaging to all organic materials (clothing, wood) as well as most metals.

Battery Types

All lead/acid batteries are not basically the same. The basic types are starting or automotive, marine and deep-cycle batteries. That last category name has been seriously abused in recent years by marketers of hybrid batteries that are not true deep-cycle but a cross between a starting battery and a deep-cycle. These will have plates that are slightly thicker than starting batteries, but much thinner than deep cycle batteries.

The most important criteria that determines battery type and performance is the thickness and composition of the battery plates, the factor that most affects cost.

Battery service life is primarily determined by how many times it is cycled, and whether it has been designed to withstand frequent and significant discharging. Cycling means each period of discharging and subsequent recharging. Equally important is how far a battery is discharged before recharging. Automotive batteries are designed to tolerate discharges at around 5% before recharging and will soon fail if deeply discharged, whereas deep cycle batteries are designed to discharge to 50% or more without being harmed.

Starting/Automotive As its name implies, starting batteries are used to start and run engines. These have different characteristics since engine starting requires very high bursts of amperage for short periods. Starting or automotive batteries have have a large number of very thin (0.40"), highly porous plates so as to provide the maximum surface area to yield that high high burst amperage. The down side of this type of battery construction is that it does not tolerate deep discharging well, and will fail after a relatively small number of deep discharge cycles (about 400 versus 2,000 for deep cycle). Starting batteries are commonly found in outboard and many entry level boats.

These are also frequently inappropriately labeled as "marine" batteries or auto/marine. Automotive batteries are meant to be constantly charged by an alternator so as to avoid discharge rates more than 5%. Starting batteries are usually rated by CCA (cold cranking amps) or simply CA (cranking amps), and more often than not have NO rating imprinted on the label. One method of identifying starting batteries is by their price: they are always much lower priced than true marine or deep-cycle batteries, as well as their lack of any rating. There are literally hundreds of brand names of this type and many are of very poor quality.

Marine It seems as if every battery manufacturer today sells "marine" batteries but, as mentioned earlier,  many such take considerable liberty with the term. Some marine batteries are deep cycle, others are hybrids, while others are pure hokum. True marine batteries are designed for dual use of engine starting and house service and are therefore hybrids (not true deep cycle). These will have spongy, porous plates that are significantly thicker than automotive batteries. They will be larger and heavier than auto batteries. A true marine battery will tolerate up to 50% discharge, whereas a deep-cycle and industrials tolerates up to 80%, whereas an auto battery will quickly die at such discharge rates. Numerous batteries found in small boats will be labeled "auto/marine" and the only way to tell the type is by cutting it open and examining the plates unless you are buying a reputable brand, but it's still a pretty good bet that any battery so labeled isn't going to be very good. There are also very many brand names of this type, and also many of low quality.

Deep-Cycle These batteries are distinguished by having much thicker plates (1/4" or 0.270" for Surette), nearly seven times thicker than an automotive battery, but high quality batteries will have solid lead plates versus others made of a lead powder composite. Lead powder plates allow for much more rapid charging but also deteriorate much faster, whereas solid or more dense and thicker plates are slower charging but have much longer service life.

Deep cycle batteries withstand greater abuse and thousands of charging cycles and have much greater service life than the other two types. They do not, however, have as great cranking or burst power, being designed to provide power over longer periods of time. Best for use with inverter systems. They are identifiable by their cost of 2-3 times that of other types and 20 hour AH ratings. True deep cycle batteries are usually only found in larger, higher end boats and yachts due to their greater cost, as well as the huge power demands of larger boats. The number of brand names of this type is relatively small since the cost is higher. Good quality ones are usually not found in discount stores or mass retail outlets.

When deep cycle batteries are used in boats, it is necessary to have considerably greater amperage than that required by the engine starter. This is almost never a problem since these batteries are used in banks of more than one battery per bank. When you get up to sizes like 4D and 8, 125 & 250 AH respectively, even a single battery is more than adequate because the amperage is so high.

Golf Cart batteries are generally a quasi-deep cycle similar to marine, and though not as good as batteries with solid plates, they are better than the auto/marine types. Usually set up in banks of six volt batteries, these have a greater number of plates to provide longer periods of use under a constant power demand and deep discharging. T-105, US2200 and GC-4 are common identifiers. These batteries can discharge up to 80% without being damaged. They are not better for use with inverters than true deep cycle batteries.

RV Batteries This name has recently begun appearing on batteries found in boats. Within the industry, there is no common battery type known as "RV" but it can be assumed that, like the "auto/marine" designation, it is a hybrid somewhere between a cranking and deep-cycle battery.

Industrial Batteries "Industrial" or "commercial" has long been used as a designation for deep cycle batteries used in fork lifts, sweepers, floor cleaners and similar battery powered machinery. Similar to golf cart but usually true deep cycle types with much heavier and pure lead plates up to around 0.270" thick. These batteries can discharge up to 80% without being damaged.

Yet another type name has crept into the lexicon recently, is the RV type. Most RV types sold are cranking batteries or hybrids as indicated by their higher cranking power but lower reserve power.

Obviously, the deep-cycle is the preferred battery type for marine use but for it's one drawback of being less able to provide high cranking power. This is overcome simply by increasing battery size.

Gel Cells

The primary difference between gel cells and flooded acid batteries is that the electrolyte in gel cells has been gelled by the addition of silica gel, turning the liquid into a thickened mush the same way napalm is gelled gasoline. Once hailed as the messiah of marine batteries, gel cells have since revealed their weakness to being damaged by heat and overcharging as these batteries cannot be fast charged by ordinary fast chargers and require much slower charging rates. Gel batteries sustain a far lower number of charging cycles than wet cell batteries, 2,000 versus 500 cycles for gel cells. This makes them less than ideal for marine applications. Additionally, they do not hold up well in hot engine rooms. The added cost has not proved worth the meager benefit of not spilling acid. Despite the common misperception, the gel cell electrolyte does evaporate over time.

AGM Batteries

AGM stands for Absorbed Glass Mat which contains the electrolyte absorbed in a mesh of Boron-Silicate glass fibers. Thus there is no fluid electrolyte to leak or spill nor will they suffer from freeze damage. There are two big advantages of this type. First, it can be charged with conventional chargers without fear of damage from modest overcharging. Second, water loss is reportedly reduced by 99% because hydrogen and oxygen are recombined within the battery. Further, this type has a modestly lower self discharge rate of 1-3% versus up to 15% with standard lead-acid batteries. The AGM is a true no maintenance battery. It otherwise has similar characteristics as the standard lead-acid battery. They have yet to see much use in boats, probably due to the higher cost. Widely used in battery back up power systems and solar systems.

The down side is the cost of around 2-3 times comparable standard batteries. Thus their greatest benefit is for installations where it is hard or impossible to ventilate charging fumes such as the interiors of sail boats.

Sealed or maintenance-free batteries

This battery type has sealed, but still vented cells because all batteries need to be vented to prevent gas build up and exploding during charging. Will not immediately leak if overturned but will over time. They are designed in such a way as to recover a large portion of the electrolyte that is normally lost through gassing of a normal wet cell. Even so, these batteries will loose electrolyte over time, causing premature failure due to overcharging.

HydroCaps and Water Mizers

These two after market devices fit in place of ordinary wet cell caps and are designed to reduce electrolyte loss from recharging by recapturing the escaping fluid. Both are widely reported to be quite effective. HydroCaps are about twice as effective as Water Mizers as the HydroCaps recombine escaping hydrogen and oxygen into water and cost twice as much (about 6.50 each) as Water Mizers. Good for boat owners who want to maintain their batteries carefully. Particularly good for very heavy battery use and deeper discharges. Recommended for large, non maintenance free batteries.

Sealed or Not Sealed? Most deep cycle batteries are not sealed, or may have removable recovery caps as described above. This is because deep cycle types will last a long time in which some electrolyte loss is inevitable and you want to be able to add water as needed. If you care about battery maintenance, unsealed or types with recovery caps are the best choice.

Battery Size

Unfortunately, battery manufacturers play a lot of games with battery sizes and ratings, making it very difficult for us to identify battery power. This is because of two factors that can be manipulated for marketing purposes. The most important things to know a bout a battery (other than voltage) is how much power and for how long. As discussed above, there are also legitimate reasons why manufacturers will favor one aspect over another, as in the need for high cranking power or longer discharge rates.

The physical dimensions of a battery are loosely relative to it's power. A battery with more or larger plates in it naturally has to be physically larger, and so does a battery with thicker plates like the deep cycle battery. This is why automotive batteries can be rather small, and yet have high CCA ratings but very low reserve power.

Group Size This is a rating promulgated by Battery Council International that defines nothing more than the physical, external size of the battery. It's purpose is to determine what size battery will fit in a given space; it has nothing to do with power rating.

Battery Types

Battery manufacturers often refer to their range of products rather inappropriately as "types". One manufacturer defines types as lead-acid versus NiCad, while another refers to 1D, 3D, 4D and 8D, or group number batteries as sizes. As near as I have been able to determine, 4D and 8D were model names of the Surette Battery Company that have since fallen into generic usage. The 4D is a 150 A.H. battery and the 8D, around 250 A.H. The 4D and 8D sizes are commonly referred to as boat sizes. Alternatively, there are the BCI types which are group sizes that have nothing to do with ratings, only physical dimensions.

Battery Ratings

Amp-Hour battery rating: AH is a common battery rating for batteries. Amp-hour rating of battery capacity is calculated by multiplying the current (in amperes) by time (in hours) that the current is drawn. Variations of the amp-hour battery rating is the most used rating. It most commonly signifies a deep cycle, marine or industrial battery.

Example: A battery which delivers 2 amperes for 20 hours would have a 40 amp-hour battery rating (2 x 20= 40). This is known as the 20-hour rating versus other ratings based on times such as 5, 8 and 100 hours, but also at different amperage rates. Such ratings are given based on what is considered most useful for the intended application. A battery intended to supply low amperage for long periods, for example, would use the 100 hour method, whereas a 5 hour rating would likely be for a high amperage rate. The 20 hour method is most common.

Cold Cranking Amperage  rating: CCA is the discharge load in amps which a battery can sustain for 30 seconds at 0 degrees F. and not fall below 1.2 volts per cell (7.2V on 12V battery). This battery rating measures a burst of energy that a car needs to start on a cold morning. This rating is used mainly for rating batteries for engine starting and tells you that you are looking at a starting battery. Example: the battery in my car is rated at 580 CCA. What does that mean to you and me? Well, probably nothing for it's meaning is relative to the ratings in other batteries. It says nothing other than an indication of starting power unless one is up to doing some serious math.

Reserve Capacity rating: RC is the number of minutes a new, fully charged battery at 80 degrees F will sustain a discharge load of 25 amps to a cut-off voltage of 1.75 volts per cell (10.5V on 12V battery). This battery rating measures more of a continuous load on the battery and is a much better indicator of how it will operate bilge pumps. An RC number given in the specification indicates that it is more than just a cranking battery and probably a hybrid starting battery. This is a very useful rating for a boater.

Reserve capacity is directly, though not completely, related to battery plate size and quality. As a general rule, cranking batteries have little reserve capacity after cranking operation unless they have thicker plates. If they have thicker plates, it will have a lower CCA rating.

MCA Marine Cranking Amps is a proprietary rating that is the same as CCA. It's an indicator that the battery is most likely an ordinary automotive cranking battery sold as suitable for boats.

Warm temperature affects lead-acid batteries positively, but cold temps negatively. These batteries in hot engine rooms are not negatively affected as higher temperatures actually increase voltage.

Ratings By Month and Warranties Increasingly consumer batteries are being sold with month/life ratings, such as 24, 48, 60, etc. As with all advertising, the words are better than the reality, particularly when you don't read the fine print. The bold print giveth and the fine print taketh away. Virtually all of the batteries that I have investigated that use month/life advertising, do not make any warranty that the battery (s) will last that long. Only the "60 month" moniker merely suggests that.

Virtually all "consumer" or mass market batteries have "pro rata" warranties, and that only for "defects in materials or workmanship. The vast majority of marine batteries investigated have 24-30 month warranties on a pro rata basis. That means that if the battery lasts 18 months on a 24 month warranty, you'll have to pay 75% of the cost of a new one while the manufacturer chips in 25%, assuming there is a defect and you did not fail to keep it properly charged.

The following warranty examples from a mass market battery (marine) labeled as 60 month:

ABC Battery Company warrants only to the original purchaser that: 1) this battery is free of defects in material and workmanship for the number of months indicated on the label, and 2) prior to installation or use, the state of charge of this battery has been maintained at a level equal to or greater than the minimum level considered necessary under industry standards for batteries to perform effectively upon their use or installation. If adjustment is necessary due to a defect in material or workmanship, or state of charge below minimum industry standards prior to installation or use, and the battery is NOT MERELY DISCHARGED after installation or use, then upon return of the battery to an authorized dealer: b) Within twelve (12) months from the date of original purchase, all marine batteries of the following types: HD24-DP, 24M-HD, 24M-RD, 24M-XHD, SRM-24, SRM-27, SRM-27B, SRM-29, will be replaced free of charge (except for taxes, where applicable).

The following is a warranty from Rolls-Surette:

Failure within 24 months from the date placed in service yields FREE REPLACEMENT, not including freight charges from the factory to the applicable destination. After the first 24 months of service, defective batteries will be adjusted for a period of up to 60 months prorated from the date first in service at prices in effect at time of adjustment.

Reading the warranty will often reveal the quality of the battery. A broader warranty usually means a better quality unit.

Typical Service Life Under Deep Cycle Use*

Cranking battery 12-18 months

Marine 1-4 years

Gel Cell 2-5 years (excluding Florida)

Golf Cart 2-6 years

Deep Cycle 4-6 years

Surette Deep Cycle 6+ years**

* Assumes proper installation and maintenance, and a properly calibrated charger. Based in part on personal observation from surveys as well as opinions of other experts. The range of time is dependent on frequency and degree of use.

** Surette batteries are often found in large yachts where short battery life is rarely a problem, in part due to high grade chargers and frequent maintenance.

My Recommendations for Boat Batteries

Outboard boats can get away with using automotive cranking batteries so long as there is no heavy power demand equipment) this does not include navigation equipment like radios, GPS, fish finder, etc., as these use little power. Equipment such as live bait well pumps, trolling motors, spotlights, electric down riggers, video chart recorders and so on demand deep-cycle batteries. However, to avoid annual battery replacement, deep cycle batteries will perform best when charging is completely reliant on engine alternators since cranking batteries do not tolerate deep discharges well. Further, if you're going offshore where there may be high demand on bilge pumps, BEWARE that cheap automotive batteries aren't going to run your pumps for very long, particularly after engine failure. Offshore operators should use higher capacity deep-cycle batteries.

Because of the high power demands on batteries in cruisers while engines aren't running or being charged by chargers, cranking batteries are a poor choice unless a boat has no appreciable other DC equipment. Boats with DC refrigerators, radar, anchor windlasses and other heavy power demands are best served by true deep cycle batteries. They are the primary reason why so many small boat owners have to replace batteries so often. MY advice is to avoid batteries labeled "auto-marine."

Sport fishermen typically have very high power demands so that only deep cycle batteries can be expected to perform well.

The question of whether you should buy deep-cycle versus marine batteries is fairly well answered by the increased service life of true deep cycle batteries versus those labeled "marine". Larger size deep-cycle batteries have no problem handling engine starting and go on providing reserve power for other things even without charging. Because boat batteries are subject to a lot of abuse, spending the money for higher quality deep-cycle batteries is usually well worth the extra cost.

Most dedicated battery resellers (those that serve business, industry and marine) typically quote prices at an installed rate. That means that they will deliver the batteries to your boat and install them and insure that they are installed properly. If you've ever tried to move batteries in and out of your engine room, you know that this is no easy task. 8D batteries weigh up to 190 lbs. Thus, the prices when quoted may at first seem very high, but are a lot less so when you realize that this includes installation and disposal of your old batteries. (We are now required to pay an environmental impact fee for battery disposal, which pretty much cancels out the salvage value that we used to get for old batteries.)

Battery Charging

Charging is a complicated issue that I'm not going to get into here beyond saying that battery charging becomes a problem when engines aren't operated long enough to complete a full charge, such as infrequent use and frequent starting and stopping. This happens as a result of short runs, as in fishing. For boats that are always on shore power systems when not running, this isn't a problem. Outboard boat owners most often suffer from battery failure due to incomplete charging. Achieving a complete charge will take several hours at least, so when you're operating for shorter periods, it is likely batteries aren't being fully charged.

Incomplete charges have a cumulative effect; that is, after incomplete charging, the battery is partly depleted and this leads to yet further depletion and longer charging times. It may only take two incomplete charging cycles for a battery to ultimately fail to start an engine, or even become damaged. The reason car batteries fail so frequently is due to short hops that result in cumulative incomplete recharging.

There is really no such thing as quick charging when talking about completing a full charge. A quick charge may bring a battery up sufficient charge (75%) to start an engine, but full charging takes much longer at lower amperage to complete the final 25%.

Battery Testing

The problem with any simple method of testing batteries is that it is only good for proving the negative. That is, you can prove that a battery has low power or is bad, but without a load tester you can't prove the overall condition. If you have wet cell batteries, using the hygrometer is useful under controlled conditions, like before charging when the electrolyte is well mixed. After charging the electrolyte tends to concentrate near the top and give false readings. But with sealed batteries all you can do is test the voltage which will only tell you the present state of charge, not the likely remaining useful life.

The voltage on a fully charged battery should be about 12.7-12.8 volts. If it's higher, the charger is on. Batteries will usually fail to start an engine at 12 volts or less. This is dependent on the age of the battery. A new, but depleted battery may only fail to start at a voltage as low as 11.5 volts.

Be wary of electric panel meters; they are often very inaccurate. Use a multi meter to test the batteries and then reset the panel meters if they are adjustable. Also, be aware that with the engines running, the helm voltmeters are reading through the alternator and are showing the charge rate, not battery state. Read these meters without starting the engines.

High Quality Battery Manufacturers

The Surette and Rolls battery companies are well known as being the Cadillac of batteries. It is a fair assumption that battery quality is directly related to price. If you are seeking nothing but low cost, you can be guaranteed to end up with a low quality battery, and low quality batteries are never worth what you pay for them. You are unlikely to find the highest quality batteries sold at common retail outlets where price is always the primary consideration.

Some manufacturers that have reputations for high quality products are listed below. Note that none of them are cheap.

Douglass (Guardian)












Sonnenschein (Prevailer)


Price examples (as of 3/03):


Rolls 4D, 165 AH, 5 yr. warranty            


Rolls 90 AH, 5 yr. Warranty                   


Delco 4D, 250 RC                                 


Sonnenschein Prevailer GEL, 95 AH       


Sonnenschein Prevailer GEL, 165 AH     


Guardian 4D, 170 AH                             


Guardian 8D, 220 AH                             


Dekka Auto/marine, Group 31, 120 RC    


Dekka 4D                                              


Trojan marine, 100 AH                            


Trojan 150 Ah deep-cycle                        


Optima AGM, 55AH, 120 RC                    


Optima AGM 75AH, 1555 AH                    


Sears DieHard Gr. 27 (no rating)                


Interstate Gr 27,550 CCA                

Link to Optima, Trojan, Surette, Lifetime source


Battery Storage Capacity Ratings

Two standard ratings are used to measure a battery's storage capacity.

Amp Hours

The Amp Hour rating tells you how much amperage is available when discharged evenly over a 20 hour period. The amp hour rating is cumulative, so in order to know how many constant amps the battery will output for 20 hours, you have to divide the amp hour rating by 20. Example: If a battery has an amp hour rating of 75, dividing by 20 = 3.75. Such a battery can carry a 3.75 amp load for 20 hours before dropping to 10.5 volts. (10.5 volts is the fully discharged level, at which point the battery needs to be recharged.) A battery with an amp hour rating of 55 will carry a 2.75 amp load for 20 hours before dropping to 10.5 volts.

Reserve Minutes

Reserve minutes is the number of minutes a battery will carry a 25 amp load before dropping to 10.5 volts. (10.5 volts is the fully discharged level, at which point the battery needs to be recharged.)

Installing a New Battery Bank  


The silence of this anchorage is only disturbed by the sounds of the captain's angst when he discovers the engine won't turn over due to a dead battery.
It’s a windless Sunday morning and you awake to the sounds of nature coming to life all around. The whole family has really enjoyed this remote cove where you’ve been snugly anchored for the weekend. Ah, life on the boat! You eat breakfast, savor your coffee in the cockpit, and then start readying the crew for the trip home. As the kids strike the sail cover and stow their gear, you turn the key to start the auxiliary engine.

Click. Click. And then you hear nothing.

‘Oh no!’ you wince. There’s only one thing happening here. The battery is dead!

This is a pretty common scenario as we sailors often misjudge just how much power gets used on our boats when we’re at anchor and unplugged from the dock. Reading lights, anchor lights, marathon margarita sessions with the 12-volt blender, all of these things take their toll. But by adding a second, isolated battery or bank of batteries you can avoid this scene. It’s an easy, smart move, and in almost all cases you can do it yourself. Here are the steps you need to follow to make this electrical upgrade.

Choosing a Battery    Batteries come in many different sizes and types, and are rated according to their capacity. When selecting a battery for your second battery bank, your first consideration is how you plan to use that battery. If your new battery bank will be used for cranking the engine, then you’ll want to choose a battery designed for that purpose. Cranking batteries supply a high amount of power for a short period of time and are not intended to be deeply discharged. For powering house loads, such as lights, electronics or refrigeration, a deep cycle battery is your best choice. This type of battery is designed to last when put through the rigors of being deeply discharged and then recharged over and over again. Deep-cycle batteries are manufactured using several different technologies, all of which are acceptable for marine use.


The battery box aboard the authors' boat doubles as the navigator's seat. More importantly, it contains the batteries securely, and provides the necessary ventilation through a stainless-steel plate.
You can choose between a traditional Wet Cell battery that will require periodic topping off with distilled water, or opt for a maintenance free Gel or Absorbed Glass Mat (AGM) battery. The Gel and AGM batteries provide the benefit of a longer life when compared to traditional Wet Cell batteries. As they require no maintenance, they’re also a good choice if your mounting location doesn’t provide easy access. They are, however, much more expensive than traditional Wet Cell batteries and these benefits need to be weighed against their higher initial cost.

When choosing a battery size and capacity for your own boat, your anticipated power consumption along with the amount of space available for mounting the battery itself are important factors to consider. If the purpose of your upgrade is to allow you to overnight more comfortably on a small boat, then the addition of a single Deep Cycle battery like a group 27 or group 31 should suffice. If you’re anticipating extended stays away from the dock, or have dreams of serious cruising, you’re going to want to choose a series of heavier, larger, and more highly rated batteries for your new bank. On Serengeti, our 46-foot cruising boat, our house bank consists of six wet cell six-volt golf cart batteries. Our second battery bank, which is reserved solely for emergency cranking, is comprised of a single group 31 battery. For more information about how to estimate your own daily power needs, you may want to check out the article, Creating a 12-Volt Spreadsheet, by Tom Woods, here at SailNet.

Mounting the Battery    In general, batteries are best mounted in a level position when your boat’s not heeled over. With Wet Cell batteries this is imperative, and with the others it just makes sense to keep them level if you can. All batteries should be tightly strapped down to eliminate the chance of movement or shifting while underway. One of the reasons for this is that batteries are heavy, but the other equally important reason is that the terminals on top of the battery must be protected from incidental contact with anything metal. You can accomplish this most easily by installing rubber boots over the lugs and terminals, or by placing the whole unit in a covered and ventilated battery box. A battery box offers the additional benefit of containing any acid should it accidentally spill out of a Wet Cell battery. Special plastic battery boxes are readily available in various sizes to meet this requirement.

"If allowed to accumulate in a non-vented area, hydrogen gas can become the fuel for an explosion should a spark somehow enter the equation."
As Wet Cell batteries charge, hydrogen gas is given off in the process. In low concentrations, this does not present a problem. However, if allowed to accumulate in a non-vented area, this hydrogen gas can become the fuel for an explosion should a spark somehow enter the equation. For this reason, it’s imperative to choose a mounting location that will allow any gas to dissipate. If you locate your battery down low in a locker or under a seat, it’s a good idea cut a hole and add a locker vent if one is not already present.

A battery is best located away from any bilge water, and in a spot where it will not adversely affect how the boat sits in the water. Also, a shorter cable run from the battery to the switch will be more efficient and less expensive. Lastly, keep in mind that you’ll need access to the batteries from time to time to check water levels and confirm that all connections remain tight.

Making Battery Cables    Making your own battery cables is very simple providing you have a few basic tools. You’ll need a hacksaw, cable lugs, heat shrink tubing, a hair dryer, lithium grease, a lug crimper or swage-it tool, and a razor knife, along with your new battery cable. The first step in making your own battery cables is to spec out the wire diameter needed for your application. Wire diameter is a function of the amount of power the cable will carry and the round-trip length from the battery to the load (battery switch in this case) and back. Once you have this information, you can use a simple chart like the one below to determine your own battery cable size.

Minimum Wire Size (AWG) Selector Table




Round-Trip Length of Conductor (Feet)
  10 20 30 40 60 80 100 120 140
1 16 16 16 16 16 14 14 14 12
2 16 16 16 14 14 12 10 10 8
5 16 14 12 10 10 8 6 6 6
10 14 10 10 8 6 6 4 4 2
15 12 10 8 6 4 2 2 1 0
20 10 8 6 6 4 2 2 1 0
25 10 6 6 4 2 2 1 0 2/0
30 10 6 4 4 2 1 0 2/0 3/0
40 8 6 4 2 1 0 2/0 3/0 4/0
50 6 4 2 2 0 2/0 3/0 4/0  
60 6 4 2 1 2/0 3/0 4/0    
70 6 2 1 0 3/0 4/0      
80 6 2 1 0 3/0 4/0      
90 4 2 0 2/0 4/0        
100 4 2 0 2/0 4/0        

If you want your new battery cables to last the life of your boat, be sure to use only tinned wire. Tinned wire is a lot more expensive but it does not corrode and lose its conductivity over time like regular copper. Because of this relatively high cost per foot, it’s important to measure carefully so that you don’t buy too much cable, or even worse find out that you’re just inches short. To help us get our lengths exactly right, we like to use a section of plastic hose to simulate each cable run. We’ve found that hose like this bends and mimics the exact path a cable will take better than any tape measure or piece of string. After we mark each end of the hose, we know that we’ve precisely determined the length we’ll need for the finished cable.

The positive cable that will run to your new battery switch will need to be red in color.  Your negative battery cable should be black in color. This black cable does not run to your new battery switch, but should terminate at a common ground point along with your existing negative battery cable.


Installing a new battery is more simple than you might think, and it requires a minimum of toolsjust those shown here.
To make sure your connections remain tight, both ends of the battery cables are fitted with lugs. Lugs are specially designed pieces of metal that crimp onto the ends of the cable to provide a flat circle that slides over a threaded stud and is held in place with a nut. When purchasing lugs, make sure you get the right diameter hole so that the lug will fit correctly onto the threaded terminal. The terminals on batteries are sometimes different sizes so that you don’t inadvertently switch the positive and negative cables by mistake. Check your individual battery and battery switch carefully to determine the lug sizes you will need.

Fitting a lug to a cable end is pretty easy. Simply strip back about one and a quarter inches of the insulation with a razor knife, then slide a three-inch long piece of heat shrink tubing onto your cable. The lug is placed over the exposed wire and snugly crimped in place using a lug crimper or swage-it tool. Apply a dab of lithium grease around the connection, and then slide the heat shrink tubing over the greased connection that you’ve just made. Using a heat gun or hair dryer, apply heat to shrink the tube down tight. The lithium grease helps to repel moisture from the connection and will further prolong its integrity.

"For most sailors, the lower capacity switches can handle far more current than we’d ever use on board."
Battery Switches    Battery switches that are designed to control two battery banks normally have four positions: OFF, 1, BOTH, or 2. This type of switch allows positions for drawing power from or supplying charging current into each battery bank individually, or both at the same time. Some switches are rated for higher amps than others, but for most of us sailors the lower capacity switches can handle far more current than we’d ever use on board.

One feature that you may want to look for when shopping for a battery switch is a model that offers an alternator field disconnect. A field disconnect turns your alternator off when the battery switch is in the OFF position. When using a standard battery switch without this feature you risk frying the alternator diode should a crewmember inadvertently turn the battery switch to OFF while the engine is running.

For cost and efficiency, battery switches are normally mounted somewhere near your batteries. For convenience, try to locate your switch in an area that’s both easy to see and use. Many switches are designed to be surface mounted so that the incoming and exiting cables can be visible or hidden from view behind a bulkhead depending upon your preferences. 


The battery switch above, which the authors installed on their own boat, does not have an alternator field disconnect, so they're always careful to shut the engine down before they switch the battery to "Off." 
When it comes time to wire the battery switch, remember that we’re only going to be dealing with the positive (red) cables. Detailed wiring instructions are almost always included with the switch, so it’s just a matter of following those directions. Looking at the backside of a standard battery switch capable of switching two battery banks, you’ll find three connections that need to be made.  The lug on the positive (red) cable from battery No. 1 attaches to the terminal marked 1. The positive (red) cable from battery No. 2 likewise attaches to terminal 2. The third lug on the back of the switch is reserved for the power out cable. Depending upon how your boat is set up, this power out lead could run to a positive bus bar, your electrical panel, and possibly to your engine’s starter motor as well.

So, with your new battery securely in place and a handy battery switch installed that allows you to control the power in and out of each battery, you’re all set. It will only take a weekend to free you forever from that dread of turning that engine key only to hear a disappointing click. We promise this addition will help you get your boat away from the dock more often and you’ll quickly begin to discover many more of the hundreds of wonderful places that only your sailboat can take you.



Suggested Reading:

Battery Bank Design by Kevin Jeffrey

Trouble Shooting Your Electrical System by Tom Wood

Standard On Board Charging Systems by Tom Wood

SailNet Store Section: Battery System Components


Last Updated on March 24, 2004

Car and Deep Cycle battery answers to Frequently Asked Questions (FAQs), tips, information, references and hyperlinks are contained on this free consumer oriented Web site about car, motorcycle, power sports, truck, boat, marine, RV (recreational vehicle) starting and deep cycle applications.

Eurobat Car Battery Construction

Car Battery Construction (Source: Eurobat)

Car and Deep Cycle Battery Frequently Asked Questions (FAQ) 5.0

This consumer oriented FAQ contains answers about lead-acid batteries used to start car, motorcycle, truck, boat, RV, power sports, motor home, tractor and other engines. It also answers questions about golf cart, EV, traction, motive, solar, standby, stationary, UPS, network, industrial and other lead-acid batteries used in deep cycle applications. It covers testing, jump starting, buying, installing, overnight draining, charging, removing sulfation, storing and other topics about Car and Deep Cycle batteries.

Battery Manufacturers and Brand Names List

This frequently updated list contains hyperlinks to lead-acid battery manufacturer's sites, battery brand names, and private labeling information.

Battery References and Information Links List

This frequently updated list contains hyperlinks to resources and other related information to and about lead-acid batteries, for example, charging systems, regulators, isolators, inverters, desulfators, cables, glossaries, business directories, test and monitoring systems, associations, books, magazines, history, etc.

Updated INformation at

Link to Jim McKinney's Site  another interesting discussion of batteries and electrical issues.


Source"  (below)

  • Starting (sometimes called SLI, for starting, lighting, ignition) batteries are commonly used to start and run engines. Engine starters need a very large starting current for a very short time. Starting batteries have a large number of thin plates for maximum surface area. The plates are composed of a Lead "sponge", similar in appearance to a very fine foam sponge. This gives a very large surface area, but if deep cycled, this sponge will quickly be consumed and fall to the bottom of the cells. Automotive batteries will generally fail after 30-150 deep cycles if deep cycled, while they may last for thousands of cycles in normal starting use (2-5% discharge).

  • Deep Cycle Batteries

    Battery Basics and Information

    The links below are on this page - you can also just scroll down if you want to read them all.

    What is a Battery?
    Types of Batteries
    Battery Lifespan
    Starting, Marine, or Deep Cycle?
    Deep Cycle Battery as a Starting Battery?
    What Batteries are made of
    Industrial Batteries (fork lift type)
    Sealed Batteries
    Battery Size Codes
    Gel Cells (and why we don't like them)
    AGM Batteries (and why we do like them)
    Temperature Effects
    Cycles vs Lifespan
    Amp Hours - what are they?
    Battery Voltages
    Battery Charging (Here is where we get into the real meat)
    Charge Controllers (for wind/solar)
    Mini Factoids - Some small facts about batteries

    The subject of batteries could take up many pages. This is a basic overview of batteries commonly used with photovoltaic power systems. These are nearly all various variations of Lead-Acid batteries. For a very brief discussion on the advantages and disadvantages of these and other types of batteries, such as NiCad, NiFe (Nickel-Iron), etc. go to our Batteries for Deep Cycle Applications page. These are sometimes referred to as "deep discharge" batteries. The correct term is deep cycle.


    What is a Battery?

    A battery, in concept, can be any device that stores energy for later use. A rock, pushed to the top of a hill, can be considered a kind of battery, since the energy used to push it up the hill (chemical energy, from muscles or combustion engines) is converted and stored as potential kinetic energy at the top of the hill. Later, that energy is released as kinetic and thermal energy when the rock rolls down the hill. Common use of the word, "battery," however, is limited to an electrochemical device that converts chemical energy into electricity, by use of a galvanic cell. A galvanic cell is a fairly simple device consisting of two electrodes (an anode and a cathode) and an electrolyte solution. Batteries consist of one or more galvanic cells.

    A battery is an electrical storage device. Batteries do not make electricity, they store it, just as a water tank stores water for future use. As chemicals in the battery change, electrical energy is stored or released. In rechargeable batteries this process can be repeated many times. Batteries are not 100% efficient - some energy is lost as heat and chemical reactions when charging and discharging. If you use 1000 watts from a battery, it might take 1200 watts or more to fully recharge it. Slower charging and discharging rates are more efficient. A battery rated at 180 amp-hours over 6 hours might be rated at 220 AH at the 20-hour rate, and 260 AH at the 48-hour rate. Typical efficiency in a lead-acid battery is 85-95%, in alkaline and NiCad battery it is about 65%.

    Practically all batteries used in PV and al but the smallest backup systems are Lead-Acid type batteries. Even after over a century of use, they still offer the best price to power ratio. A few systems use NiCad, but they are not recommend except in cases where extremely cold temperatures (-50 F or less) are common. They are expensive to buy, and very expensive to dispose of due the the hazardous nature of Cadmium. There is little direct experience with the NiFe (alkaline) batteries, but from what has been learned from others, they are not recommended - one major disadvantage is that there is a large voltage difference between the fully charged and discharged state. Another problem is that they are very inefficient - you lose from 30-40% in heat just in charging and discharging them. Many inverters and charge controls have a hard time with them. It appears that the only current source for new cells is from Hungary.

    It is important to note here that ALL of the batteries commonly used in deep cycle applications are Lead-Acid. This includes the standard flooded (wet) batteries, gelled, and AGM. They all use the same chemistry, although the actual construction of the plates etc can vary considerably. NiCads, Nickel-Iron, and other types are found in some systems, but are not common due to their expense and/or poor efficiency.

    Back to top

    Major Battery Types

    Batteries are divided in two ways, by application (what they are used for) and construction (how they are built). The major applications are automotive, marine, and deep-cycle. Deep-cycle includes solar electric (PV), backup power, and RV and boat "house" batteries. The major construction types are flooded (wet), gelled, and AGM (Absorbed Glass Mat). AGM batteries are also sometimes called "starved electrolyte" or "dry", because the fiberglass mat is only 95% saturated with Sulfuric acid and there is no excess liquid.

    Flooded may be standard, with removable caps, or the so-called "maintenance free" (that means they are designed to die one week after the warranty runs out). All gelled are sealed and a few are "valve regulated", which means that a tiny valve keeps a slight positive pressure.  Nearly all AGM batteries are sealed valve regulated. Most valve regulated are under some pressure - 1 to 4 psi at sea level.

    Lifespan of Batteries

    The lifespan of a battery will vary considerably with how it is used, how it is maintained and charged, temperature, and other factors. In extreme cases, it can vary to extremes - there have been L-16's killed in less than a year by severe overcharging, and a large set of surplus telephone batteries that sees only occasional (5-10 times per year) heavy service that are now over 20 years old. There have been gelled cells destroyed in one day when overcharged with a large automotive charger. Golf cart batteries have been destroyed without ever being used in less than a year because they were left sitting in a hot garage without being charged. Even the so-called "dry charged" (where you add acid when you need them) have a shelf life of at most 18 months, as they are not totally dry.

    These are some general (minimum - maximum) typical expectations for batteries used in deep cycle service:
    Starting: 3-12 months
    Marine: 1-6 years
    Golf cart: 2-6 years
    AGM deep cycle: 4-8 years
    Gelled deep cycle: 2-5 years
    Deep cycle (L-16 type etc): 4-8 years
    Industrial deep cycle (Crown and Rolls 4KS series): 10-20+ years
    Telephone (float): 1-10 years (these are special purpose "float service", but often appear on the surplus market as "deep cycle").
    NiFe (alkaline): 2-25 years
    NiCad: 1-20 years

    Back to top

    Starting, Marine, and Deep-Cycle Batteries




    Some common battery size codes used are: (ratings are approximate)



    34 to 40 Amp hours

    12 volts

    Group 24

    70-85 Amp hours

    12 volts

    Group 27

    85-105 Amp hours

    12 volts

    Group 31

    95-125 Amp hours

    12 volts


    180-215 Amp hours

    12 volts


    225-255 Amp hours

    12 volts

    Golf cart & T-105

    180 to 220 Amp hours

    6 volts


    340 to 380 Amp hours

    6 volts


    Gelled electrolyte

    Gelled batteries, or "Gel Cells" contain acid that has been "gelled" by the addition of Silica Gel, turning the acid into a solid mass that looks like gooey Jell-O. The advantage of these batteries is that it is impossible to spill acid even if they are broken. However, there are several disadvantages. One is that they must be charged at a slower rate (C/20) to prevent excess gas from damaging the cells. They cannot be fast charged on a conventional automotive charger or they may be permanently damaged. This is not usually a problem with solar electric systems, but if an auxiliary generator or inverter bulk charger is used, current must be limited to the manufacturers specifications. Most better inverters commonly used in solar electric systems can be set to limit charging current to the batteries.

    Some other disadvantages of gel cells is that they must be charged at a lower voltage (2/10th's less) than flooded or AGM batteries. If overcharged, voids can develop in the gel which will never heal, causing a loss in battery capacity. In hot climates, water loss can be enough over 2-4 years to cause premature battery death. It is for this and other reasons that we no longer sell any of the gelled cells except for replacement use. The newer AGM (absorbed glass mat) batteries have all the advanges (and then some) of gelled, with none of the disadvantages.

    AGM, or Absorbed Glass Mat Batteries

    A newer type of sealed battery uses "Absorbed Glass Mats", or AGM between the plates. This is a very fine fiber Boron-Silicate glass mat. These type of batteries have all the advantages of gelled, but can take much more abuse. One brand recommended is the Concorde (and Lifeline, made by Concorde) AGM batteries. These are also called "starved electrolyte", as the mat is about 95% saturated rather than fully soaked.

    AGM batteries have several advantages over both gelled and flooded, at about the same cost as gelled:

    Since all the electrolyte (acid) is contained in the glass mats, they cannot spill, even if broken. This also means that since they are non-hazardous, the shipping costs are lower. In addition, since there is no liquid to freeze and expand, they are practically immune from freezing damage.

    Nearly all AGM batteries are "recombinant" - what that means is that the Oxygen and Hydrogen recombine INSIDE the battery. These use gas phase transfer of oxygen to the negative plates to recombine them back into water while charging and prevent the loss of water through electrolysis. The recombining is typically 99+% efficient, so almost no water is lost.

    The charging voltages are the same as for any standard battery - no need for any special adjustments or problems with incompatible chargers or charge controls. And, since the internal resistance is extremely low, there is almost no heating of the battery even under heavy charge and discharge currents. The Concorde (and most AGM) batteries have no charge or discharge current limits.

    AGM's have a very low self-discharge - from 1% to 3% per month is usual. This means that they can sit in storage for much longer periods without charging than standard batteries. The Concorde batteries can be almost fully recharged (95% or better) even after 30 days of being totally discharged.

    AGM's do not have any liquid to spill, and even under severe overcharge conditions hydrogen emission is far below the 4% max specified for aircraft and enclosed spaces. The plates in AGM's are tightly packed and rigidly mounted, and will withstand shock and vibration better than any standard battery.

    Even with all the advantages listed above, there is still a place for the standard flooded deep cycle battery. AGM's will cost 2 to 3 times as much as flooded batteries of the same capacity. In many installations, where the batteries are set in an area where you don't have to worry about fumes or leakage, a standard or industrial deep cycle is a better economic choice. AGM batteries main advantages are no maintenance, completely sealed against fumes, Hydrogen, or leakage, non-spilling even if they are broken, and can survive most freezes. Not everyone needs these features.

    Back to top

    Temperature Effects on Batteries

    Battery capacity (how many amp-hours it can hold) is reduced as temperature goes down, and increased as temperature goes up. This is why your car battery dies on a cold winter morning, even though it worked fine the previous afternoon. If your batteries spend part of the year shivering in the cold, the reduced capacity has to be taken into account when sizing the system batteries. The standard rating for batteries is at room temperature - 25 degrees C (about 77 F). At approximately -22 degrees F (-27 C), battery AH capacity drops to 50%. At freezing, capacity is reduced by 20%. Capacity is increased at higher temperatures - at 122 degrees F, battery capacity would be about 12% higher.

    Charge voltage vs Temperature for AGM and Flooded batteriesBattery charging voltage also changes with temperature. It will vary from about 2.74 volts per cell (16.4 volts) at -40 C to 2.3 volts per cell (13.8 volts) at 50 C. This is why you should have temperature compensation on your charger or charge control if your batteries are outside and/or subject to wide temperature variations. Some charge controls have temperature compensation built in (such as Morningstar) - this works fine if the controller is subject to the same temperatures as the batteries. However, if your batteries are outside, and the controller is inside, it does not work that well. Adding another complication is that large battery banks make up a large thermal mass. Thermal mass means that because they have so much mass, they will change internal temperature much slower than the surrounding air temperature. A large insulated battery bank may vary as little as 10 degrees over 24 hours internally, even though the air temperature varies from 20 to 70 degrees. For this reason, external (add-on) temperature sensors should be attached to one of the POSITIVE plate terminals, and bundled up a little with some type of insulation on the terminal. The sensor will then read very close to the actual internal battery temperature. 

    Click to see larger graph of Battery Capacity vs TemperatureEven though battery capacity at high temperatures is higher,  battery life is shortened. Battery capacity is reduced by 50% at -22 degrees F - but battery LIFE increases by about 60%. Battery life is reduced at higher temperatures - for every 15 degrees F over 77, battery life is cut in half. This holds true for ANY type of Lead-Acid battery, whether sealed, gelled, AGM, industrial or whatever. This is actually not as bad as it seems, as the battery will tend to average out the good and bad times. Click on the small graph to see a full size chart of temperature vs capacity.

    One last note on temperatures - in some places that have extremely cold or hot conditions, batteries may be sold locally that are NOT standard electrolyte (acid) strengths. The electrolyte may be stronger (for cold) or weaker (for very hot) climates. In such cases, the specific gravity and the voltages may vary from what we show.

    Cycles vs Life

    A battery "cycle" is one complete discharge and recharge cycle. It is usually considered to be discharging from 100% to 20%, and then back to 100%. However, there are often ratings for other depth of discharge cycles, the most common ones are 10%, 20%, and 50%. You have to be careful when looking at ratings that list how many cycles a battery is rated for unless it also states how far down it is being discharged. For example, one of the widely advertised telephone type (float service) batteries have been advertised as having a 20-year life. If you look at the fine print, it has that rating only at 5% DOD - it is much less when used in an application where they are cycled deeper on a regular basis. Those same batteries are rated at less than 5 years if cycled to 50%. For example, most golf cart batteries are rated for about 550 cycles to 50% discharge - which equates to about 2 years.

    How depth of discharge affects cycle life on batteriesBattery life is directly related to how deep the battery is cycled each time. If a battery is discharged to 50% every day, it will last about twice as long as if it is cycled to 80% DOD. If cycled only 10% DOD, it will last about 5 times as long as one cycled to 50%. Obviously, there are some practical limitations on this - you don't usually want to have a 5 ton pile of batteries sitting there just to reduce the DOD. The most practical number to use is 50% DOD on a regular basis. This does NOT mean you cannot go to 80% once in a while. It's just that when designing a system when you have some idea of the loads, you should figure on an average DOD of around 50% for the best storage vs cost factor. Also, there is an upper limit - a battery that is continually cycled 5% or less will usually not last as long as one cycled down 10%. This happens because at very shallow cycles, the Lead Dioxide tends to build up in clumps on the the positive plates rather in an even film. The graph above shows how lifespan is affected by depth of discharge. The chart is for a Concorde Lifeline battery, but all lead-acid batteries will be similar in the shape of the curve, although the number of cycles will vary.

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    Battery Voltages

    All Lead-Acid batteries supply about 2.14 volts per cell (12.6 to 12.8 for a 12 volt battery) when fully charged. Batteries that are stored for long periods will eventually lose all their charge. This "leakage" or self discharge varies considerably with battery type, age, & temperature. It can range from about 1% to 15% per month. Generally, new AGM batteries have the lowest, and old industrial (Lead-Antimony plates) are the highest. In systems that are continually connected to some type charging source, whether it is solar, wind, or an AC powered charger this is seldom a problem. However, one of the biggest killers of batteries is sitting stored in a partly discharged state for a few months. A "float" charge should be maintained on the batteries even if they are not used (or, especially if they are not used). Even the "dry charged" batteries (those sold without electrolyte so they can be shipped more easily, with acid added later) will deteriorate over time. Max storage life on those is about 2-3 years.

    Batteries self-discharge faster at higher temperatures. Lifespan can also be seriously reduced at higher temperatures - most manufacturers state this as a 50% loss in life for every 15 degrees F over a 77 degree cell temperature. Lifespan is increased at the same rate if below 77 degrees, but capacity is reduced. This tends to even out in most systems - they will spend part of their life at higher temperatures, and part at lower. The old myth about not storing batteries on concrete floors is just that - a myth.

    State of Charge

    State of charge, or conversely, the depth of discharge (DOD) can be determined by measuring the voltage and/or the specific gravity of the acid with a hydrometer. This will NOT tell you how good (capacity in AH) the battery condition is - only a sustained load test can do that. Voltage on a fully charged battery will read 2.12 to 2.15 volts per cell, or 12.7 volts for a 12 volt battery. At 50% the reading will be 2.03 VPC, and at 0% will be 1.75 VPC or less. Specific gravity will be about 1.265 for a fully charged cell, and 1.13 or less for a totally discharged cell. This can vary with battery types and brands somewhat - when you buy new batteries you should charge them up and let them sit for a while, then take a reference measurement. Many batteries are sealed, and hydrometer reading cannot be taken, so you must rely on voltage. Hydrometer readings may not tell the whole story, as it takes a while for the acid to get mixed up in wet cells. If measured right after charging, you might see 1.27 at the top of the cell, even though it is much less at the bottom. This does not apply to gelled or AGM batteries. 

    "False" Capacity

    A battery can meet all the tests for being at full charge, yet be much lower than it's original capacity. If plates are damaged, sulfated, or partially gone from long use, the battery may give the appearance of being fully charged, but in reality acts like a battery of much smaller size. This same thing can occur in gelled cells if they are overcharged and gaps or bubbles occur in the gel. What is left of the plates may be fully functional, but with only 20% of the plates left... Batteries usually go bad for other reasons before reaching this point, but it is something to be aware of if your batteries seem to test OK but lack capacity and go dead very quickly under load.

    On the table below, you have to be careful that you are not just measuring the surface charge. To properly check the voltages, the battery should sit at rest for a few hours, or you should put a small load on it, such as a small automotive bulb, for a few minutes. The voltages below apply to ALL Lead-Acid batteries, except gelled. For gel cells, subtract .2 volts. Note that the voltages when actually charging will be quite different, so do not use these numbers for a battery that is under charge.

    Amp-Hour Capacity

    All deep cycle batteries are rated in amp-hours. An amp-hour is one amp for one hour, or 10 amps for 1/10 of an hour and so forth. It is amps x hours. If you have something that pulls 20 amps, and you use it for 20 minutes, then the amp-hours used would be 20 (amps) x .333 (hours), or 6.67 AH. The accepted AH rating time period for batteries used in solar electric and backup power systems (and for nearly all deep cycle batteries) is the "20 hour rate". This means that it is discharged down to 10.5 volts over a 20 hour period while the total actual amp-hours it supplies is measured. Sometimes ratings at the 6 hour rate and 100 hour rate are also given for comparison and for different applications. The 6-hour rate is often used for industrial batteries, as that is a typical daily duty cycle. Sometimes the 100 hour rate is given just to make the battery look better than it really is, but it is also useful for figuring battery capacity for long-term backup amp-hour requirements.

    Why amp-hours are specified at a particular rate:

    Because of something called the Peukert Effect. The Peukert value is directly related to the internal resistance of the battery. The higher the internal resistance, the higher the losses while charging and discharging, especially at higher currents. This means that the faster a battery is used (discharged), the LOWER the AH capacity. Conversely, if it is drained slower, the AH capacity is higher. This is important because some folks have chosen to rate their batteries at the 100 hour rate - which makes them look a lot better than they really are. Here are some typical battery capacities from the manufacturers data sheets:

    Battery Type

    100 hour rate

    20 hour rate


    Trojan T-105

    250 AH

    225 AH


    US Battery 2200


    225 AH

    181 AH

    Concorde PVX-6220

    255 AH

    221 AH

    183 AH

    Surrette CH-375 (L-16)

    429 AH

    344 AH

    282 AH

    Trojan L-16

    400 AH

    360 AH


    Surrette CS-25-PS

    974 AH

    779 AH

    639 AH

    State of Charge

    Here are no-load typical voltages vs state of charge

    (figured at 10.5 volts = fully discharged, and 77 degrees F). Voltages are for a 12 volt battery system. For 24 volt systems multiply by 2, for 48 volt system, multiply by 4. VPC is the volts per individual cell - if you measure more than a .2 volt difference between each cell, you need to equalize, or your batteries are going bad. These voltages are for batteries that have been at rest for 3 hours or more. Batteries that are being charged will be higher - the voltages while under charge will not tell you anything, you have to let the battery sit for a while. For longest life, batteries should stay in the green zone. Occasional dips into the yellow are not harmful, but continual discharges to those levels will shorten battery life considerably. It is important to realize that voltage measurements are only approximate. The best determination is to measure the specific gravity, but in many batteries this is difficult or impossible.

    State of Charge

    12 Volt battery

    Volts per Cell


































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    Battery Charging

    Battery charging takes place in 3 basic stages: Bulk, Absorption, and Float.



    Bulk Charge - The first stage of 3-stage battery charging. Current is sent to batteries at the maximum safe rate they will accept until voltage rises to near (80-90%) full charge level. Voltages at this stage typically range from 10.5 volts to 15 volts. There is no "correct" voltage for bulk charging, but there may be limits on the maximum current that the battery and/or wiring can take.

    Absorption Charge: The 2nd stage of 3-stage battery charging. Voltage remains constant and current gradually tapers off as internal resistance increases during charging. It is during this stage that the charger puts out maximum voltage. Voltages at this stage are typically around 14.2 to 15.5 volts.

    Float Charge: The 3rd stage of 3-stage battery charging. After batteries reach full charge, charging voltage is reduced to a lower level (typically 12.8 to 13.2) to reduce gassing and prolong battery life. This is often referred to as a maintenance or trickle charge, since it's main purpose is to keep an already charged battery from discharging. PWM, or "pulse width modulation" accomplishes the same thing. In PWM, the controller or charger senses tiny voltage drops in the battery and sends very short charging cycles (pulses) to the battery. This may occur several hundred times per minute. It is called "pulse width" because the width of the pulses may vary from a few microseconds to several seconds.

    Chargers: Most garage and consumer (automotive) type battery chargers are bulk charge only, and have little (if any) voltage regulation. They are fine for a quick boost to low batteries, but not to leave on for long periods. Among the regulated chargers, there are the voltage regulated ones, such as Iota Engineering and Todd, which keep a constant regulated voltage on the batteries. If these are set to the correct voltages for your batteries, they will keep the batteries charged without damage. These are sometimes called "taper charge" - as if that is a selling point. What taper charge really means is that as the battery gets charged up, the voltage goes up, so the amps out of the charger goes down. They charge OK, but a charger rated at 20 amps may only be supplying 5 amps when the batteries are 80% charged. To get around this, Statpower (and maybe others?) have come out with "smart", or multi-stage chargers. These use a variable voltage to keep the charging amps much more constant for faster charging.

    Suggested chargers are the Iota Engineering battery chargers and the Statpower Truecharge "smart" chargers.

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    Charge controllers

    A charge controller is a regulator that goes between the solar panels and the batteries. Regulators for solar systems are designed to keep the batteries charged at peak without overcharging. Meters for Amps (from the panels) and battery Volts are optional with most types. Some of the various brands and models that we use and recommend are listed below. Note that a couple of them are listed as "power trackers" - for a full explanation of this, see the page on "Why 75 watts does not equal 75 watts".

    Most of the modern controllers have automatic or manual equalization built in, and many have a LOAD output. There is no "best" controller for all applications - some systems may need the bells and whistles of the more expensive controls, others may not.

    These are some of the charge controllers that are recommend at this time for all systems. Exact model will depend on application and system size and voltage.

    Trace C12, C40 (not C30 or C30a)
    Morningstar Prostar and SunSaver
    Solar Converters Inc (power point trackers)
    Pulse (now part of Trace)
    Fire Wind & Rain (max power point trackers)

    Using any of these will almost always give better battery life and charge than "on-off" or simple shunt type regulators

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    Battery Charging Voltages and Currents:

    Most flooded batteries should be charged at no more than the "C/18" rate for any sustained period. "C/8" is the battery capacity at the 20-hour rate divided by 8. For a 220 AH battery, this would equal 26 Amps. Gelled cells should be charged at no more than the C/20 rate, or 5% of their amp-hour capacity. The Concorde AGM batteries are a special case - the can be charged at up the the Cx4 rate, or 400% of the capacity for the bulk charge cycle. However, since very few battery cables can take that much current, it isn't recommend you try this at home. To avoid cable overheating, you should stick to C/4 or less.

    Charging at 15.5 volts will give you a 100% charge on Lead-Acid batteries. Once the charging voltage reaches 2.583 volts per cell, charging should stop or be reduced to a trickle charge. Note that flooded batteries MUST bubble (gas) somewhat to insure a full charge, and to mix the electrolyte. Float voltage for Lead-Acid batteries should be about 2.15 to 2.23 volts per cell, or about 12.9-13.4 volts for a 12 volt battery. At higher temperatures (over 85 degrees F) this should be reduced to about 2.10 volts per cell.

    Never add acid to a battery except to replace spilled liquid. Distilled or deionized water should be used to top off non-sealed batteries. Float and charging voltages for gelled batteries are usually about 2/10th volt less than for flooded to reduce water loss. Note that many shunt-type charge controllers sold for solar systems will NOT give you a full charge - check the specifications first. To get a full charge, you must continue to apply a current after the battery voltage reaches the cutoff point of most of these type of controllers. This is why it is recommended the charge controls and battery chargers listed in the sections above. Not all shunt type controllers are 100% on or off, but most are.

    Flooded battery life can be extended if an equalizing charge is applied every 10 to 40 days. This is a charge that is about 10% higher than normal full charge voltage, and is applied for about 2 to 16 hours. This makes sure that all the cells are equally charged, and the gas bubbles mix the electrolyte. If the liquid in standard wet cells is not mixed, the electrolyte becomes "stratified". You can have very strong solution at the top, and very weak at the bottom of the cell. With stratification, you can test a battery with a hydrometer and get readings that are quite a ways off. If you cannot equalize for some reason, you should let the battery sit for at least 24 hours and then use the hydrometer. AGM and gelled should be equalized 2-4 times a year at most - check the manufacturers recommendations, especially on gelled.

    Battery Aging

    As batteries age, their maintenance requirements change. This means longer charging time and/or higher finish rate (higher amperage at the end of the charge). Usually older batteries need to be watered more often. And, their capacity decreases.

    Mini Factoids



    Nearly all batteries will not reach full capacity until cycled 10-30 times. A brand new battery will have a capacity of about 5-10% less than the rated capacity.

    Batteries should be watered after charging unless the plates are exposed, then add just enough water to cover the plates. After a full charge, the water level should be even in all cells and usually 1/4" to 1/2" below the bottom of the fill well in the cell (depends on battery size and type).

    In situations where multiple batteries are connected in series, parallel or series/parallel, replacement batteries should be the same size, type and manufacturer (if possible). Age and usage level should be the same as the companion batteries. Do not put a new battery in a pack which is more than 3 month old or has more than 75 cycles. Either replace with all new or use a good used battery.

    The vent caps on flooded batteries should remain on the battery while charging. This prevents a lot of the water loss and splashing that may occur when they are bubbling.

    When you first buy a new set of flooded (wet) batteries, you should fully charge and equalize them, and then take a hydrometer reading for future reference. Since not all batteries have exactly the same acid strength, this will give you a baseline for future readings.

    When using a small solar panel to keep a float (maintenance) charge on a battery (without using a charge controller), choose a panel that will give a maximum output of about 1/300th to 1/1000th of the amp-hour capacity. For a pair of golf cart batteries, that would be about a 1 to 5 watt panel - the smaller panel if you get 5 or more hours of sun per day, the larger one for those long cloudy winter days in the Northeast.

    Lead-Acid batteries do NOT have a memory, and the rumor that they should be fully discharged to avoid this "memory" is totally false and will lead to early battery failure.

    Inactivity can be extremely harmful to a battery. It is a VERY poor idea to buy new batteries and "save" them for later. Either buy them when you need them, or keep them on a continual trickle charge. The best thing - if you buy them, use them.

    Some Peukert Exponent values (not complete, just for info). Don't have a lot of data. Trojan T-105 = 1.25; Optima 750S = 1.109; US Battery 2200 = 1.20.


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