NOTE: This article deals with FLOODED marine batteries, not AGM or GEL!


The Problem: The problem, as I see it, is one of dishonest marketing where multiple battery types wear the same clothes (sticker) and will perform differently when placed into a deep cycling situation. Sadly the standards board in the USA, Battery Council International (BCI), allows for this foolishness.


Does this mean a Group 24, 27 or 31 battery can't and won't work? Absolutely not, it just means that when compared to a battery that is actually designed, built and intended for deep cycling activity, such as golf car or sweeper scrubber or fork truck batteries a Group 24, 27 or 31 flooded battery simply is not the same, cycle-wise, despite the "sticker" suggesting to a buyer that it is.


Unfortunately the industry has no real marketing definition of what "deep cycle" means so any battery they feel they want to slap a "deep cycle" sticker on, gets one all the way from a Group 24 12V battery up to and including massive 15-20 year expected life 2V cells used in stationary applications. A 2V cell is obviously more "deep cycle" than even golf car, fork truck or sweeper scrubber batteries but all may wear the same "deep cycle sticker. If you buy batteries more carefully, you can get more out of them for your dollar.


As a marine electrician I get to see, replace and test lots and lots of batteries. Because of this I also need to know quite a bit about batteries, how they work, how they charge, how they cycle, which ones are best suited for the application and above all else, how they are marketed. Suffice it to say this last point, the marketing, is my biggest bone of contention.


I also test batteries for their actual Ah capacity ratings on a near daily basis. As I type this I have two 20 hour tests in-process. When I began doing actual 20 hour testing about 17-18 years ago my set up was crude but it worked ok. I quickly began to see a sharp differentiation between how 12V Group 24, 27, 31, 4D & 8D batteries were holding up when compared to industrial use deep cycle batteries such as golf cart, L16's J305's etc.. As time moved on I invested more money into better capacity testing & charging equipment to get much more repeatable and more accurate capacity data. My test stations even include temp controlled water-baths so battery temp is not a factor.


On occasions I hit a point where I become so frustrated with bovine-dung marketing that I must simply explain myself. What believe should be obvious, is not always so obvious, especially when the marketing departments muddy the waters.


This article is one of those cases where I have simply had enough of the bovine-dung and have to get this out there so boaters will have a better understanding of what they are really getting, or not getting, when they plunk down a pile of greenbacks for a set of "deep-cycle" batteries. Let this rant begin.......(wink)


First let me summarize this article in a sentence or so. This is so you can bypass the wordy diatribe if you must..


Group 24, 27 & 31 & most *4D & *8D flooded 12V "Marine Batteries" are NOT really a deep-cycle battery when compared to a battery specifically designed from the ground up as a "deep cycle" battery. Simple stuff. There are 6V and 12V batteries that are much better suited to cycling duty than Group 24, 27, 31 etc. batteries. If you have the height to fit 6V or 12V golf car, sweeper scrubber or industrial batteries you certainly can get a lot more for your money in terms of cycle life.


*There are only two companies I know of producing 4D or 8D deep cycle batteries, comparable to Golf Car batteries in terms of cycling, and they are Dyno and Rolls.


Let's go through how far too many boat owners buy batteries;


#1 Walks into West Marine or battery dealer and says "I need a new battery for my boat." Dealer or West Marine associate says "What size?" and the owner says "They look like that." The associate then asks whether the boat owner will be starting the motor only or starting the motor and using electronics with it? Owner answers yes about starting and electronics and walks out with a 12V group 24, 27 or 31 "Marine Dual Purpose" battery for a deep-cycling application.... This transaction = FAIL


#2 Owner walks into said retailer and says "I want a house bank battery with the most capacity." Associate looks at the reserve capacity "RC" or Reserve Minutes rating and tells the buyer "Here's a Group 24 130Ah battery.". Owner leaves thinking he just bought a massive 130Ah Group 24 12V battery. Sadly, no such beast exists on this planet. D'oh... This transaction = FAIL


There are plenty more scenarios I could play out here but the main point is that the average boater has little understanding of how to buy & choose a battery properly suited for deep-cycling use. Deep cycle use is drawing the battery to 50% state of charge, on a fairly regular basis. The boat owner however is not alone in this because, even the associates at many battery retailers don't have a clue about the products they are selling.


It is not my intent to pick on West Marine in this article but seeing as they have the published data in their catalog, which most battery buyers entirely miss or gloss over on, I thought I would walk through the battery buying process for the average flooded battery buyer..


Some Key Points:


*Ampere Hours (Ah's) and Reserve Capacity / Reserve Minutes (RC) are not one in the same nor are they interchangeable. You buy house bank cycling batteries based on the Ampere Hour rating at the 20 hour rate not the Reserve Capacity or Reserve Minutes rating. If the battery does not have a 20 hour Ampere Hour rating it is generally best to simply walk away, or find out who actually made the battery.


When I say "who made the battery" I am talking about the physical manufacturer, not who put the sticker on it, but who physically manufactured it. You then call them and find out the 20 hour Ah capacity rating for that battery. Some batteries are tested for 20 hour rate but the sticker shops leave this data off.


Worse yet is that some Group 24, 27 31 etc. batteries have their 20 hour rating "calculated" from reserve minute or reserve capacity testing. What? All this means is the battery you may be buying was never physically tested for an actual Ah rating and they used a mathematical calculation to try and predict the 20 hour rate. An RC test is at 25A and an Ah capacity test is one which allows the battery to run for 20 hours at the 20 hour discharge rate. For a 100Ah battery this would be 5A. That same battery when tested for reserve capacity would be discharged at a 25A rate. The mathematical translations to Ah capacity do not always work out accurately, and many Group 24, 27, 31 etc. batteries I have tested have a very, very tough time delivering their actual Ah capacity rating even when new and cycled for break in.


*Not all marketed "Deep Cycle" batteries are in fact Deep Cycle batteries. The vast majority of 12V flooded batteries sold as "Marine Deep Cycle" are not really what I would consider a deep cycle battery. I often refer to 12V flooded deep cycle batteries as light cycling or pseudo deep cycle batteries. While there are a few exceptions to this rule, such as 12V golf cart batteries and 12V sweeper/scrubber batteries, you won't find them most marine retailers like a West Marine.


As I said above Group 24, 27, 29, 30, 31, & most 4D & 8D flooded batteries are simply not a deep cycle product when compared to a battery such as a golf car battery, sweeper/scrubber battery or any other industrial application deep cycle batteries. GC2 6V batteries usually cost less than 12V batteries per ampere hour and also usually yield double the lab cycle life rating or more..


*The term "Deep Cycle" has been badly bastardized by the industry to the point that it is almost entirely meaningless. You as the buyer should educate yourself as to what really represents a good value before buying batteries..




Deep Cycle Battery Questions to Be Answered:


#1 What is the 20 hour Ah rating?

#2 What is the "lab rated" cycle life of the battery when compared to other batteries within its own brand?

#3 What is the Cost per Ah?

#4 What is the cost per lab-cycle?

#5 What is the largest Ah capacity battery I can fit?

#6 Can the manufacturer provide you a Peukerts constant so a battery monitor can be programmed?

#7 Is this a good quality battery made by a reputable manufacturer?

#8 Who actually manufactured this battery? (Not who put the sticker on it?)

#10 What is the warranty? (do not get hung up on this)



What is the most important part of this equation that is almost always left out? If you guessed step #2, you win!!!!




Lets use these steps to buy the pictured batteries above:


Group 24 12V:

#1 Group 24 Deep Cycle = 75 Ampere Hours or 150Ah's for two. (you'd need two of them for any usable house bank)

#2 Group 24 Deep Cycle = 350 lab rated cycles to 50% DOD

#3 Cost per Ah = $2.07 For a 150Ah bank

#4 Cost Per Lab Deep Cycle = 97¢ *per cycle - *expect half the rated cycles/real world = $1.94/Cycle




Group GC2 6V (requires two for a 12V bank):

#1 Group GC2 6V = 230 Ampere Hours (two 6V batteries in series = 12V bank)

#2 Group GC2 Deep Cycle = 850 (took mid range) "lab rated" cycles to 50% of capacity

#3 Cost per Ah = $1.74 For a 230Ah bank

#4 Cost Per lab deep cycle = 47¢ *per cycle - *expect half the rated cycles/real world = 97¢ per lab Cycle


In virtually the same foot print as two Group 24 12V impostor deep cycle batteries, only slightly taller, two 6V batteries wired in series for 12V will yield approximately 850 lab cycles vs. 350! It will also yield 80 more amp hours of capacity in virtually the same foot print! The cost per Ah (West Marine prices) is considerably less and the cost per cycle is approx half.


My point here? Don't forget to look into the lab rated cycle life and then buy the batteries that can theoretically deliver you the most cycles. Remember lab cycles are a theoretical best case for "real world" use but they can be used a guidance within a brand. In other words lets stop focusing on what the "sticker" says, hell they all say "deep cycle", and focus on the expected durability of the battery in a cycling application.


Fittment of Batteries:


Unless the builder has left you no additional height at all in the battery compartment a GC2 6V or GC-12 golf car battery can very often fit where Group 24, 27 or 31 batteries did. A GC-2 (T-105) golf car battery will fit into virtually the same foot print as a Group 24 battery only it will be slightly taller..