So what's the bottom line between golf cart batteries (GC2), L16's, J305's, GC-12/T1275/J-150/9C12/921's and light-cycling 12V Group 24, 27, 31 etc.? Below is the answer one of the largest US battery makers gave to me when I asked this very pointed *question. (*Source email string between Marine How To and battery manufacturer testing engineer.)


Question asked by Marine How To.com:
"If the GC2, GC-12 or L16's were cycled using the same cycle life testing as the 12V Group 24, 27, 31 etc. what would that outcome, in cycle life, actually look like? Is it fair to suggest a GC2, GC12 or L16 battery has double to triple the lab cycle life, to 50% DOD, than the 12V 24, 27 & 31 etc. "deep cycles" do?"


Senior Battery Engineer - Major US Battery Manufacturer:
"Yes, for packs of equivalent energy content (voltage * capacity) the Golf Car types and L16's are 2-3 times better than the DC automotive sizes (24, 27 and 31)."


Wow.. Straight from the source, golf car batteries are 2-3X better than 12V 24, 27 & 31. These flooded 12V Group 24, 27 & 31 batteries are simply marketed in a rather misleading manner..


To make an even fairer comparison I ran the numbers on a group 24 deep cycle test and a GC2 cycle test by the same manufacturer. Both batteries were tested at approximately 33% of their 20 hour Ah rating, and it can't get more apples to apples, in terms of comparable testing, than this.


The Group 24 "deep-cycle" delivered just 350 lab cycles and the GC2 golf cart battery delivered 1000 cycles when tested exactly the same way. Both batteries tested at the same discharge rate to the same end of life point yet the golf cart battery delivered 650 more cycles than the 12V Group 24 "deep cycle".


I am going to review some terms I am using in this article.


Lab Rated Cycle Life:


This term simply denotes what the battery manufacturer see's in their own white coat, white glove, laboratory. The batteries are cycled under ideal conditions, and quite often to no industry standardized test across brands. Some might test the battery to BCIS-02 and some to BCIS-07. Some might use the 20 hour discharge rate and some might use the 5 hour or 2 hour rate for cycle life testing.


For what it's worth some battery makers never even conduct a 20 hour test and instead this data is "calculated" from other tests that take them much less time. The batteries which are most likely to never undergo an actual 20 hour capacity tests? You guessed it "automotive" G-24, 27, 31 etc.types (I am quoting the battery engineer with this quote)


Battery testing procedures are guided by the Battery Council International or BCI for short. The problem is there are many different cycle life testing procedures and manufacturers rarely tell you which one they use or are testing to for the specific battery in question.


I was recently at a solar trade show and two of the big gun battery makers referenced in this article were there. I asked each manufacturer to tell me which BCI test standards they used for 12V marine G-24, 27 & 31 batteries vs. golf car or other deep cycle batteries. The answers I got.... BLANK STARES.


One manufacturer then suggested the golf car batteries might be tested at a 2 hour rate and the marine batteries at a different rate. If this is so then the spread between golf cart batteries and 12V deep cycle batteries can grow even wider in the real world. Ouch!!!

Light-cycling batteries are already half of the rated cycles of a golf car type battery and they "may" be being treated with kid gloves to boot when you actually compare testing. At a best case the 12V G-24, 27 & 31 battery might be half the cycles of an actual deep cycle in the lab but they may actually be worse than half the rated cycles depending upon the testing used hence the battery engineers statement of golf car type batteries being 2-3 times better for cycling use...


This is just a sampling of some BCI Tests:

BCIS-05 - Capacity Testing of Electric Vehicle & Cycling Batteries

BCIS-06 - Constant Current Cycle Life Testing of Deep Cycle Batteries

BCIS-07 - Cycle Life Testing of Batteries for Golf Carts

BCIS-08 - Cycle Life Testing of Deep Cycle Marine/RV Batteries


What tests were used and the differences between these tests can result in massive variances, brand to brand, and also in their cycle life claims. This is why it is nearly impossible to use cycle life data across brands or manufacturers. Through in-house testing battery manufacturers can produce cycle life data across their Oown products based on their own testing.


Remember these lab tests are under ideal conditions and the battery is repeatedly cycled up to full and down to the chosen DOD then back up again, all at controlled temps, discharge rate, charge rate, rest times and all back-to-back. These numbers have very little meaningful relationship to real world cycling performance. Out in the real world marine batteries rarely, if ever, deliver even half the lab rated cycles, and there is good reason for this. In general most boaters can expect 10% to as high as 60% of the lab cycles out in the real world.


The only useful lab data is lab data within a brand or manufacturer of batteries. For example, if Deka/East Penn says their 6V GC2 golf cart battery is 700-1000 cycles, when compared to their 12V Group 24, 27 or 31 deep cycle, using the same test criteria such as BCI 2, this can be good guidance, and I stress the term guidance. It is only good guidance within the Deka/East Penn brand as to which of their batteries could be expected to deliver the most cycles when used in a deep cycling application.


However, if Trojan suggests their 12V Group 24, 27 or 31 deep cycle marine battery (SCS-150, SCS-200 & SCS-225) have 600 cycles, you CAN NOT and SHOULD NOT compare that data to another brand or manufacturer. It is beyond meaningless to do so. Remember these companies rarely use independent external labs or standardized cycle life test procedures in direct A to A test scenarios across brands.



Real Word Cycle Life:


This one is almost impossible to nail down other than to say, from years of experience, that the average marine battery won't usually deliver even half the cycles, in the real world, as they do in the lab. Cut lab numbers in half, at the very top end, and you'll be in better shape expectation wise. Why?


In the real world we have so many factors that inhibit lab like results that it is nearly impossible to get anywhere close to this data. By industry standards a flooded battery is dead when it can no longer deliver 80% of its rated Ah capacity. Will a battery still work beyond this point? Absolutely, and they do, but the catastrophic failure rate, such as an internal short, starts to increases pretty steeply as the rated Ah capacity drops below this 80% figure. The rate of internal failures I see in marine batteries is highest in 4D & 8D then group 24, 27 & 31 flooded batteries. AGM & GEL much less so but it happens. It also happens in golf car batteries but to a much lesser extent than I see it in "automotive" case batteries. Why? I don't have a good answer other than to point at the separators and space below the plates. Separators are thicker in industrial batteries than they are in automotive cased flooded batteries. The last few GC2's and group 24, 27 or 31 batteries I have autopsied showed the separators to be nearly twice as thick in the golf car batteries than they are in the automotive cased batteries.


In a recent survey of "Marine Battery Use" on Sailboatowners.com, more than 1000 sailors responded, a truly exceptional response. 79.5% of responders indicated they cycled their house banks to 50% DOD less than 25 times per year. Stack that data point on-top of the fact that 82% of responders reported less than 6 years of marine battery life and that means the vast majority of boaters are getting less than 150 real world cycles out of their marine batteries... Ouch! While there was a large spread in cycle life between golf cart and group 24, 27 & 31 type batteries, in terms of longevity, none of these batteries tend get within half the lab ratings.


Keep in mind that many of these companies claim 1200 "lab cycles", as seen right in this article, and yet these batteries are still failing in less than 300 cycles of marine deep cycle use. Lab cycle life is simply a fairy tale in the marine world. The lower the lab cycle life number the worse the batteries generally perform in a real world application.


Remember that golf and industrial batteries can be murdered too it just usually takes a bit longer.



Unfortunately we have no widely used test equipment that can accurately tell us the as-now 20 hour ampere hour capacity. It is a real crap shoot and there are lots of tools that try but all fail miserably, except for an actual 20 hour capacity test, which very few are even willing to pay for.


I do offer physical 20 hour capacity testing for my customers with expensive GEL or AGM batteries, or even expensive industrial flooded batteries, but the testing cost, per battery, for a typical sub $200.00 flooded battery, is simply not realistic. As far as I know I am the only marine electrician in the US offering this type of testing and it is EYE OPENING & SHOCKING to many owners.


*EDIT: I know Charlie Johnson of JTB Marine in Florida & Bill Trayfors of YachtSense in Annapolis have now added the equipment to test for actual Ah capacity.


You can do a capacity test yourself but it involves some set up and equipment that some may not have on hand. It also requires time and patience to do so..


Owner to owner, dock talk, or dock gossip is also as useless as boobs on a bull and not meaningful. It could only be meaningful if you have a baseline for comparison & consistency and everyone used their batteries identically and we simply don't have this. Bottom line, ignore dock talk on battery life and learn how to test your own batteries.


I have one older customer, a delightful guy, with a beautiful classic little day sailor, he claims consistently & repeatably that he gets "7 years" out of his single 12V deep cycle battery. His basis for a failed battery is when he can no longer power his VHF radio. Ouch! He has no motor to start, never needs running lights and uses no electronics, he's old school and I like that! His batteries are most likely dead by year two but his VHF radio draws so little that he once was able to eek out seven years before it would not even power a brief 2A load to call the launch.


He is a battery murderer yet he does not believe nor understand it because he once had a battery last him seven years powering a VHF radio to call the launch. Flooded batteries are considered dead, by industry standards, when they can no longer deliver 80% of the new Ah capacity or 80Ah for a 100Ah rated battery. Would 80% be dead for powering a VHF radio? Absolutely not. However, in many other applications, for which the battery was intended, it would be in marginal shape at best, and would be more prone to failure.


Interestingly his last replacement battery failed internally at year 3 and became a 10 volt battery instead of 12V. Of course he still gets "7 years" out of his batteries and see's no reason to change his charging habits. Of course cost is not an issue for him, so why should he. I can nearly guarantee that if you were to talk to him today, on the dock, he still gets seven years because it did happen once.... This is a prime example of why dock talk is not meaningful in battery life conversations. If everyone capacity tested their batteries, and was honest about when they hit 80% of rated capacity, then we might have meaningful conversations about battery life. We'd also have a lot of owners saying "holy $hit" my batteries are dead in two years.. (head bonk)


In the real world batteries don't die naturally they are murdered by their owners. Here's some of the how and why with regards to the reasons lab data does not compare to the real world.




What Kills Batteries?



Temperature Abuse- Heat MURDERS BATTERIES, the engine room is the last place any battery should be.

Poor Charging Practices

Absorption voltages set too low

Inadequate absorption voltage duration (under absorbing or premature floaulation)

Incomplete or Under Charging - Alternator only for mooring sailed boats

Over Charging - Old school non-smart chargers

Improper Charging - Poorly designed battery chargers, there are MANY.

No Temp Compensated Charging - If a battery charges at 14.4V this is max at 77F, not at 90F or 100F plus

Lack Of Maintenance - Exposed plates sulfate in short order

Contamination - Happens when batteries are not properly serviced and external contaminates get in

Improper Flooded Battery Orientation - With respect to sail boats

Improper Bank Wiring - Poor wiring practices can cause interbank imbalances

Lack of Proper Equalization Charges -

Batteries Sitting at Less Than 100% SOC - Especially for for days or weeks

Chronic "PSOC" operation - Partial State of Charge Operation