This is one of many articles we like from Compass Marine.
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What is a Deep Cycle Battery?
Light Cycle vs. Deeper Cycle
NOTE: This article deals with FLOODED marine batteries, not AGM or GEL batteries.
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, 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 a manufacturer feels they want to slap a “deep cycle” sticker on, gets one. This range of deep cycle labeled batteries spans all the way from a Group 24 12V automotive cased 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 life 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 occasion I hit a point where I become so frustrated with bovine-dung marketing that I must simply explain myself. What I 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 only two companies I know of producing 4D or 8D deep cycle flooded batteries, comparable to Golf Car batteries in terms of cycling, and they are Dyno and Rolls.
There are 2V, 6V and 12V flooded batteries that are better suited to cycling duty than the typical Group 24, 27, 31 , 4D or 8D automotive cased 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.
Let’s go through how far too many boat owners buy batteries;
#1 Boat owner 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 Boat owner walks into West Marine or battery dealer and says “I want a house bank battery with the most capacity.” Associate looks at the reserve capacity (RC) or Reserve Minutes (RM) rating and tells the buyer; “Here’s a Group 24 130Ah battery.“. The boat owner leaves thinking he just bought a massive 130Ah Group 24 12V battery. Sadly, no such battery 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 boat owner often has little understanding of how to buy & choose a battery properly suited for deep-cycling use.
Deep cycle use, for most battery manufacturers, is considered drawing the battery to 50% state of charge, on a fairly regular basis. The boat owner however is not alone in their lack of education 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 want to purchase 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 the re-seller who simply slapped a sticker on it. You then call the manufacturer and find out the 20 hour Ah capacity rating for that battery, if it even has one. Some batteries are tested for a 20 hour rating but the sticker brands often leave this data off.
Worse yet is that some Group 24, 27 31 etc. batteries have their 20 hour rating “calculated” from a 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 is usually conducted 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 from RC to Ah capacity do not always work out accurately. Many Group 24, 27, 31 etc. batteries, that 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 such as a West Marine.
As I mentioned 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 on a per ampere hour basis 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:
- What is the 20 hour Ah rating?
- What is the “lab rated” cycle life of the battery when compared to other batteries within its own brand?
- What is the Cost per Ah?
- What is the cost per lab-cycle?
- What is the largest Ah capacity battery I can fit?
- Can the manufacturer provide you a Peukerts constant so a battery monitor can be programmed?
- Is this a good quality battery made by a reputable manufacturer?
- Who actually manufactured this battery? (Not who put the sticker on it?)
- 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:
· Group 24 Deep Cycle = 75 Ampere Hours or 150Ah’s for two. (you’d need two of them for any usable house bank)
· Group 24 Deep Cycle = 350 lab rated cycles to 50% DOD
· Cost per Ah = $2.07 For a 150Ah bank
· 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):
· Group GC2 6V = 230 Ampere Hours (two 6V batteries in series = 12V bank)
· Group GC2 Deep Cycle = 850 (took mid range) “lab rated” cycles to 50% of capacity
· Cost per Ah = $1.74 For a 230Ah bank
· 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 light 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, based on West Marine prices, is considerably less and the cost per lab cycle is approx half.
My point here? Don’t forget to look into the lab rated cycle life and then purchase batteries that can theoretically deliver you the most cycles. We must remember that lab cycles are a theoretical best case for “real world” use. bThis cycle life data can be used as 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.
Fitment 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 GC2 (T-105) golf car battery will fit into virtually the same foot print as a Group 24 battery only it will be slightly taller..
Dual Purpose vs. Deep Cycle
Pictured here is the lab rated difference between a golf cart battery and a Dual Purpose Group 24, 27 or 31.
The West Marine Advisor – Cycle Life
Just in case you wanted to see where you can actually reference the *data points am discussing, in regards to the West Marine Sea Volt/ East Penn product, you can find it in the West Marine catalog. This chart is on page 379 of the 2014 West Marine catalog..
When buying batteries for deep cycling use, always be sure you can get your hands on manufacturer cycle life data to compare their batteries across types. No data? No buy…….
*Data Points: I have confirmed this data to be accurate, with West Marine, to the best of their knowledge. The data in this image comes straight from West Marines supplier, East Penn Manufacturing which is also known as Deka.
East Penn Golf & L16 Chart
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:
Q: “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?”
A: 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).”