Join Date: Jan 2003
Location: Maine Coast
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Battery Confusion - Amp Hours vs. Reserve Capacity
A number of times on this and other forums, but twice just in the last two days, I have come across situations where a battery owner is confusing reserve capacity (RC) with amp hours (Ah) and wanted to quickly address this.
Battery life/longevity is a constant topic of frustration for sailors. What makes this even more frustrating is the confusion between Reserve Capacity (RC) and Amp Hours (Ah). There is much more to batteries but I do not want to drag that out here.
RC and Ah are NOT one in the same as I often see people use them interchangeably. Usually, most batteries that do not have an amp hour rating are also not usually a purpose designed deep cycle battery. Some still are, but have just not been Ah tested.
A safe bet is to always by a battery with a Ah capacity rating. This ensures that the battery has been tested and rated for such. You still can use the RC/Reserve Capacity number but this number should generally be divided by two to arrive at a close Ah capacity.
For example the Trojan AGM 4D battery is rated at 325 RC and 165 Ah. If you divide by 2 you would get 162.5 Ah's. This is usually a close approximation to the Ah capacity of a given battery.
RC is generally almost DOUBLE what the usual Ah rating is. On another forum, a rather astute and knowledgeable owner stated that their battery had XXX Ah's. The reality is that this battery was never rated in Ah capacity and only in RC so this bank was nearly HALF the size it was thought to be, which could prove to be an expensive miscalculation.
Mistaking RC for Ah can be a dangerous and costly mistake. You could kill your batts, and be left dead in the water. This is especially true if after removing 150 AH from a 300 RC bank, thinking you are only at half discharge, when you are actually closer to flat dead.
There are a few theories on how to determine Ah capacity from RC but none that I have found spot on. Some say to divide RC by 2 then to add 16 to that number while others just say to divide by 2.
Personally I prefer to buy batteries rated for Ah capacity rather than to attempt a guess, as any guess could throw off a battery monitor or energy budget or could leave you depleting your bank beyond 50% DOD which can considerably shorten its life..
If you buy a specific group size battery eg: group 24, 27, 29/31, 4D, 8D etc. and the one you're buying has a significantly higher "rating" than others of the same physical size that should be a red flag that you are looking at an RC measurement rather than an Ah capacity measurement..
Just a head up.
-Maine Sail / CS-36T
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Last edited by Maine Sail; 04-09-2010 at 07:58 PM.