In dealing with Interstate batteries over an issue of selection, they told me for a rough estimate of amp hours, multiply the RC minutes (Reserve Capacity) by 0.60. (I think I have that right by memory.) So RC=650 * .60 = 390AH. Perhaps that is useful.
Reserve capcity or reservve minutes is the number of minutes the battery will supply at a 25A load before hitting 10.5V.
Due to Peukert constants being different among batteries any translation can only be a rough approximation..650 reserve minutes would be a HUGE battery. Don't confuse MCA, CCA and RC... RC and Ah are not the same and conversions are rarely spot on accurate.
A number of times on this and other forums I have come across situations where a battery owner is confusing RC with Ah or MCA or CCA..
Battery life/longevity is a constant topic of frustration for sailors and there are lots of factors that come into play, which I will not bore you with here. What makes this even more frustrating is the confusion between Reserve Capacity (RC) and Amp Hours (Ah). Reserve Capacity, or more accurately Reserve Minutes, is how long the battery can support a 25 amp load before dropping to 10.5 volts.
RC and Ah are not one in the same. Usually, most batteries that do not have an amp hour rating are often not a purpose designed deep cycle battery. However in many cases a battery is private labeled and may have a 20 hour rate but they chose to omit it in the labeling. Some still are, but have not been Ah tested or the vendor relabeling the battery just chooses not to publish the 20 hour Ah rate..
A safe bet is to always by a battery with a 20 hour 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. There is not really accurate conversion factor I have found.
Keep in mind that the new Wal*Mart rating is pretty much meaningless. For the group 29/31 it says 114Ah @1.. That is about as poor and meaningless a rating as it gets.
A group 31 battery should produce between 100Ah and 130Ah depending upon brand. The 20 hour load for a 100Ah battery would be 5A and the 20 hour load for the 130Ah battery would be 6.5A.
At a 1A load a 100Ah battery would produce 149Ah's with a Peukert of 1.25
At a 1A load a 130Ah battery would produce 208Ah's with a Peukert of 1.25
If in fact the Wal*Mart battery is 114Ah (20 hour rate) then the load at which it will deliver its 20 hour rate is 5.7A not 1A.... Due to the piss poor labeling this rating is either meaningless or completely incorrect. No one at Wal*Mart or JCI can confirm the specifications on these batteries, I have tried to get them...
Back to RC calculations. 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 a close approximation to the Ah capacity of THIS given battery but still not exact.
If we take the Duracell 29HM from earlier in this thread we can see that it has 105Ah and a RC of 185 minutes. If we divide by 2 we get 92.5Ah's. If we divide by 2 and add 16 we get 108.5. Still neither hit this exactly. If we do 185 x .60 we wind up at 111Ah's... RC and Ah don't exactly translate due to Peukert....
We as boaters need to be careful because RC/RM is generally almost DOUBLE what the usual Ah rating is. I have even seen rather astute and knowledgeable owners who've stated that their battery had XXX Ah's. The reality is that this battery was never rated in Ah capacity and only in RC/RM so this bank was nearly HALF the size the owner thought it was. This can prove to be an expensive miscalculation.
Using the 29HM from above again, "I have a 185Ah battery so that means 50% SOC is 92.5Ah that I can remove." Remove 92.5Ah from a 105Ah battery and were are you!!!!:cry:
Mistaking RC/RM for Ah can be a dangerous and costly mistake and I actually see it quite frequently. You could kill your batts and be left dead in the water especially if your think after removing 150 AH from a 300 RC bank 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/RM 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. Some say RC X .60 etc.. None have come up exact when applied to multiple examples of known battery Ah ratings. Why? Each battery has is own Peukert constant. A battery with a Peukert of 1.11 wont translate the same as a battery with a Pekert of 1.49...
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 (depth of discharge) which can drastically 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 may be looking at an RC/RM measurement rather than an Ah capacity measurement..
