A number of times on this and other forums I have come across situations where a battery owner is confusing RC with Ah and wanted to quickly address this.
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. 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 also not usually a purpose designed deep cycle battery. 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.
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 but still not exact.
RC/RM is generally almost DOUBLE what the usual Ah rating is. On another post, 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/RM so this bank was nearly HALF the size it was thought to be which could prove to be an expensive miscalculation.
Mistaking RC/RM for Ah can be a dangerous and costly mistake. 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. 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 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 are looking at an RC/RM measurement rather than an Ah capacity measurement..
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. 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 also not usually a purpose designed deep cycle battery. 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.
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 but still not exact.
RC/RM is generally almost DOUBLE what the usual Ah rating is. On another post, 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/RM so this bank was nearly HALF the size it was thought to be which could prove to be an expensive miscalculation.
Mistaking RC/RM for Ah can be a dangerous and costly mistake. 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. 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 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 are looking at an RC/RM measurement rather than an Ah capacity measurement..
