Dennis N replied:

Mark,

Dug this formula up using Google [wanted to know myself seeing you asked]

Cam from a University of Hawaii electronics website. hope it is of use:

Many batteries today are rated in CCA or RC. CCA is Cold Cranking Amps. CCA is approximately equal to the RC of a battery times five. [1000 CCA is about 190 RC]. RC is Reserve Capacity. You can convert RC to amp/hours by the following formula:

Amp/Hours = (Reserve Capacity / 2) plus 16

Steve replied:

Beware using the formula too literally. It is for standard construction batteries. Note that there are significant differences in the construction of Deep cycle to starting batteries. For example a standard 75 amp/hour starting battery may have 1000CCA (if your starting a landcruiser ;). However, a deep cycle 75 amp battery may only be able to deliver 500CCA...as it is not meant for high amp starting. Best way is to check the labels or web sites.

porl replied:

As per the last response - the formula may work for certain types of batteries but deep cycle batteries are fundamentally different in construction to starting batteries and for that very reason they are measured in CCA and not RC. An good example of the last reply is my deep cycle yellow top optima - it has a rating of 65ah but at 0 degrees centrigrade can put out from memory something like 950CCA - which certainly does not fit the formula proposed - but that of course is perhaps because the optima is not a wet celled battery. etc etc. Also, don't use a starting battery as a deep cycle battery to draw down to its DOD (depth of discharge) or you will get a fraction of its expected lifespan.