If the battery has a reserve capacity rating instead of an amp hours rating, it's almost certainly a thin plate starting battery rather than a thick plate deep cycle battery. If it's a thin plate, then draining it deeply will cause it to lose storage capacity (sulfated) faster than a thick plate battery and it won't last as long (less total discharge/recharge cycles).
With lead-acid batteries, the life of the battery is directly related to the depth of discharge. For example you can see in this graph from Panasonic, that if you take their lead-acid batteries down to only 30% DoD, you'll get around 1200 cycles, but if you take them down 100%, you'll only get 200 cycles:
So, to supply a given load of X, a bigger battery will be drained by a lower percentage, and would last for more cycles. I.e., To supply a 50ah load, a 100ah battery would be drained 50%, whereas a 200ah battery would only be drained 25%. The bigger battery will last longer (more cycles before it's toast).
Of course, if you use a thin plate starting battery in a thick plate deep cycle application - you aren't gonna get nearly as many cycles anyway.
Last edited by dwh; 03-29-2011 at 12:33 PM.
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