How can I convert Amp Hours at the 20 Hour Rate to Reserve Capacity?
To convert Amp Hours (AHC20) to Reserve Capacity, divide the known C20 Ah rating by .54 (maximum) and .58 (minimum) to get a good average result. More ampere-hours (or RC) are better in every case when looking for a deep-cycle battery. This conversion method will allow you to convert and compare competitive data GENERALLY.
75Ah / ,54 = 138 minutes of reserve capacity at a 25A draw or 75Ah / ,58 = 129 minutes of reserve capacity at a 25A draw.
Therefore, a battery rated at 75AH C20 can be estimated to provide between 138 and 129 minutes of reserve capacity at a 25A draw.