Battery Reserve Capacity to Amp-Hours
The amp-hours behind a battery's reserve-capacity minutes: BCI / SAE J537 reserve capacity is the minutes a fully charged 12 V battery at 80 F sustains a 25 A draw to a 10.5 V cutoff, so amp-hours at that rate = 25 x RC/60 -- an RC of 120 minutes is 50 Ah. This RC-rate capacity is lower than the 20-hour-rate amp-hours on a deep-cycle label (Peukert's effect), so the two are not interchangeable; cold reduces it further. A comparison aid; the battery rating and a load test govern.
Formula and source
amp_hours = 25 x reserve_capacity_minutes / 60.
BCI / SAE J537 reserve capacity, by name; the battery's published rating and a load test govern.
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