Insert battery capacity (Ah), charger current (A), and start/end charge. Click Calculate to obtain charge time.

The formula is hours = Ah × (end% − start%) / 100 / A. For example, a 100 Ah battery from 20% to 80% on a 20 A charger is 100 × 0.6 / 20 = 3 hours. This is the constant-current estimate. Charging the last stretch to 100% on lithium often takes longer because current tapers.

Insert battery capacity (Ah), the time you have (hours), and start/end charge. Click Calculate to obtain charger current (A).

The formula is A = Ah × (end% − start%) / 100 / hours. For example, 100 Ah from 0% to 100% in 5 hours needs 100 / 5 = 20 A.