Battery charge time calculator

Charge time is the amp-hours to replace divided by charge current, divided again by charging efficiency. A 100 Ah battery at 20% charged with a 10 A charger at 85% efficiency takes about 9.4 hours to fill.

This calculation covers the bulk phase, where the charger delivers its full current. Real chargers then move to an absorption phase at falling current and, for lead-acid, a float phase — and the last 20% of a lead-acid charge can take as long as the first 80%. Lithium behaves far better, holding near-constant current almost to the top. The C-rate figure is a useful sanity check: charging above 0.5 C stresses most lead-acid batteries, whereas lithium iron phosphate commonly accepts 1 C without complaint.