Charging
Battery Charging Time Calculator
Estimate how long a charger takes to bring a battery from one state of charge to another.
Formula
Ah(needed) = C × (SoC(to) − SoC(from))
t = Ah(needed) ÷ (I × η)
η ≈ 0.85 lead-acid · 0.95 lithium (assumption)Worked example
100 Ah AGM from 20% to 100% with 10 A → 80 Ah ÷ (10 × 0.85) ≈ 9.4 h.
Limitations and warnings
- Lead-acid chargers taper current in the absorption stage; the last ~20% usually takes noticeably longer than this linear estimate.
- Efficiency values are typical assumptions, not measured data.
Four values that are often confused
- Nominal capacity (Ah)
- Charge a battery delivers under rated conditions, usually over 20 hours (C20) at 25 °C.
- Usable capacity
- The part of the nominal capacity you can use without damaging the battery — about 50% for lead-acid, 80–90% for LiFePO4.
- Cranking current (CCA)
- Current deliverable for a short time in the cold to start an engine. It measures power, not stored charge, and cannot be converted to Ah.
- Stored energy (Wh)
- Ah × voltage. The right unit to compare batteries of different voltages.