Battery Charge Time Calculator
Enter battery Ah, charger amps and the charge window to get realistic charge time — including the C-rate flag that protects battery life.
Last updated: 2026-09-15
How the calculation works
- The charge window converts to Ah; dividing by charger current gives time at the constant-current rate.
- The C-rate (charger A ÷ capacity Ah) is checked: above 0.5C needs explicit fast-charge support.
- Efficiency inflates the energy the charger must actually deliver.
Formula
Ah needed = capacity × window ÷ efficiency Time = Ah ÷ charger A C-rate = A ÷ capacity
| Symbol | Meaning | Unit |
|---|---|---|
C | C-rate (current relative to capacity) | — |
η | Charge acceptance efficiency | — |
Worked example
100 Ah battery from 20% to 100% on a 20 A charger at 90% efficiency: 80 Ah ÷ 0.9 = 89 Ah to restore → 4.4 hours at C/5. A 50 A charger on the same battery: 1.8 hours at 0.5C — fast, but verify the battery supports it.
Interpreting the result
The C-rate is the battery's comfort metric: lead-acid prefers ≤ 0.2C (C/5) for full absorption; lithium handles 0.5–1C happily. The calculator's figure is the constant-current phase — lead-acid's absorption phase above ~80% stretches the last 20% enormously, so 0–80% time is the honest comparison number.
Assumptions
- Constant current until the target SOC (real chargers taper).
- Battery at moderate temperature.
Limitations
- Lead-acid absorption phase can take as long as the bulk phase — total time is longer than calculated.
- Cold batteries charge slowly or not at all (lithium BMS blocks sub-zero charging).
Frequently asked questions
How long to charge a 100Ah battery?
At 20 A from 20–100%: about 4.5–5 hours (lithium) or 6–8 hours (lead-acid, with absorption taper).
Can I charge a battery too fast?
Yes — above the C-rate limit, heat and gassing (lead-acid) or lithium plating (cold Li-ion) damage the battery. Match charger current to the battery's spec.