Battery charging time formula
Ah to add = capacity (Ah) × (target % − current %)
Charge time (h) = Ah to add ÷ (charger A × efficiency)
That simple formula is right for the part of the charge at full current. The calculator adds two corrections:
- Charge efficiency. Some energy is lost as heat, so more amp-hours go in than the battery stores: about 95% efficiency for lithium and 85% for lead-acid.
- The taper. Above about 95% for lithium and 80% for lead-acid, the charger holds the voltage and the current drops. We count that stage at 50% of the charger’s current on average.
Worked example for the default: a 100 Ah LiFePO4 battery from 20% on a 20 A charger. The bulk stage, 20% to 95%, takes 3.95 h; the final 5% takes 0.53 h at the lower current; 4.47 hours in total.
How long does it take to charge a 100Ah battery?
Charging time for a 100 Ah battery by charger size. LiFePO4 is charged from empty; AGM from 50%, the deepest you should normally discharge it.
| Charger | LiFePO4, 0–100% | LiFePO4, 20–80% | AGM, 50–100% |
|---|---|---|---|
| 5 A | 22 h 6 min | 12 h 38 min | 16 h 28 min |
| 10 A | 11 h 3 min | 6 h 19 min | 8 h 14 min |
| 20 A | 5 h 32 min | 3 h 9 min | 4 h 7 min |
| 30 A | 3 h 41 min | 2 h 6 min | 2 h 45 min |
| 50 A | 2 h 13 min | 1 h 16 min | 1 h 39 min |
Doubling a lead-acid charger’s current does not halve the time, because only the bulk stage speeds up. For lithium, the time falls almost in proportion, as long as the charger stays within the battery’s maximum charge current.
Charge time chart by battery size
LiFePO4 batteries charged from 20% to 100%.
| Battery | 10 A charger | 20 A charger | 40 A charger |
|---|---|---|---|
| 50 Ah | 4 h 28 min | 2 h 14 min | 1 h 7 min |
| 100 Ah | 8 h 57 min | 4 h 28 min | 2 h 14 min |
| 200 Ah | 17 h 54 min | 8 h 57 min | 4 h 28 min |
| 300 Ah | 26 h 51 min | 13 h 25 min | 6 h 43 min |
Charging from solar panels or a car alternator
Solar charge controllers and DC-DC chargers are rated in amps, so enter that figure as the charger current. If you only know the panel watts, an MPPT controller delivers roughly panel watts × 0.8 ÷ battery voltage in good sun: a 200 W panel charging a 12.8 V battery gives about 12.5 A. Solar output varies through the day, so treat the result as hours of good sun, not clock hours. To size a solar battery bank, use the solar battery calculator.
Charge time is the other half of runtime: the battery runtime calculator tells you how long a full battery lasts, and the battery capacity calculator how big it needs to be. The method behind all the calculators is on how battery runtime is calculated.