Calculator KB-23
Battery charging time calculator
Simply dividing capacity by current gives the wrong answer: charging involves losses, and the current drops near the end.
Result
In brief: Calculate charging time for lead-acid, Li-ion, and LiFePO4 batteries based on capacity and current: constant-current and constant-voltage stages, C-rate, and a safe-current check. Example: Charging time: 8 h 46 min.
Calculation example
| Battery type | LiFePO4 |
|---|---|
| Capacity | 100 Ah |
| Charger current | 10 A |
| Current charge | 20 % |
| Charge to | 100 % |
| Charging time | 8 h 46 min |
|---|---|
| Charge to add | 80 Ah |
| Current relative to capacity | 0.1 C |
| Constant-current stage | 7 h 13 min |
| Topping-off stage | 1 h 33 min |
How it is calculated
η: lead-acid 0.85; Li-ion 0.95; LiFePO4 0.97
The charger maintains a constant current up to a certain charge level, then switches to constant voltage and the current gradually drops. The last 20% takes a long time with lead-acid batteries, but is barely noticeable with LiFePO4. This is an estimate: the exact time depends on the charger and temperature.
What to look for
- For lead-acid batteries, the recommended current is usually up to 0.2–0.3 C; for lithium batteries, up to 0.5–1 C.
- Lithium batteries must not be charged in freezing temperatures: cells quickly lose capacity in the cold and can fail.
Frequently asked questions
How long does it take to charge a 100 Ah battery at 10 A?
From 20% to 100%, a LiFePO4 battery takes about 8 h 46 min, while a lead-acid battery of the same capacity takes about 14 hours because of its lower efficiency and lengthy final stage.
What charging current should I choose?
For lead-acid batteries, usually up to 0.2–0.3C, or 20–30 A for 100 Ah; for lithium batteries, up to 0.5–1C unless the manufacturer specifies otherwise. A lower current is gentler on the battery.
Why do the last few percent take so long to charge?
During the final stage, the charger holds a constant voltage while the current gradually drops. This stage takes hours for lead-acid batteries, but is short for LiFePO4.
