Battery Charging Time Calculator
By Santaji GadepatilinUpdated October 28, 2026
How long a battery takes to charge depends on its capacity, the charger current and how much charge it needs — plus losses that make real charging slower than Ah ÷ A. This battery charging time calculator includes the state of charge and efficiency — see industrial battery charger float and boost charging and the main causes of battery failure.
How to use the battery charging time calculator
Enter the battery capacity, the charging current, the starting and target state of charge, and choose the battery type for charging efficiency. You get the charge needed in Ah, the estimated time and the C-rate.
Battery charging time formula
t (h) = C × (SoCtarget − SoCstart) ÷ (I × η)
Charging time for a 150 Ah lead-acid battery (20% → 100%)
| Charging current | C-rate | Time |
|---|---|---|
| 5 A | 0.03C | 30.0 h |
| 10 A | 0.07C | 15.0 h |
| 15 A | 0.10C | 10.0 h |
| 20 A | 0.13C | 7.5 h |
| 30 A | 0.20C | 5.0 h |
Add 1–3 hours for the absorption stage of lead-acid batteries.
Recommended charging currents
| Battery | Typical charge rate |
|---|---|
| Flooded / tubular lead-acid (inverter batteries) | C/10 – C/8 |
| AGM / gel lead-acid | C/5 – C/10 |
| LiFePO4 | 0.2C – 1C |
| Li-ion (phone, laptop) | 0.5C – 1C, fast charge up to 2C+ |
Charging stages (lead-acid)
- Bulk — constant current until about 80% charge.
- Absorption — constant voltage (about 14.4–14.8 V for 12 V) while current tapers.
- Float — lower voltage (about 13.5–13.8 V) to maintain full charge.
Frequently asked questions
How do I calculate battery charging time?
Time (h) = Ah needed ÷ (charging current × efficiency). Charging 120 Ah into a lead-acid battery at 15 A takes about 120 ÷ (15 × 0.8) = 10 hours.
Why does the last 20% take longer?
Chargers switch to constant-voltage (absorption) mode near full charge, and the current tapers off. Lead-acid batteries can need several extra hours to reach 100%.
What charging current should I use?
For lead-acid, about 10% of capacity (C/10) — 15 A for a 150 Ah battery. Lithium batteries accept 0.5C or more; check the datasheet.
What is C-rate?
Charging current divided by capacity. 15 A on a 150 Ah battery is 0.1C.