How to estimate charging time
Divide the amp-hours you need to put back by the charger's current, then allow for charging losses. Lithium batteries are about 95% efficient and accept full current almost to the top. Lead-acid batteries lose more energy and slow down in the absorption stage.
Ah to add = Capacity × (Target % − Start %)
Time (h) = Ah to add ÷ (Charger amps × Efficiency)
Example: 100 Ah LiFePO4 from 20% with a 20 A charger
80 Ah ÷ (20 A × 0.95) = 4.2 hours.
Recommended charge rates
| Type | Typical charge current | 100 Ah example |
|---|---|---|
| LiFePO4 | 0.2–0.5 C | 20–50 A |
| AGM | 0.1–0.25 C | 10–25 A |
| Flooded lead-acid | 0.1–0.13 C | 10–13 A |
Always follow the battery manufacturer's maximum charge current and voltage.
Frequently asked questions
How long does it take to charge a 100Ah battery?
About 4–5 hours with a 20 A charger from 20% for lithium; closer to 6–8 hours for lead-acid because of the slow absorption stage.
Can I charge faster with a bigger charger?
Up to the battery's rated charge current. Charging above it shortens battery life and can be unsafe.
How do I charge from solar?
Use the charge controller's output current as the charger current, averaged over the day's sun hours.