Battery Charge Time Calculator
How long until it's charged? Enter the battery's capacity, the charger's power, and where the charge level starts — losses included.
Needed to convert Ah into energy (Wh = Ah × V).
Lithium ≈ 90–95%, lead-acid ≈ 80–85%.
Estimated charge time
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Best-case estimate — chargers taper near full, and cold or aged batteries charge slower. Expect 10–30% longer in practice.
How charge time is calculated
The core formula is energy ÷ power:
Time (h) = capacity (Wh) × (target % − current %) ÷ (charger W × efficiency)
If your battery is rated in amp-hours, its energy is Ah × voltage — a 100 Ah 12 V battery holds 1,200 Wh. If your charger is rated in amps, its power is A × battery voltage — a 20 A charger on a 12 V battery delivers about 240 W. Charging loses 5–20% to heat and conversion, which is what the efficiency field accounts for.
The taper effect: chargers don't hold full current all the way. Lead-acid absorbs slowly beyond ~70%; lithium tapers above ~80–90%. If you only need the battery mostly full, charging to 80% is disproportionately fast — the last 20% is the slowest part.
Battery charging FAQ
How do I calculate battery charging time?
Time = energy needed ÷ effective charger power. A 100 Ah 12 V battery (1,200 Wh) from 20% to 100% needs 960 Wh; a 240 W charger at 90% efficiency does it in ≈ 4.4 hours.
How long does a 100Ah battery take to charge?
Roughly 10–11 hours from empty with a 10 A charger, 5–6 hours with 20 A — longer in practice because charging slows near full.
Why is real charging time longer than calculated?
Efficiency losses (85–95%), current taper near full charge, and temperature effects. Treat the result as best case and add 10–30%.
Can I charge faster with a bigger charger?
Only up to the battery's max charge rate: lithium typically accepts 0.5C–1C, lead-acid only 0.1C–0.25C. Check the datasheet first.