Battery Charging Time Calculator
Battery Charging Time Calculator
How long a battery takes to charge from its capacity, the charger’s current or power and the state of charge you start from — including the slow constant-voltage tail at the end.
Charging time
150 Ah lead-acid at 12 V, 30% to 100%, 15 A charger, 80% efficient, tail from 80% at 0.35×
Two stages, counted separately
- C
- battery capacity in Ah
- I
- charge current in A (charger watts ÷ battery volts)
- η
- charge efficiency: 0.8 flooded lead-acid, 0.99 lithium
- f
- average tail current as a fraction of I (1 = no tail)
Worked example
150 Ah lead-acid at 12 V, 30% to 100%, 15 A charger, 80% efficient, tail from 80% at 0.35×
Amp-hours to replace = (100 − 30)% × 150 = 105 Ah
Constant current 30 → 80%: 75 ÷ (15 × 0.8) = 6.25 h
Tail 80 → 100%: 30 ÷ (15 × 0.35 × 0.8) = 7.14 h
Total 13.39 h — against 7.00 h if you divided amp-hours by amps
Charging a 150 Ah lead-acid battery from 30% to full (80% efficient)
| Charge current | Constant current | Tail | Total |
|---|---|---|---|
| C/20 — 7.5 A trickle | 12.50 h | 14.29 h | 26.79 h |
| C/10 — 15 A (typical home inverter charger) | 6.25 h | 7.14 h | 13.39 h |
| C/5 — 30 A | 3.12 h | 3.57 h | 6.70 h |
| C/3 — 50 A | 1.88 h | 2.14 h | 4.02 h |
How long a battery takes to charge
The arithmetic starts simply: to put 105 Ah back into a 150 Ah battery at 15 A takes seven hours if every amp-hour counts. Two things stretch it. First, not every amp-hour counts — a flooded lead-acid battery returns about 80% of what you push in, the rest going to gassing and heat, while lithium returns about 99%. Second, no charger holds full current to the end.
The CC/CV tail. A modern charger runs constant current until the battery reaches its voltage limit, then holds that voltage while the current decays. Battery University measures the split for lead acid: about 70% of the charge goes in during 5–8 hours of constant current, and the remaining 30% takes another 7–10 hours of topping charge. Lithium is far quicker — the constant-current stage gets to roughly 85%, and a full charge at 0.5C to 1C takes two to three hours — but the shape is the same. This page models the tail as an average current, a fraction of the charge current, from the knee to the target. At the defaults the bulk stage takes 6.25 hours and the tail 7.14 hours: 13.39 hours in total, which is why an inverter battery flattened by a long cut is often not full again by evening.
Charge current. Divide by the capacity to get the C-rate: 15 A into 150 Ah is C/10, the rate most home inverter chargers use. Lead-acid manufacturers commonly advise around 0.3C as a maximum; above that the battery gasses and heats. Lithium accepts 0.5C to 1C comfortably, but its BMS may cut the current in the cold. If your charger is quoted in watts, enter that instead and the page divides by the battery voltage — 180 W at 12 V is the same 15 A.
Solar is different. A panel’s current follows the sun, so a solar charge controller spends much of the day below its rating; size the array and the controller with the solar panel size calculator and the solar charge controller calculator rather than assuming a steady current. To see how long the charge you just put in will last, use the inverter backup time calculator.
Frequently asked questions
How long does it take to charge a 150Ah battery?
From 30% to full at 15 A with an 80% efficient lead-acid battery, about 13.4 hours: 6.25 hours of constant current and 7.14 hours of tapering tail.
How do I calculate battery charging time?
Amp-hours to replace ÷ (charge current × charge efficiency) for the constant-current part, then add the tail, where the average current is a fraction of the charge current. Amp-hours to replace = capacity × (target SoC − starting SoC).
Why does the last 20% take so long?
Because the charger has switched from constant current to constant voltage. As the battery fills, the voltage difference driving the current shrinks, so the current decays — exponentially, near the end. Lead acid needs that time to finish converting the plates; stopping early leaves it sulphating.
Does a bigger charger charge proportionally faster?
Only the bulk stage. Doubling the current halves the constant-current time but the tail is set by the battery’s chemistry, not the charger, so the total falls by less than half.
What charge current should I use?
Around C/10 is the traditional lead-acid figure and what most inverter chargers deliver; manufacturers typically allow up to about 0.3C. For lithium, 0.5C to 1C is normal. Always check the battery’s own datasheet.
Related calculators
References
- Battery University. BU-403: Charging Lead Acid: “During the constant-current charge, the battery charges to about 70 percent in 5–8 hours; the remaining 30 percent is filled with the slower topping charge that lasts another 7–10 hours.”
- Battery University. BU-409: Charging Lithium-ion: a full charge at 0.5C–1C takes about 2–3 hours; charge efficiency is about 99%; the cell is full when the current falls to 3–5% of the Ah rating.
- Rolls Battery (Surrette). Charge Efficiency — Flooded Lead-Acid Battery: “Charge efficiency for Flooded deep cycle models is typically ~80%. This should be reduced 1% per year after the third (3) year of operation.”
- Victron Energy. SmartShunt / BMV battery monitor manual, charge efficiency factor: “We recommend setting the charge efficiency at 99%” for lithium batteries.
- Vader R. Energy Unlimited. Victron Energy. Charge efficiency “will range in between 80% and 95%”; battery ageing advances “disproportionately fast” once roughly 80% depth of discharge is exceeded.