Battery Charge Time Calculator
Estimate battery charging time from capacity, charger current and state of charge. See the ideal time, calculated minimum and assumptions.
How the battery charge time calculation works
The calculator first finds how much charge must be returned to the battery:
charge to replace (Ah) = capacity (Ah) × (target SoC − starting SoC) ÷ 100
The ideal time divides that charge by the charger current. The calculated minimum also accounts for the selected charge-efficiency assumption:
calculated minimum (hours) = charge to replace ÷ (charger current × efficiency)
This is not a guaranteed full-charge time. Lead-acid batteries may enter an absorption phase near the top of charge, when current falls and the remaining charge takes longer. Charger limits, temperature, battery condition and a BMS can also change the actual time. For this reason, the calculator says “at least” when a lead-acid target above 80% is selected without a sourced charger profile or an additional absorption time.
Charging-time examples
60 Ah lead-acid battery, from 20% to 100%, with a 5 A charger
The battery needs 48 Ah: 60 Ah × (100% − 20%) = 48 Ah. The ideal time is 9 hours 36 minutes. With the visible 95% efficiency assumption, the calculated minimum is 10 hours 6 minutes. Because the target is 100%, the variable absorption phase is not included and full charge may take longer. Open this example in the calculator100 Ah LiFePO4 battery, from 20% to 90%, with a 20 A charger
The battery needs 70 Ah. The ideal time is 3 hours 30 minutes. With the visible 99% efficiency assumption, the estimate rounds to 3 hours 32 minutes. The charger or BMS may still limit current, so treat the result as an estimate rather than a promise. Open this example in the calculatorAGM battery 100 Ah, from 50% to 100%, 10 A charger
Half the capacity to put back, so 50 Ah. A 10 A charger delivers that in 5 hours 0 minutes, though in practice count on 5 hours 16 minutes — some of the energy becomes heat rather than charge. And that is not the end of it: a lead-acid battery takes the last stretch slowly, in the absorption phase. We do not guess how long that lasts, because it depends on the charger and the temperature. Open this example in the calculatorA 12 V 7 Ah battery from a UPS or alarm panel, from 0% to 100%, 1 A charger
The small black brick inside a UPS or an alarm panel. From flat to full it needs 7 hours 0 minutes, or 7 hours 22 minutes once losses are counted. It is tempting to hook it up to a car charger and be done in an hour — do not. At this size 1 A works out to 0.14C, and that is exactly what it should be. Open this example in the calculatorCar battery 45 Ah, from 30% to 100%, 5 A charger
The car sat for a week and the starter barely turned. At 30% left there are 31.5 Ah to put back: 6 hours 18 minutes on paper, 6 hours 38 minutes with losses. Do not leave it until the weekend. A starter battery sulfates faster the longer it stands discharged, and the capacity lost that way does not come back. Open this example in the calculatorLiFePO4 battery 200 Ah, from 10% to 100%, 50 A charger
A camper set after a few overcast days. There are 180 Ah to make up and a 50 A charger to do it: 3 hours 36 minutes in theory, 3 hours 38 minutes once losses are counted. Those two minutes are, in effect, the whole advantage of lithium — LiFePO4 accepts charge at roughly 99% efficiency, lead-acid at about 95%. Open this example in the calculatorMotorcycle battery 12 Ah, from 40% to 100%, 1.5 A charger
A bike coming out of winter storage, on a motorcycle charger. The 7.2 Ah to put back work out at 4 hours 48 minutes, or 5 hours 3 minutes with losses. A car charger would do it quicker and boil the electrolyte on the way — at 12 Ah you match the current to the battery, not to your patience. Open this example in the calculatorGel battery 230 Ah, from 50% to 100%, 20 A charger
A bank for an off-grid system or a larger backup supply. The 115 Ah to put back come to 5 hours 45 minutes, or 6 hours 3 minutes with losses. Twenty amps into that capacity is only 0.09C, which is very gentle on the battery — but gentle also means slow. Plan for half a day, not an afternoon. Open this example in the calculatorA 100 Ah battery on a 2 A trickle charger, from 60% to 100%
The question that comes up most often: is a charger that small any use at all. It is — 40 Ah at 2 A is 20 hours 0 minutes, or 21 hours 3 minutes with losses. But that is almost a full day. A 2 A charger exists to keep a charged battery healthy through winter, not to rescue a flat one before work. Open this example in the calculatorA 100 Ah LiFePO4 battery on a 60 A charger, from 0% to 100%
The one example where the calculator raises a flag. Flat to full comes out at 1 hour 40 minutes, or 1 hour 41 minutes with losses — impressive, except that 60 A into 100 Ah means 0.6C, against a reference value of 0.5C. Before you claim that time as yours, check the battery datasheet and the BMS settings to see whether that current gets through at all. Open this example in the calculatorFrequently asked questions
How long does it take to charge a 60 Ah battery with a 5 A charger?
From 0% to 100%, the ideal calculation is 12 hours. With a 95% efficiency assumption, the calculated minimum is about 12 hours 38 minutes. A lead-acid absorption phase can extend the full-charge time.
How long does it take to charge a 100 Ah battery with a 10 A charger?
From 0% to 100%, the ideal result is 10 hours. With a 95% efficiency assumption, the calculated minimum is about 10 hours 32 minutes, excluding variable absorption time.
Does battery voltage change the Ah-based charging-time result?
Not directly when capacity is entered in amp-hours and charger output in amps. System voltage is used here to interpret a sourced resting-voltage profile. Voltage is required when energy is calculated in watt-hours, which belongs to the related capacity and runtime tool.
Can resting voltage determine the starting state of charge?
Only approximately and only with a profile suitable for the battery. Measure after the battery has rested, with charging and loads disconnected. The calculator labels the selected source. It does not offer a generic voltage-to-SoC conversion for LiFePO4.
Why can the final part of lead-acid charging take longer?
During absorption, the charger holds a controlled voltage while current tapers. A simple Ah/A division assumes constant current and therefore cannot predict that variable final phase.
Is a higher charging current always better?
No. The permitted current depends on the battery and charger. The calculator can flag a high C-rate, but the battery and charger documentation must decide the safe limit.
Is the result exact?
No. It is an estimate based on the inputs and visible assumptions. Temperature, battery age and condition, charger behaviour, cable losses and BMS limits can change actual time.
Does the calculator work offline?
Yes, after the application has loaded successfully and its offline files have been cached. Saved configurations remain on the device unless the user shares a parameterised link.