Free Tool
Battery Charge Time Calculator
Estimate how long it will take to charge your battery from mains, solar or a DC-DC charger, based on capacity, chemistry and how full you want it.
Charge time is roughly the battery capacity in amp-hours divided by the charge current in amps, plus about 15 to 20 percent for the final absorption phase. A 100Ah battery charged at 20A takes around six to seven hours. Your charge source (mains, solar or alternator) sets the current.
Interactive Tool
Battery Charge Time Calculator
Estimated charge time
4 hrs 51 mins
1,011Wh needed (1,200Wh × 80% ÷ 95% efficiency) ÷ 240W charge power, plus 15% for the taper to 100%
Shop battery storageEstimates account for charge efficiency and the slower absorption stage near full. Real-world charge times vary with temperature, battery age and charger behaviour.
How this is calculated
We first work out the energy your battery needs in watt hours. That is its capacity (Ah multiplied by voltage, or entered directly in Wh) multiplied by the percentage you are adding, then divided by the charge efficiency of the chemistry. LiFePO4 accepts charge at around 95% efficiency, while lead-acid AGM loses more to heat at around 85%.
Charge time is that energy divided by the charge power. For mains and DC-DC charging, power is the charger current multiplied by the battery voltage. For solar, we take 75% of the array's rated wattage to reflect real-world output from panel angle, cloud and controller losses in the UK.
Charging slows down near full as the charger tapers current during the absorption stage. When you charge to 100%, we add 15% extra time for LiFePO4 and 30% for lead-acid AGM, which taper for much longer. Stopping at 90% is often the faster, kinder option for day-to-day use.
Frequently asked questions
How long does it take to charge a battery?
Divide capacity (Ah) by charge current (A), then add 15 to 20 percent for the absorption stage where charging slows. A 100Ah battery at a 20A charge rate takes about six to seven hours from flat.
What charge rate is safe?
LiFePO4 typically accepts up to a 0.5C rate (50A for a 100Ah battery), while lead-acid prefers a gentler 0.1 to 0.2C. Always follow the maker’s spec, as too high a rate shortens battery life.
Does the charge source change the time?
Yes. Mains chargers deliver a steady current, solar varies with sun and array size, and a DC-DC alternator charger is capped by its rating. The calculator lets you enter the real current from each source.
Do LiFePO4 and lead-acid charge differently?
LiFePO4 accepts higher currents and holds voltage longer, so it charges faster and more fully. Lead-acid needs a longer, slower absorption phase to reach full charge and avoid sulphation.
