Battery Charge Time Calculator
Works out how long a battery takes to charge between two states of charge at a given charger current, including charging losses.
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The formula
hours = (Ah × ΔSoC) ÷ (charger amps × efficiency)
Charging is not free: 10–20% of the energy going in becomes heat, so replacing 80 Ah of charge takes more than 80 amp-hours of charger output.
What to watch out for
Two caveats on the answer:
- The last 10–20% takes disproportionately long. Chargers switch from constant current to constant voltage near the top, and the current tapers away. A calculation to 100% will understate the time; to 90% it is close.
- The C rate has limits. Charging at 1C means charger amps equal pack amp-hours. Most lead-acid tolerates 0.1–0.3C; LiFePO₄ commonly accepts 0.5C and sometimes 1C. Exceeding the maker's limit costs cycle life.
Frequently Asked Questions
How long to charge a 100 Ah battery with a 20 A charger?
From 20% to 100% is 80 Ah to replace, about 4.4 hours at 90% efficiency — plus extra time for the absorption stage at the top.
What is a C rate?
Charger current divided by pack capacity. 20 A into 100 Ah is 0.2C. It is the rate-independent way to express whether a charge is gentle or aggressive.
Why does the last bit take so long?
Because the charger holds voltage constant and lets current fall away, to avoid overcharging. That absorption phase can take as long as the bulk charge before it.