Battery Charge Time Calculator
Know when a pack will be ready. Enter its capacity, the charge current, an efficiency allowance, and how empty it is, and get the estimated charge time plus the C-rate you are charging at.
Efficiency is typically 85–95% for lithium. Charging to a target below 100% avoids the slow constant-voltage tail.
Formula & how it works
Charge needed is capacity × (target% − start%). Time is charge needed ÷ (current × efficiency), in hours. C-rate is current(A) ÷ capacity(Ah).
Worked example
A 3000 mAh battery charged at 1 A from empty to full needs 3000 mAh. At 90% efficiency that is 3000 ÷ (1000 × 0.9) ≈ 3.3 hours, or about 3 hours 20 minutes. The C-rate is 1 A ÷ 3 Ah ≈ 0.33C — a gentle charge.
Charging with confidence
Efficiency stretches the clock
Some of the current going into a battery is lost to heat and internal chemistry rather than stored charge, so charging always takes a bit longer than dividing capacity by current suggests. An efficiency of around 90% for lithium captures that, giving a more truthful estimate than the ideal figure.
Watch the C-rate
C-rate tells you how aggressive a charge is relative to the battery's size. Gentle rates around a third of C are easy on cells and generate little heat; higher rates charge faster but stress the battery and demand a charger and cell rated for it. When in doubt, charge slower.
The last stretch is slow
Lithium cells take constant current up to roughly 70 to 80 percent, then taper as they approach full, which is why the final few percent drag on. Charging to a target a little below 100 percent skips that slow tail and, as a bonus, is gentler on the battery's long-term health.