Battery Charge Time Calculator
Estimate how long a battery takes to charge from its capacity, charger current and charging efficiency.
How it works
Charge needed = capacity × (100 − start%) ÷ 100 · Time = charge needed ÷ current ÷ efficiency · C-rate = current ÷ capacity
The theoretical charge time is simply the charge you need to add divided by the charging current, but real batteries never hit 100% of that: energy is lost to heat, and lithium cells taper the current sharply as they approach full, so an efficiency factor of 70–85% is realistic. The C-rate — charge current divided by capacity — describes how aggressive the charge is; 1C fills an empty pack in about an hour, and rates above 1C generate more heat and stress the cell.
Worked example
A 4,000 mAh battery at 20% needs 4,000 × 0.8 = 3,200 mAh. At 2,000 mA the ideal time is 1.6 h, but at 80% efficiency it takes 3,200 ÷ 2,000 ÷ 0.8 = 2.0 h. The C-rate is 2,000 ÷ 4,000 = 0.5C.
Frequently asked questions
Why does charging slow down near full?
Lithium-ion chargers switch from constant-current to constant-voltage mode around 70–80%, tapering the current to protect the cell. That is why the last 20% often takes disproportionately long and why fast-charge ratings usually quote a 0–80% time.
What efficiency should I use?
For phones and USB devices, 75–85% is typical once conversion and heat losses are counted. Higher currents and hot conditions push efficiency lower, so use a conservative figure like 80%.
Will a higher-current charger charge faster?
Up to the limit the battery and its charging circuit allow, yes. But the device caps the accepted current, so a charger rated far above that limit will not charge any faster — it just has spare capacity.
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This calculator is for educational purposes. Double-check important results before acting on them.