Battery Charge Time Calculator
Estimate how long it takes to charge a battery from its capacity and the charger's output current. The basic time is capacity ÷ charging current, and real charging is a little slower because no charger is 100% efficient — so the calculator also gives a realistic estimate using a charging efficiency factor.
Formula
Charge time = battery capacity ÷ (charging current × efficiency)
- The ideal charge time is simply capacity ÷ charging current (e.g. 4000 mAh ÷ 2000 mA = 2 hours).
- Real charging is slower because energy is lost as heat; dividing by an efficiency factor (around 0.8) gives a realistic figure.
- Keep capacity and current in matching units — both mAh/mA, or both Ah/A.
- Many chargers also slow down near 100% (the constant-voltage phase), so the last 10–20% can take longer than this estimate suggests.
4000 mAh battery, 2000 mA charger, 80% efficiency
Inputs
- Battery Capacity: 4000 mAh
- Charging Current: 2000 mA
- Charging Efficiency: 80 %
Ideal time = 4000 ÷ 2000 = 2 hours. Allowing for 80% efficiency: 2 ÷ 0.8 = 2.5 hours, or about 2 h 30 min.
Frequently asked questions
How do I calculate battery charge time?
Divide the battery capacity by the charging current. For a realistic figure, divide that by the charging efficiency (roughly 0.8) to allow for energy lost as heat.
Why does charging take longer than the ideal time?
Chargers aren't perfectly efficient, and most slow the current right down as the battery nears full to protect it, so real charging always takes a bit longer than capacity ÷ current.
What units should I use?
Use matching units — capacity in mAh with current in mA, or capacity in Ah with current in A. 1 A = 1000 mA.
What is a typical charging efficiency?
Everyday chargers are usually around 70–85% efficient. The exact figure depends on the battery chemistry, charger and temperature.