Battery Charging Time Calculator
Written by Thierno Sadou Diallo, formula verified per our methodology • Last checked on 9/3/2026
Charging time is estimated by dividing the battery's capacity (mAh) by the charger's current (mA), adjusted by a real-world charging efficiency (about 80% for a typical USB charge). Example: a 4000 mAh battery with a 2 A charger recharges in about 2.5 hours.
Explanation
The simplest calculation (capacity ÷ current) assumes a perfectly constant charging current from start to finish, which isn't how it works in practice: most lithium-ion batteries charge in a two-phase cycle known as CC/CV (constant current, then constant voltage). The current stays at its maximum for most of the charge, then slows down significantly over the last 20% to protect the battery, which stretches the real time out compared to a purely linear calculation. This calculator accounts for that slowdown with an adjustable charging efficiency (80% by default, a common value for a phone or laptop USB charge): the lower this efficiency, the further the real charging time drifts from the simple theoretical calculation. For DISCHARGE time (a battery's runtime in use), see our battery life calculator, which answers the reverse question.
Example: a 4000 mAh phone on a 2 A charger
Inputs
Capacity: 4000 mAh. Charging current: 2000 mA. Efficiency: 80%.
Calculation
Effective charging current: 2000 × 80% = 1600 mA. Charging time: 4000 ÷ 1600 = 2.5 hours.
Result
This battery recharges in about 2 hours and 30 minutes.
Frequently asked questions
Why is the real time longer than capacity ÷ current?
Because a lithium-ion battery doesn't charge at a constant current throughout: past about 80% charge, the current gradually decreases (constant-voltage phase) to avoid damaging the battery. This calculator's charging efficiency accounts for that slowdown, more realistic than a purely linear calculation.
Does a more powerful charger always charge faster?
Only up to a point: the battery itself has a maximum accepted charging current (limited by its internal controller), beyond which a more powerful charger provides no further speed gain. Using a charging current above that accepted maximum doesn't speed up the charge, contrary to intuition — and once unplugged, our battery life calculator takes over to estimate how long that charge will last.
Where does the default 80% efficiency figure come from?
It's a common order of magnitude for a phone or laptop USB charge, reflecting the end-of-charge slowdown and a small share of heat loss. The real efficiency varies with the fast-charging technology used and the battery's condition; adjust the slider if the manufacturer provides a more precise figure for your device.