Battery Charging Time Calculator
Estimate charging time from battery energy, starting and target charge, charger power, losses and simultaneous device use.
Estimated constant-power charging time
Use average negotiated charger power rather than assuming the charger label is sustained. Battery ageing, temperature, charge taper and device power changes can extend runtime. This estimates charging, while UPS Runtime estimates discharge.
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How this calculator works
Convert the charge-percentage change into watt-hours to add. The charger supplies energy, while losses and any device running during charging reduce the power reaching the battery. Divide required energy by that net charging power. If the device consumes as much as the effective charger supplies, the model cannot reach a higher charge. This is a constant-power estimate; real charging often tapers near full capacity.
Formula
Energy added = battery Wh × (target% − start%)/100. Net charging W = charger W × efficiency/100 − device W. Estimated hours = energy added / net charging W.
Assumptions & limitations
Use average negotiated charger power rather than assuming the charger label is sustained. Battery ageing, temperature, charge taper and device power changes can extend runtime. This estimates charging, while UPS Runtime estimates discharge.
Worked example
Charging a 60 Wh battery from 20% to 80% adds 36 Wh. A 30 W charger at 90% efficiency supplies 27 W to charging with no device load, giving 36 ÷ 27 ≈ 1.3333 hours; the loss-free ideal is 1.2 hours.
Our approach to calculations ↗Common questions
Why is my real charge slower than the estimate?
The model holds power constant. Negotiated limits, tapering, heat and changing device consumption can reduce average charging power.
Can I enter mAh instead of Wh?
Convert first: Wh = mAh × nominal voltage ÷ 1,000. The voltage matters; mAh alone is not energy.
What if the device load exceeds charger output?
The entered scenario has no net charging power. Reduce the load or use a higher sustained charging-power assumption.