Battery Charging Time Calculator - Estimate Full Charge Time
Estimate battery charging time from capacity, charger output, current charge percentage, and charger efficiency.
Estimated Time to Full Charge
4h 42m
Energy needed: 40.00 Wh at 8.50 effective W
How to Calculate Battery Charging Time
Charging time depends on how much energy the battery still needs and how quickly the charger can deliver it, minus losses from charging inefficiency. This calculator finds the remaining energy needed (capacity × the uncharged percentage), divides it by the charger's effective power output (charger wattage × efficiency), and returns the estimated time to reach a full charge.
Example
A 50 Wh battery at 20% charge needs 50 × 0.8 = 40 Wh to reach full. A 10W charger running at 85% efficiency delivers 10 × 0.85 = 8.5 effective watts, so the estimated charging time is 40 ÷ 8.5 ≈ 4.7 hours (about 4h 42m).
Common Use Cases
- Estimating how long a phone, laptop, or power bank will take to charge from a given charger.
- Planning charging schedules for e-bikes, drones, or other portable battery-powered devices.
- Comparing how much faster a higher-wattage charger would charge the same battery.
FAQs
Why isn't charging 100% efficient?
Some energy is always lost as heat during charging, in the charger's power conversion circuitry and the battery's internal chemistry. A default of 85% efficiency is a reasonable general estimate for many consumer lithium-ion devices, though actual efficiency varies by charger and battery quality.
How do I convert mAh to Wh?
Watt-hours equal milliamp-hours divided by 1,000, multiplied by the battery's nominal voltage (Wh = mAh ÷ 1000 × V). This calculator does that conversion automatically when you select mAh as your capacity unit and provide the battery's voltage.
Why does real-world charging often take longer than this estimate?
Many devices use a tapering charge curve that slows down significantly above about 80-90% to protect battery health, so the last portion of a charge often takes proportionally longer than this calculator's constant-power estimate suggests.