How to use the Battery Charging Time Calculator
Enter the battery capacity, charger power, starting percentage, and target percentage. The calculator estimates how much energy must be added and how long that will take after efficiency losses and charging taper.
For phones, mAh is the most common battery rating. The calculator converts mAh to watt-hours using the nominal battery voltage because charger power is measured in watts.
If you are comparing phone screens while choosing a device, use the Phone Screen Size Comparison Calculator. If battery life and display sharpness matter together, check the Phone PPI Calculator.
Battery charging time formula
battery Wh = battery mAh x nominal voltage / 1000energy to add = battery Wh x (target % - start %) / 100ideal time = energy to add / (charger watts x efficiency)practical time = energy to add / (charger watts x efficiency x taper factor)The ideal formula assumes steady power. The practical estimate adds a taper factor because phones usually reduce charging power as the battery gets closer to full.
Assumptions and methodology
This Battery Charging Time Calculator is an estimate, not a brand-specific charging curve. It is designed to answer practical planning questions such as whether a short top-up is enough, how much a faster charger helps, and why the last part of charging can take longer.
- Battery energy is calculated in watt-hours so it can be compared with charger watts.
- The default 3.85 V is a common nominal value for phone batteries, but you can adjust it if your specification lists a different voltage.
- The typical model assumes faster charging below 80% and slower charging near full.
- Actual results can vary because charger labels show maximum output, not guaranteed average power into the battery.
Example calculations
5000 mAh phone with a 20 W charger
A 5,000 mAh battery at 3.85 V stores about 19.3 Wh. Charging from 20% to 80% adds about 11.5 Wh.
With a 20.0 W charger, 85% efficiency, and typical charging taper, the estimated charging time is about 53 min. The ideal no-taper minimum is about 41 min, which is why simple watt-hour math often looks too optimistic.
Battery charging time chart
These examples use a 3.85 V battery, 85% charging efficiency, and the typical phone charging model from 20% to 80%. Use the calculator above for your exact capacity, charger, and target charge.
| Scenario | Battery | Charger | Energy added | Estimated time |
|---|---|---|---|---|
| 3,000 mAh phone, 20 W, 20-80% | 3,000 mAh | 20 W | 6.9 Wh | 32 min |
| 4,000 mAh phone, 20 W, 20-80% | 4,000 mAh | 20 W | 9.2 Wh | 43 min |
| 5,000 mAh phone, 20 W, 20-80% | 5,000 mAh | 20 W | 11.5 Wh | 53 min |
| 5,000 mAh phone, 30 W, 20-80% | 5,000 mAh | 30 W | 11.5 Wh | 36 min |
| 5,000 mAh phone, 45 W, 20-80% | 5,000 mAh | 45 W | 11.5 Wh | 24 min |
| 8,000 mAh tablet, 30 W, 20-80% | 8,000 mAh | 30 W | 18.5 Wh | 57 min |
| 10,000 mAh tablet, 45 W, 20-80% | 10,000 mAh | 45 W | 23.1 Wh | 47 min |
Popular battery charging time questions
If you already know the battery size or charger wattage, these focused answers give a quicker starting point and link back to the full calculator when you need custom settings.
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FAQ
How do I calculate battery charging time?
Convert battery capacity to watt-hours, multiply by the percentage you want to add, then divide by effective charging power. For phones, effective power is usually lower than the charger label because of efficiency losses and charging taper.
How long does it take to charge a 5000 mAh phone battery?
Using the default 3.85 V battery, 20 W charger, 85% efficiency, and a typical phone charging model, charging from 20% to 80% takes about 53 min. A full 0% to 100% charge takes about 1 hr 37 min in this estimate.
Why does phone charging slow down after 80%?
Phones reduce charging power at higher battery levels to control heat and protect battery health. That is why the last 20% can take much longer than the first 20%, even with the same charger.
Is charger wattage the same as charging speed?
Not always. A charger may be labeled 30 W or 65 W, but the phone must support that power, the cable must allow it, and the phone may only use peak power for part of the session.
How do I convert mAh to Wh for a phone battery?
Use Wh = mAh x voltage / 1000. For example, a 5000 mAh battery at 3.85 V stores about 19.3 Wh of energy.
Can I use this calculator for tablets and power banks?
Yes, if you know the battery capacity and charger power. For power banks, using Wh is often clearer because different products may use different cell voltages and output voltages.
Why is my real charging time different from the calculator?
Real charging time can change with battery temperature, cable quality, adapter protocol, phone software, background use, battery age, and whether the charger reaches the advertised power with your device.