Charging Time Calculator
Why a "Fast Charger" Doesn't Halve Your Wait in Half the Advertised Time
Charging a battery is never 100% efficient — some of the energy delivered by the charger is lost as heat during the chemical conversion, particularly as the battery approaches full charge and its charge controller tapers the current. This calculator estimates real-world charging time from a battery's capacity, the charger's output current, the starting and target charge percentage, and an efficiency factor that accounts for that loss.
The Formula
The time needed is the energy still required divided by the rate it's delivered, adjusted for efficiency:
Time (h) = Capacity Needed (mAh) / Charger Current (mA) / Efficiency
The default efficiency assumption is 85%, a reasonable mid-range estimate for lithium-ion charging losses; you can override it if you know your device's actual figure.
Where This Calculation Matters
- Comparing chargers — converting a charger's current or wattage rating into an actual expected time to full, rather than trusting marketing shorthand like "fast charging."
- Trip and travel planning — working out whether a device will reach a usable charge level in the time available before departure.
- Partial-charge habits — estimating the time needed for a quick top-up (say, 20% to 80%) rather than a full charge, which many battery health guides recommend over charging to 100%.
- Power bank and accessory shopping — deciding whether a portable charger's output current is fast enough for your use case.
Charging Time Examples (0% to 100%, 85% efficiency)
| Charger Output | Charging Time |
|---|---|
| 1000 mA (~5W) | 4.71 hours |
| 2000 mA (~10W) | 2.35 hours |
| 3000 mA (~15W) | 1.57 hours |
Real charging curves taper current as the battery nears 100%, so the last portion of a charge is typically slower than this linear estimate suggests — charging to 80% is usually faster per percentage point than the final stretch to 100%.
How to Use This Calculator
- Enter the Battery Capacity (mAh).
- Enter the Charger Current (mA).
- Optionally set Start Charge % (default 0) and Target Charge % (default 100).
- Optionally set Charging Efficiency % (default 85).
- Select Calculate to get the estimated time in hours.
Related Calculations
Curious how long that same battery will last once charged? Use the Battery Life Calculator. Working with capacitors instead of batteries? See the Capacitor Charge Calculator.