Battery Charge Time Calculator
A 4,000 mAh battery charging from 20% to 80% on a 2,000 mA charger at 85% efficiency takes about 1 hour and 25 minutes. This battery charge time calculator estimates how long that kind of top-up actually takes using your battery capacity in milliamp-hours, your current and target charge percentage, and the charger's rated output current. It also factors in charging efficiency, since real chargers never deliver their full rated current as usable capacity. Use it to plan phone charging before a trip, check whether a power bank can refill a device overnight, or compare how much faster a higher-current charger would be for the same battery. Enter the battery's rated capacity, where it sits now, where you want it to end up, and the charger's current rating, and the calculator returns the estimated charge time along with the milliamp-hours that still need to go into the battery.
Quick answer
Charge time in hours equals the milliamp-hours you need to add, divided by the charger current in milliamps times the efficiency factor.
Estimated Charge Time
1.4118hours
Time (hours and minutes)
1h 25m
mAh still needed
2400mAh
What this tells you
- •Charge time in hours equals the milliamp-hours you need to add, divided by the charger current in milliamps times the efficiency factor.
- •The milliamp-hours you need to add equals the battery capacity times the percentage-point gap between your current and target charge level, divided by 100.
- •Efficiency defaults to 85%, which reflects typical losses from heat and from the charging curve slowing down as the battery approaches full.
- •A higher-current charger shortens the estimate in direct proportion, so doubling the charger's rated current roughly halves the time.
- •The gap between current and target percentage matters as much as the charger itself. Topping up the last 10% takes a tenth of the time that a full 0% to 100% charge takes, all else equal.
- •This is a linear estimate. Real charging slows near 100% because most batteries use a constant-current, constant-voltage curve that tapers the closer it gets to full.
How to Use
- 1Enter the battery's rated capacity in milliamp-hours (mAh). This number is usually printed on the battery, in the device specs, or in the power bank's listing.
- 2Enter the current charge percentage, meaning how full the battery is right now, from 0 to 100.
- 3Enter the target charge percentage you want to reach. It must be higher than the current percentage, since the calculator only estimates time to charge up, not down.
- 4Enter the charger's rated output current in milliamps (mA). A charger listed as 2A outputs 2,000 mA. A USB port listed as 500 mA outputs 500 mA.
- 5Adjust the efficiency percentage if you have a reason to. Leave it at the 85% default for a typical wired charger, or lower it for wireless charging or old cables, which lose more energy to heat.
- 6Click Calculate to see the estimated charge time as decimal hours, as a readable hours-and-minutes breakdown, and the milliamp-hours still needed to reach your target.
How It Works
Formula
Charge time (h) = Capacity (mAh) x (Target% - Current%) / 100 / (Charger current (mA) x Efficiency)The calculator works in two steps. First it finds how many milliamp-hours still need to go into the battery: capacity times the percentage-point gap between the target and current charge level, divided by 100. A 4,000 mAh battery going from 20% to 80% needs 4,000 x 60 / 100 = 2,400 mAh. Second, it divides that figure by the usable current the charger actually delivers, which is the charger's rated current multiplied by an efficiency factor. Efficiency is a decimal fraction, so 85% becomes 0.85. A 2,000 mA charger at 85% efficiency delivers 2,000 x 0.85 = 1,700 mA of usable charging current. Dividing 2,400 mAh by 1,700 mA gives 1.4118 hours, or 1 hour and 25 minutes. The efficiency factor exists because no charging system converts 100% of its rated current into stored capacity. Some energy is lost as heat in the cable, connector, and battery management circuitry, and charging speed naturally drops as the battery nears full under most lithium-ion charge curves. An 85% default reflects a reasonable wired-charger estimate, but you can lower it for wireless charging, damaged cables, or older hardware, or raise it slightly for well cooled, high-quality fast-charging setups.
Calculation note: values are processed in the order shown above, using the current input units.
Worked Examples
Phone battery from 20% to 80%
This is the classic partial top-up. The battery needs 2,400 mAh to close a 60 percentage-point gap. At 85% efficiency, the 2,000 mA charger delivers 1,700 mA of usable current, so 2,400 / 1,700 = 1.4118 hours, which rounds to 1 hour and 25 minutes.
Power bank from 10% to 100%
A near-empty 10,000 mAh power bank needs 9,000 mAh to reach full. At 90% efficiency, the 1,000 mA charger delivers 900 mA of usable current, so 9,000 / 900 works out to an even 10 hours. This is why large power banks are usually left charging overnight.
Wireless charging pad from 30% to 90%
Wireless charging loses more energy to heat than a cable does, so this example uses a lower 75% efficiency instead of the 85% default. The battery needs 1,800 mAh, and the 1,000 mA pad delivers 750 mA of usable current, giving 1,800 / 750 = 2.4 hours.
Time to Add 50% Charge to a 5,000 mAh Battery
This table holds the battery capacity and the amount to add fixed at 2,500 mAh (50% of 5,000 mAh) at 85% efficiency, and varies only the charger's rated current, so you can see how much a faster charger actually saves.
| Charger current | Estimated time | Typical source |
|---|---|---|
| 500 mA | 5h 53m | Older USB-A port or basic accessory charger |
| 1000 mA | 2h 56m | Standard 1A wall adapter |
| 1500 mA | 1h 58m | Mid-range 1.5A wall adapter |
| 2000 mA | 1h 28m | Common 2A phone or tablet charger |
| 3000 mA | 0h 59m | Fast-charging USB-C adapter (negotiated current) |
Actual fast-charging phones often negotiate a higher current than a plain wall adapter, and taper it down automatically as the battery approaches full, so real results can run faster early and slower near 100% than this simple estimate.
Common mistakes
- Entering amps as milliamps or the reverse. A charger rated 2A should be entered as 2000 mA, not 2. Off-by-1000 errors are the most common mistake with this calculator.
- Forgetting that percentage points, not raw percentages, drive the math. Going from 40% to 90% is a 50 percentage-point gap, not a 90 percent charge from scratch.
- Assuming the charger's rated current is always what the device actually draws. Many phones cap the current they accept below the charger's rated maximum, especially over older cables or with basic (non fast-charging) adapters.
- Leaving efficiency at 85% for every scenario. Wireless charging, damaged cables, and cheap adapters lose more energy to heat, and lower efficiency belongs in the input, not ignored.
- Treating the result as an exact countdown. This calculator assumes a constant charging rate for simplicity. Real lithium-ion batteries charge faster in the middle of the range and taper down noticeably in the final 10 to 20%, so the real time near full often runs longer than a straight-line estimate suggests.
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