About the Battery Life Calculator
The Battery Life Calculator estimates how many hours a battery-powered device will run before it needs a recharge, based on the battery's rated capacity, the device's current draw, and a usable-efficiency factor. It is aimed at anyone sizing a battery for a project or checking whether a device's advertised runtime is realistic.
How It Works
Enter the battery's capacity in milliamp-hours, the device's current draw in milliamps, and a usable efficiency percentage, defaulting to 85 percent and adjustable between 1 and 100 percent, that accounts for voltage sag, discharge losses, and batteries rarely being run down to absolute zero. The calculator multiplies capacity by the efficiency fraction and divides by current draw to get runtime in hours, then also converts that figure to minutes.
Formula & Methodology
To do this by hand, convert the efficiency percentage to a decimal by dividing by 100, multiply it by the battery's rated capacity in mAh to get a realistic usable capacity, then divide that usable capacity by the device's current draw in mA. The result is in hours; multiply by 60 for minutes.
Examples
Default 3000 mAh battery
With a 3000 mAh battery, a 250 mA current draw, and 85 percent efficiency, runtime is (3000 x 0.85) / 250, equal to 10.2 hours, or 612 minutes.
Higher-drain device
A 5000 mAh battery powering a 500 mA load at 80 percent efficiency gives (5000 x 0.80) / 500, equal to 8 hours of runtime, or 480 minutes.
Advantages
- Gives a fast runtime estimate without needing to run a full discharge test.
- The adjustable efficiency factor keeps the estimate closer to real-world performance than dividing rated capacity by current draw alone.
- Makes it easy to compare how battery choice or a device's power draw changes expected runtime.
Common Mistakes
- Using the battery's full rated capacity with no efficiency discount, which overstates real-world runtime since batteries rarely deliver 100 percent of their rated mAh under load.
- Entering a peak or idle current draw figure instead of the device's average current draw across a full use cycle.
- Mixing up units by entering a capacity in Ah where the field expects mAh, inflating the runtime estimate by a factor of 1000.
Edge Cases to Watch For
- Current draw must be greater than zero; the calculator returns an error rather than a divide-by-zero result if it is left at 0.
- The efficiency input is restricted to between 1 and 100 percent, since a value outside that range would not represent a physically meaningful fraction of usable capacity.
- The formula assumes a constant current draw for the whole discharge; a device cycling between an idle low-power state and active high-power bursts needs an average current draw figure, not the peak, or the estimate will run short.
Common Use Cases
- Hobbyists and makers sizing a battery for an Arduino, Raspberry Pi, or other electronics project.
- Shoppers comparing manufacturer runtime claims against a device's known current draw.
- Engineers estimating standby versus active runtime for battery-powered sensors or remote equipment.