Calculator
Battery Life Calculator
Estimate runtime from battery capacity and current draw.
How is battery life calculated?
A battery’s capacity (in milliamp-hours, mAh) tells you how much charge it holds; dividing that by your device’s current draw (in milliamps, mA) gives an estimated runtime in hours. Real devices rarely hit the theoretical number exactly, so this calculator includes an efficiency factor to bring the estimate closer to real-world behavior.
Frequently asked questions
What is mAh?
Milliamp-hours measure electrical charge capacity — a 3000mAh battery can theoretically supply 3000mA for one hour, or 1000mA for three hours, or 100mA for 30 hours. It's a measure of total charge, not power, so it doesn't by itself tell you how long a specific device will run.
Why is real-world runtime lower than the rated capacity?
Rated capacity is measured under ideal lab conditions. Real devices lose runtime to voltage sag under load, reduced capacity in cold temperatures, and gradual capacity loss as a battery ages through charge cycles. The efficiency percentage in this calculator is a rough way to account for all of that at once.
How do I find my device's average current draw?
Manufacturer datasheets often list current draw for different modes (idle, active, transmitting). For DIY projects, a multimeter in series with the circuit gives a direct reading; for consumer devices, average draw is usually estimated by dividing rated battery capacity by the manufacturer's stated runtime.
Does temperature really affect battery life?
Yes, significantly for most rechargeable chemistries (like lithium-ion) — cold temperatures temporarily reduce usable capacity, sometimes by 20% or more in freezing conditions, while excessive heat accelerates long-term capacity degradation. Room-temperature operation generally gets closest to rated capacity.
Real-world runtime is usually lower than the rated capacity suggests — the efficiency factor accounts for voltage sag, temperature and battery age.