Battery Runtime Calculator
Enter a battery's capacity, the current it's supplying, and an efficiency factor to estimate its runtime.
How battery runtime is estimated
The basic formula divides a battery's capacity (in milliamp-hours) by the current it's supplying (in milliamps), giving the theoretical runtime in hours. This calculator also applies an efficiency factor to account for real-world losses from internal resistance, voltage drop as the battery discharges, and temperature effects.
A typical efficiency factor for consumer batteries under moderate loads is 80–90%. High-drain devices or extreme temperatures can lower it further, so treat the result as an estimate rather than a guarantee.
Frequently asked questions
Why isn't the runtime just capacity divided by current?
That gives the theoretical maximum, but real batteries lose some capacity to internal resistance, voltage sag, and temperature — the efficiency factor adjusts for these real-world losses.
What efficiency factor should I use?
80–90% is typical for consumer batteries under moderate loads. Use a lower value for high-drain devices, cold environments, or older/degraded batteries, since those conditions reduce the usable capacity further.
Does this work for any battery chemistry?
The formula is chemistry-agnostic as long as you use the battery's rated capacity in mAh and an appropriate efficiency factor — though different chemistries (like lithium-ion vs. alkaline) typically have different realistic efficiency ranges.