Try it
Understand the result
What's going on
A battery’s printed capacity is a promise made under ideal lab conditions. Real circuits draw current unevenly, waste some of it as heat, and rarely get to fully drain a cell - so a quick estimate needs to knock a chunk off the label number to be realistic.
The formula
Life (h) = Capacity (mAh) × Efficiency / Load (mA)
Show the derivation, mnemonic, and worked example
Build it up
Capacity in milliamp-hours literally means “this many milliamps, for this many hours, before it’s empty.” Divide that capacity by how many milliamps your circuit actually draws and you get the raw number of hours it could theoretically run. Then discount that by an efficiency factor - regulators, self-discharge, and non-ideal discharge curves usually eat 10 to 20% of the rated capacity in practice.
Worked example
Worked example
Capacity = 2,000 mAh, Load = 120 mA, Efficiency = 85%
Life = 2,000 × 0.85 / 120
Life ≈ 14.2 hours
