Battery Life Calculator

Estimating runtime

To first order, runtime is the usable charge divided by the average current draw:

where $C$ is the rated capacity (mAh), $I_{\text{load}}$ the average current (mA) and $\eta$ a derating factor. A datasheet capacity is measured under ideal conditions; in practice you rarely get all of it, so $\eta$ of 0.8–0.9 keeps the estimate honest.

This assumes a roughly constant average current. For duty-cycled or sleepy MCU designs, compute the time-weighted average first: $I_{\text{avg}}=D{I}_{\text{active}}+(1-D)I_{\text{sleep}}$, then feed that in. Peukert effects and temperature shift real life further — treat this as a planning figure, not a guarantee.