How it works
Dividing the charge the battery can supply (mAh) by the average current (mA) gives the runtime in hours. The usable capacity falls with aging, temperature and high-rate discharge, so a derating factor is applied.
Runtime (h) = capacity (mAh) × (derating / 100) / average current (mA)Days = hours / 24Iavg = (Iact × tact + Isleep × tsleep) / (tact + tsleep)
Worked example
A 2000 mAh battery with 80 % derating and an average of 50 mA lasts 32.0 h, about 1.3 days. A sensor node that runs 30 mA for 2 s and sleeps at 10 µA for 58 s has an average current of about 1.01 mA. With the same battery it runs about 1585 h, or about 66 days.
Design tips
- A sleep current in the microamp range looks small, but over a long sleep period it dominates the average. Check the regulator Iq, MCU leakage and sensor standby current.
- Do not design from the average current alone. If the peak current during the active period drops the voltage across the battery's internal resistance, the device may reset.
- With a DC-DC converter, the battery current is the output power divided by the input voltage, which is larger than the output current. Include the conversion efficiency.
- The most reliable check is to measure the real average current with a current probe or a precision power analyzer and compare it with the calculation.
Limitations
- Conversion efficiency, self-discharge and temperature-dependent capacity are not included. The derating factor provides a rough correction.
- At high discharge rates capacity falls (the Peukert effect). Check the discharge curves in the datasheet.
- End-of-discharge voltage differs by battery chemistry, so the usable capacity must be judged against the cutoff voltage.
Frequently asked questions
How much derating should I use?
Usable capacity falls with temperature, aging and high-rate discharge. At room temperature with a low discharge rate, 80 to 90 % is a common starting point. Check the discharge curve at the lowest temperature and the maximum load in the datasheet.
Why does a small sleep current affect battery life so much?
When sleep lasts a long time, the sleep component makes up most of the average current. For example, 10 µA flowing for 58 seconds takes a large share of the average even if the active period is short.
How does a DC-DC converter change the calculation?
The battery-side current is the output power divided by the input voltage and the conversion efficiency. Calculate the average battery-side current including the efficiency, then apply the same formula.