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Writing a power budget

Battery life comes down to arithmetic: how much current each state draws, for how long, and how often. Work it out before choosing a battery.

Foundation

Average current of a device that sleeps and wakes once a minute A device sleeps at 5 µA and wakes for 50 ms at 10 mA once a minute. Weighted by time, that averages about 13 µA, and the sleep current alone is over a third of the average. Current Not to scale Time 10 mA for 50 ms once a minute average about 13 µA sleep 5 µA Where the 13 µA average comes from sleep, 5 µA 50 ms wake-ups Sleep current alone is over a third of the average. Capacity ÷ average current is the best case, not a forecast. Self-discharge, temperature, peaks and the cut-off voltage all shorten it. Measuring the bursts A multimeter can read the steady sleep current but shows only a slow average of the bursts. Use a scope with a current probe or shunt, or an instrument that samples current fast.
A device that sleeps at 5 µA and wakes for 50 ms at 10 mA once a minute averages about 13 µA, and sleep is over a third of that.

Key points

  1. Average current is each state's current weighted by the fraction of time spent in it. A device that sleeps at 5 µA and wakes for 50 ms at 10 mA once a minute averages about 13 µA, and the sleep current is over a third of that.1
  2. Battery capacity divided by average current gives the best case, not a forecast. Self-discharge, temperature, high current peaks and the voltage at which the circuit stops working all shorten it.123
  3. A multimeter on its microamp range can measure a steady sleep current, but it cannot follow millisecond wake-ups and shows only a slow average. To see the active bursts, measure current against time with an oscilloscope and a current probe or small shunt resistor, or with an instrument that samples current quickly.1

Common mistakes

  • Counting the radio but forgetting what is always on: the regulator's own current, pull-up resistors, an LED, or a sensor left powered.

Where to go next

Sources

  1. AN092: Measuring Bluetooth Low Energy Power Consumption (SWRA347a) · Texas Instruments · Reference
  2. Coin Cells and Peak Current Draw (SWRA349) · Texas Instruments · Reference
  3. Lithium Coin Handbook and Application Manual · Energizer · Reference