# 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.

Page: https://dharanova.com/learn/power-budget · Track: Power · Level: Foundation · Facts last checked 7 October 2026

Diagram: Current against time, not to scale: a flat sleep line at 5 µA with a 50 ms burst to 10 mA once a minute, and a dashed average line at about 13 µA. A bar splits that average into sleep, 5 µA, and the wake-ups; sleep alone is over a third of it. Notes: capacity divided by average current is the best case, not a forecast, as self-discharge, temperature, peaks and the cut-off voltage shorten it; a multimeter can read the steady sleep current but shows only a slow average of the bursts, so use a scope with a current probe or shunt, or an instrument that samples current fast.

## 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. [1][2][3]
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

- [Batteries for small devices](https://dharanova.com/learn/batteries.md): Coin cells, Li-ion and LiFePO4: their voltages, their capacity, and when the capacity on the label actually holds.
- [LDOs, buck converters and PMICs](https://dharanova.com/learn/regulators.md): How a battery or adapter is turned into the steady voltages a chip needs, and why the choice of regulator matters for battery life.
- [Sleep modes and waking up](https://dharanova.com/learn/low-power-modes.md): A battery device spends most of its time asleep. What stays on, what wakes it up, and what is lost while it sleeps.

## Sources

1. [AN092: Measuring Bluetooth Low Energy Power Consumption (SWRA347a)](https://www.ti.com/lit/an/swra347a/swra347a.pdf), Texas Instruments
2. [Coin Cells and Peak Current Draw (SWRA349)](https://www.ti.com/lit/wp/swra349/swra349.pdf), Texas Instruments
3. [Lithium Coin Handbook and Application Manual](https://data.energizer.com/pdfs/lithiumcoin_appman.pdf), Energizer