How long will my battery last?
You have a sensor that wakes up, measures, transmits, then goes back to sleep. The awkward question: will it last three months or three years? And if you stick a small solar panel on it, is that enough? It takes ten minutes with a multimeter and this page. Ready? Let’s go!
A quick reminder before we start
A battery is a tank of milliamp-hours (mAh): 3,000 mAh is 3,000 mA for one hour, or 3 mA for a thousand hours. Each phase of the sensor drains the tank in its own way: a large, very short current (the radio transmission), a moderate regular current (the measurement), and a permanent trickle (sleep). We compute what each phase consumes over one hour, add them up, and divide the tank by the total. Spoiler: sleep often wins.
Four boards, four fates
Typical orders of magnitude to feel the gap: click, compare, then replace with your own measurements. The result updates by itself. As soon as you change a value you switch to “Custom”; your saved configurations stay in this browser.
The trap
On the µA range, the multimeter inserts several hundred ohms into the circuit. When the radio starts and draws 60 mA, the voltage collapses across that resistance and the sensor reboots in a loop, or blows the multimeter fuse. You then measure nonsense.
The right setting
Always start on the mA (or 10 A) range for start-up and active phases. For sleep, short the multimeter with a wire while the sensor goes to sleep, switch to the µA range, then remove the wire. Auto-ranging meters switch range in the middle of a spike: force the range by hand.
Watch out!
The result is an estimate: cell capacity drops with cold, transmissions get longer when the network is far (SF12, retries) and a sensor that loses the network can loop at full current. Size with a margin, then check in the field by watching the battery voltage reported by the sensor.
In short
- Three phases, three measurements: measurement and transmission in mA, sleep in µA.
- Consumption of a phase over one hour = current × (duration × times per hour) ÷ 3,600.
- Battery life = usable capacity ÷ total consumption per hour.
- On the µA range the meter reboots the sensor: short wire during the active phases.
- Solar panel: count the sun hours of the worst month, and aim for 1.5 times the consumption.
- Keep a margin: cold, distant network, retries.
And you, which phase drains your battery? If it is the transmission, the next page tells you why.
The transmission duration mostly depends on the spreading factor: see LoRa airtime.
Missing a tool?
Tell us what you are trying to do: if we already have it on the bench, we will put it online.