How the battery life calculator works
A battery-powered sensor spends most of its time asleep, wakes up to measure, sends the result over the radio and goes back to sleep. Each of these states draws a different current for a different time. The calculator adds the charge of the three states per cycle, divides by the cycle time for the average current, adds the battery’s self-discharge, and divides the usable capacity by the total.
It also shows which state uses most of the charge, so you know where saving effort pays off.
Average current
For a node that sleeps at 5 µA, measures for 100 ms at 10 mA and transmits for 50 ms at 40 mA once a minute, the charge per cycle is 0.30 mA·s asleep, 1 mA·s measuring and 2 mA·s transmitting. The average is 55 µA. The radio is 61 % of the total: halving the transmit time or sending every other measurement saves far more than a lower sleep current.
With a 2400 mAh cell of which 80 % is usable and 1 % self-discharge per year, the result is about 3.8 years.
Usable capacity
The rated capacity is measured at a small steady current, room temperature and a stated cut-off voltage. Your node gets less if it is cold, if it draws high pulses, or if its electronics stop above the battery’s cut-off voltage. The usable share covers this; 70 to 85 % is a common planning value, and the battery datasheet’s capacity curves for your temperature and load give a better one.
Peak current and battery chemistry
Lithium thionyl chloride (Li-SOCl₂) cells have a high capacity and a very low self-discharge, which makes them common in meters and long-life sensors. Bobbin-type cells deliver only small pulse currents, and a radio burst can pull their voltage below the cut-off, especially in the cold. A capacitor or a hybrid layer capacitor across the cell supplies the peaks. Check the peak current shown by the calculator against the battery’s pulse capability.
Self-discharge and long lives
Self-discharge is a share of the capacity lost per year whether the node draws current or not. For very low average currents it can exceed the load, and for lives beyond about ten years the battery’s shelf life and passivation, not the load, usually set the limit.
Frequently asked questions
How do I measure the currents?
Use a meter or current profiler that captures the sleep current in microamps and the short bursts in milliamps. A multimeter on the mA range averages the bursts and misses the sleep current.
Does the interval have to be constant?
No. For a node that sends more often on an alarm, calculate both cases and weight them by how long each lasts.
What about the sensor’s warm-up time?
Many sensors need time after power-up before the first valid reading. Include that time, at the sensor’s current, in the measuring time.