How it works
A low-power device repeats one cycle: wake, measure, maybe transmit, then sleep until the next wake. Its average current is the charge per cycle divided by the cycle length:
I_avg = (I_active × t_active + I_radio × t_radio + I_sleep × t_sleep) / interval
life = capacity × usable % / I_avg
The tool also shows how much of the average comes from sleep. When it dominates, sleep current is the place to optimise: regulator quiescent current, pull-ups, sensors left powered. When active time dominates, wake less often or finish faster.
Usable capacity
Coin cells deliver far less than their rating at high pulse currents and in the cold; a 10 mA radio burst on a CR2032 can cost 20–30 % of its capacity. Add self-discharge (≈1 %/year for lithium primaries, much more for Li-ion) and the regulator’s cut-off voltage. 70–85 % usable is a realistic starting point.