Making batteries last.
WHAT CONSUMES POWER
Radio transmission, overwhelmingly Processing Sensors Indicators and displays Leakage in the circuit
WHY TRANSMISSION DOMINATES
Radio power exceeds processing power by a large factor.
WHAT FOLLOWS
Reducing transmissions extends life more than any other change.
WHAT SLEEP MODES PROVIDE
Consumption reduced to a tiny fraction while idle.
WHAT THE DEVICE SHOULD DO
Sleep almost always, waking to measure and transmit.
WHAT DUTY CYCLE MEANS
The proportion of time awake.
WHAT DETERMINES BATTERY LIFE
Average current, which is dominated by that proportion.
HOW TO CALCULATE IT
Current in each state, multiplied by the time in it, summed.
WHAT TO MEASURE RATHER THAN CALCULATE
Actual consumption, with an instrument.
WHY
Estimates omit leakage, peripherals left enabled, and unexpected wake-ups.
WHAT COMMONLY WASTES POWER
Peripherals not disabled before sleep Indicators left on Radio retries from poor signal Wake-ups from unhandled interrupts
WHY RETRIES MATTER
A device in poor coverage may consume several times more than one in good coverage.
WHAT TO PLAN
Battery capacity for the intended life, plus margin for cold and age.
