Test: Low power (sleep, wake and the coin cell)

Work in progress

This is a standalone test page for a demonstration under development. It is not yet part of a chapter. It draws on the BLE, Zigbee and Thread, RTC and buses labs.

Power & Batteries

Low Power: Sleep, Wake and the Coin Cell

derekmolloy.ie

1. Duty cycle2. Sleep states3. Wake sources4. Leaks5. Batteries and bursts6. Measuring it7. The whole budgetSandbox

Duty cycle: where the charge goes

Next: Sleep states

A battery sensor spends almost all its time asleep, at a few microamps, and wakes for a few milliseconds at a few milliamps. Those two currents differ by a factor of a thousand or more, so the trace is drawn on a log scale. What empties the battery is charge: current × time. The average current is the sleep current plus the charge of each wake-up divided by the time between them, and battery life is the capacity divided by that average. Change the numbers and watch which term wins.

Challenge: Wake every second and still run a CR2032 for more than five years.

Charge per wake-up

Current × time awake. Here mA × ms gives microcoulombs (µC).

3 mA × 1 ms = 3.0 µC

Average current

Sleep current, always, plus the charge per wake-up ÷ the time between wake-ups.

3.00 µA + 3.0 µC ÷ 10 s = 3.30 µA

Battery life

Usable capacity ÷ (average current + the cell’s self-discharge).

CR2032: 6.1 years

sleepsensorCPUradioleaks· log scale: each grid line is ten times the one below · bursts are drawn at least 2 px wide

Show 10 ms at 5 mA every 10 s← Start here

Reset

Awake current

1 mA3 mA5 mA10 mA

Awake for

0.1 ms0.5 ms1 ms10 ms

Every

1 s10 s60 s

Asleep at

400 nA1.50 µA3.00 µA600 µA

Awake

0.010 % of the time

Average

3.30 µA

Asleep share

91 %

CR2032 lasts

6.1 years

The CR2032’s 225 mAh is derated to 85 % for a pulsed load, and the cell loses about 1 % a year to self-discharge, which on its own is about 0.26 µA. 600 µA is what the chip draws idling with its clocks running: forget to really sleep and nothing else matters.