Monday, October 5, 2015

Pebble watch programming to extend battery life.

These are my observations to date on battery life with the Pebble classic watch. I figure the data will be of interest to other watch face developers.

For my own Pebble watchface I wanted:
  • Simple digital display with seconds counter.
  • Show current date.
  • Large characters to be readable in poor light conditions.
Once it was running, I wanted to measure and optimize battery life. The watch reports battery state in 10% increments, so I track the level changes as a function of time.
Pebble has some hints online at . Basically, they say to do as little as possible when servicing the timer ticks, and minimize the portion of the screen that's updated.
The plot here was obtained with the watchface shown in my previous post: . The green lines are the first version: I call the routines to update everything on the screen every second. For the blue line, I update the seconds display every second, but the rest of the display only updates every minute. For the red line, I update the seconds every second (obviously), the hour and minute update when the minute changes, and the day updates every hour. The short magenta line is like red, except I turn the backlight off (instead of auto) and turn the watch off at night. Bluetooth was off in all cases.
  • The time from end-of-charging to 90% battery level varies widely, from a few minutes to many hours. My guess is that the battery keeps trickle charging even after the display shows charge is complete: the shortest times to 90% were when I took the watch off charge immediately, and the longest were when it was connected for an hour or so after the indicated completion of charge.
  • That said, minimizing unnecessary screen updates appears to make a significant difference in power use: an additional day or so before the battery drops to 40% remaining power.
  • I probably won't bother keeping the backlight off or turning the watch off at night: the savings don't seem that great and the inconvenience is significant.
  • The screen backlight was in auto mode: it comes on for a few seconds when the watch shakes. It did occasionally come on during all tests (except magenta). No sure how much difference it made.
Again, this is for the Pebble Classic; no idea how it works on other Pebbles.

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