I have finally found the time to put together some graphs from one week of running BatteryChart.
With BLE being more or less constantly active (and connected) during the day, while off at night, the watch lasts two days, with a few percent left.
The SoC device graph is only a rough estimation, since the widget only records values only every ten minutes and thus the LCD is naturally enabled more often than displayed there.
You will notice that the battery level seems to drop faster while the watch is not being used at night. However, you need to keep in mind that the percentage is calculated and takes the die temperature into account which also drops, which most likely explains the faster decline.
Looks like the discharge percent calculation in Bangle's firmware is linear, but a Li battery's discharge curve is not linear.
And coincides somewhat with your finding: Here (scroll to fig2 the state-of-charge - voltage graph looks close to linear until ~20%, and drops quickly.
That's great! Thanks for those graphs!
And yes, it'll be exactly as @AkosLukacs says - battery percentage is currently linear: https://github.com/espruino/Espruino/blob/master/libs/banglejs/jswrap_bangle.c#L1114
It'd be interesting to see if it could be tweaked slightly to provide a more linear reading.
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