Reflect non-linear relation of battery capacity to voltage

[danielkucera]

https://lygte-info.dk/info/BatteryChargePercent%20UK.html

[bergzand]

Also relevant: https://forum.pine64.org/showthread.php?tid=8147

[wosym]

Maybe you can fit an equation on the observed measurements?
e.g.
y = 2.358007 + (0.003753138 - 2.358007)/(1 + (x/3.722494)^51.16191)
with y the remaining capacity in Ah, x the voltage in V.
(Function fit to the first graph of the first link)

[bergzand]

I've applied a piecewise linear function over the data linked here. It might require some tuning, but it is probably better than the simple linear equation used previously.

See 82dc83c