I have a way to measure battery power

[StephenG1CMZ]

It seems that on the Android, the HP Prime battery status only updates when the screen is tapped.
Until then, it continues to display the previous status.

So it may not be possible to program a "beep" if the charging lead is accidentally disconnected, or monitor the mains supply.

Detail: plug/unplug the lead - Prime display doesnt change (Android battery status does).
Tap the screen once - display updates charging/discharging status.
Triple-tapping the screen (Android optional accessibility screen zoom): charging status doesnt update.

The no-update affects the charging/discharging indication - I don't know yet whether it also affects the charge percentage.

Android 5.0.2
LG G3 s.