by John F
Warns you of both low and high battery level with toast notification and sound. Charging your battery to 100% all the time can damage it, and it's better to keep the charge somewhere between about 30 and 90 percent for extended battery life. Batman gives a subtle audio signal (in its default setting) when outside these limits, getting low and discharging, or over the higher limit while on charge. The charge is checked every 5 minutes and the "pip" signal is repeated once for every percentile over- or under-charged, so it becomes more noticeable (and you can count the pips and tell exactly how low or high it is).
The flow checks first whether your device is in airplane mode and doesn't give any warnings if it is. I added this feature as a way to avoid being disturbed at night, for instance if my phone is on the bedside table and I didn't realise it was getting low. Airplane conveniently avoids calls and SMS as well so I use it at night anyway.
You should find all the relevant blocks in the flow if you want to alter the high/low values, repetition interval, sound signal, toast notifications, etc., although I intend to add features to edit these from within the program.
It is not currently an auto-start app, so you should start it manually after a reboot. It has a running notification, which also reports the low-high% values set.
Suggestions very welcome in the comments. Enjoy.