In the case where the device holder does not have access to the app, the device should periodically begin to “nag” the user to recharge it, both with an alert tone and a voice prompt.
The app could provide settings for when this self-preserving “nag” feature would start and how frequently it would prompt the user. For instance, an aggressive setting might begin nagging every 5 minutes when the battery is at 25%, but increase to every minute when battery reaches 10%.
One might want to exclude these alerts during sleeping hours.
The alert should play at maximum volume, overriding the device’s volume settings.