changes in vehicle with MissionPlanner low battery warning

MissionPlanner has a feature to warn at a voltage level, using the "MP Alert on Low Battery" tick box. If the user changes between a 3S and a 6S vehicle then this warning will persist and be incorrect for the new aircraft.
An alternative would be to use the UI to set BATT_LOW_VOLT which is stored on the vehicle, so changes with the vehicle.
Alternatively it could be a percentage above the BATT_LOW_VOLT, so the user starts getting a warning at (for example) 10% above the low voltage.

Start by tracing the MissionPlanner "MP Alert on Low Battery" setting and how BATT_LOW_VOLT is read or stored for the selected vehicle. The issue presents multiple possible behaviors, so confirm the intended approach before changing the warning; done means switching between 3S and 6S vehicles no longer leaves an incorrect low-battery warning.

Written by the indexing model from the issue text.