"Another option would be to use battery A to hold in a relay in series with a pot and N/O contact. The pot is set so that the relay drops at < 12V
A changeover contact on the relay switches from battery A (energised) to battery B (de-energised). The technician would change out battery A then...
My boss ask to do a project. I have a pay phone that is powered by a 12 volt battery but the company which owns the phone says the battery gets to a low voltage before they change it making the phone not work properly so the company ask us to make something to connect two independent batteries...
