Voltage Sensitive Relay

[@jackpinebearcat1845]

I had a commenter ask what the yellow wire is for. The comment has disappeared for some reason, but the answer is...it allows manual activation of the VSR. Wire a switch in line to 12 volts using the yellow wire, and when the switch is "on" the relay closes and the two batteries are joined. This allows using both batteries at once to start the vehicle, or to use the second as a backup.