So the battery tender can only charge one battery at a time?
Normally, you leave it alone. The system is designed to be very simple. For normal use, you leave the system in the "DUAL ON" mode (green LED), and leave it alone - nothing to switch unless you have a starting / charging problem; the vehicle starts on the main battery, and then both batteries are charged while running. When you turn off the vehicle, the batteries are isolated from each other. If you have a dead battery, you can self-jump by switching to the "EMER ON" mode (red LED), which connects both batteries to start. If you have an alternator failure, you can switch to "DUAL OFF" mode (no LED), which prevents the aux battery from draining when the vehicle is on, giving you about twice the distance w/o the alt...