I’m sure many know this trick and issue but the boat next to mine didn’t so I thought I would post it.
The symptom started with their Xantrex Inverter having trouble starting. It would simply shut down as soon as it looked like it was beginning to start. The battery monitor on the panel really showed the problem – a DC voltage of 12.2 volts on fully charged batteries – not enough for the Xantrex to work. Though the problem was first recognized as the Xantrex not working, there would ultimately be other problems showing up as well with electronics not working or the engine not starting.
Using a digital volt meter, I could show him that the voltage from the battery terminals was 12.9 volts simply by putting the red lead on the positive terminal and the black on the negative terminal. What was not obvious to the captain