Where does this wire go?

The first is that I bought a voltmeter to use to know how much charge is in the batteries. I think we will wire it in so we can know when the charge is getting low. I don’t want any surprises.

The second item is that the charger that I thought was dead seems to be working. It appears that it was the batteries that made the charger look dead. It is supplying some current to the new batteries and I’ll check the charge in a bit. I expect I’ll be happily surprised.