That looks like a good approach. When controller disables charging current, the power cables look just like sense lines. The mppt algorithm may measure PV open circuit. At that time charging current is also disabled. That's a good time to sense battery voltage.
To make this work, a dedicated line to batteries is required that's not shared with other loads. That's usually the case anyways.
Iota converter does something similar. It senses battery voltage when ac power is off. When power gets turned on, converter knows exactly which mode is required.
Who builds 3048?
Sal
