Hmmm. Resources left on the board are: One Arduino I/O pin (the TX one), one unused relay. That could be used for one of the six channels to decrease the loa

Hmmm. Resources left on the board are: One Arduino I/O pin (the TX one), one unused relay. That could be used for one of the six channels to decrease the load resistance slightly by splicing in a parallel load resistor. By measuring the voltage drop from momentarily increasing the load, the ESR could be computed, at least roughly, for that one battery position.