The grey and black wire are attached together and terminate in what is possible a temperature sensor. This is what may turn the charging process off should the battery overheat.

The yellow wire terminates at the second battery pack, the white at the first. The voltage at the yellow wire is 8Volt. The white does not carry any voltage, thus possibly part of the charging circuit

One more view of the termination on the top circuit board:

position of the grey/black wire (temperature sensor?).

position of the yellow/white wires (charging circuit?)