This was one of those reverse engineering projects that had to be revisited a few times. The circuitry is unusual and it took a while before the puzzle clicked into place. It's always that bit harder to probe surface mount components on a double sided PCB if trying to probe both sides at once.
That said, the whole design can be summarised as follows. It has two independent batteries that each has its own charge monitoring processor. One processor is the master and deals with pack status communication, while also communicating with the other battery's processor via a bidirectional opto-isolated data link.
The method of detecting individual cell status by gating a voltage divider feeding an analogue to digital convertor (ADC) is weird due to the odd resistor values for each cell. My best guess is that the design is trying to fit to nice round figures in the software.
The NTC temperature pin can have either of the battery thermistors connected to it, plus potentially do simple bidirectional communication with the tool or charger.
The two rows of pads are probably for testing and maybe programming of the packs during manufacture. It would make sense to have the reset circuit in their vicinity.
Update. I couldn't quite work out the weird communication circuitry on the NTC/T pin. It turns out that it was different to my schematic, and the two 750K resistors are a divider driving the gate of a MOSFET that is switched by an external 12V communication signal, and then converts it to a logic level signal for the processor. The thermistor pin receives data and the ID pin sends it by switching an external 12V signal to 0V.
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5 окт 2024