What I don't understand though is what the actual issue is - the voltage measurement is low but as far as I can see the hybrid board doesnt actually measure the voltage, it is just an isolator that passes the voltage along the circuit to be measured later.
The hybrid board should read the voltage across pins 1 & 2 (HV and VL) and output the same voltage across pins 3 & 4 while maintaining isolation between the pack voltage and the 12 volt system. The main pcb will have a ADC that measures this voltage and then convert that voltage to the actual voltage seen at the MCU (also referred to as the condenser voltage) The issue seems to be that the ISA215N can go out of calibration or in my case, fail all together, so that the voltage it outputs doesn't match it's input voltage. It seems that the MCU voltage/condenser voltage needs to be within around 12 volts of pack voltage reported by the BMS or a fault will be reported and the car will shut down.
The AD202KN is an isolation amplifier that we are using in unity gain mode to mimic the ISA215N. It seems to have a slight voltage offset that means the MCU reports around 6 volts less than actual battery voltage. Today I took a reading at 28% SOC and had an 8 volt difference 334v - 326v but it was back to the 'normal' 6v difference at 74% SOC, 352v - 346v. So far this voltage difference hasn't caused any problems, but that's only 2 vehicles so far (I've now done 285 miles on mine, since the repair).
This repair isn't a magic bullet, many other possible problems have to be ruled out first. Read through all of this thread and other related threads if you have a P1A15 error. A quick check list would be;
12 volt battery condition and connections
Full diagnostic scan, fix any other errors first, particularly CAN bus errors
Try AC charging the car and then scan again and fix any of those errors
Disconnect Heater and AC compressor
Measure voltage at MCU (condenser voltage) with min/max DVOM