Sorry not explained very well.
Yes the 24 (3.4Volt) cells are in series and I could measure accordingly.
ie cell 1 @ nominal 3.4 volts then cell 2 as 6.8volts etc up to the 81.6 volts as you have indicated and then in the sketch calculate the differences and hence monitor each cell.
A few points about any connecting to the traction battery pack:
It is important that I maintain the electrical isolation of this pack and do not want to reference to the chassis ground in anyway. An aux 12 volt battery is used (normal required by law) to power vehicle lights, horn, warning indicators and certain other items. This is connected to chassis ground as in a conventional ICE vehicle. This aux 12 volt source is used to power the battery management system (BMS) in my conversion as well. The Arduino could be powered from either source but I would prefer the aux 12 volt option. In DIY EV conversions, it is considered best practice to not bring the high traction pack voltage inside the passenger compartment in any way (for safety reasons).
So I would somehow like to isolate the cell measurements from the displaying device inside the vehicle totally.
Hence the thought about isolated differential inputs that can handle the voltage. Either via a series string measurement and simply calculating the difference as mentioned above or by using 24 separate floating/isolated inputs.
Hope this explains a bit more?
Regards
Bruce