@Rob
Good point
If the cells are being read out in a short amount of time (a few milliseconds) there shouldn't be a problem I guess.
If the BMS gives me a warning while I'm accelerating or braking (regenerating) I could guess there might be another reason than actual imbalance.
I guess It would be quite reliable when cruising at a more or less fixed speed (even at top speed = limited to 110kph).
I can also choose/adapt the tolerance for the difference between voltages.
I could also program the arduino to give a secondary alarm if the voltage of one cell drops below a safe point.
An external power supply shouldn't be a problem (I also have a 12V circuit in the Vectrix).
I'd make the circuit so I can put it on when I want (not all the time)