AWOL:
What happens to your reference if the supply voltage dips?
The scooter has a 12V/500W dc-dc converter to power the arduino. The arduino will only do his state of charge thing, no blinking LEDs or other stuff. So I believe, 5V will be pretty stable and Aref too.
I'm also planning to use a supercap across 5V to be able to store the last SOC when 'ignition' is switched off.
Unfortunately, I have no clue how to detect that ignition off situation.