> The Fieri light runs on 4 rechargeable cells 2p X 2s. 
> 
> Just wire it so that the voltmeter always gets 8 volts less than the ebike battery.



Another idea is to just come off one of the parallel 4 cells and see how representative it is.

I won't do that just now as I'll rebuild a 7pX10s to a 3p × 10s. If that works I'll rebuild the original with the remaining cells.

The Fieri light runs on 4 rechargeable cells 2p X 2s.

Just wire it so that the voltmeter always gets 8 volts less than the ebike battery.