Batteries in my electric scooter.

That'll depend on how they're wired up.
I've not had an EV pack on a bench.

What I do know from using Li-ion batteries with security cameras in clusters of 4 is when one cell is duff it will not read above 48% as total capacity with fully charged batteries. So, if similar, then yes you should see a significant difference.

Not really.

When on the bench the battery charges just fine to the required voltage. Everything seems fine. When running on the bench everything behaves normally including watching the battery voltage on the in-built voltmeter on the scooter’s display.

However, this is clearly insufficient load. I did try to fake the load using the brakes, but both are electronically controlled and cut the motor when applied.

So, out on the road with the load applied (Me) the performance seems to be much the same, Perhaps a couple of MPH slower, but difficult to be sure since studying a speedo on a scooter at 30mph can easily involve face planting.

The scooter has always suffered from battery sag, as all batteries do. Again, one cannot really read the voltmeter at speed but it does also have a handy visual display (not unlike the original Vodafone signal strength display of 5 blocks). As I recall previously when the scooter was under load the display would drop an extra block below it’s actual level but would then pick up to its actual level when stopped. It might have dropped two blocks, I cannot remember for sure.

What happens now is that after a few minutes (3 or 4?) at full power it starts dropping block by block and doesn’t stop. If it gets to one block the software decides the battery is exhausted and shuts down. It will not restart until a charger is applied, but the charger need only be applied for mere moments and it starts showing the ‘correct’ voltage and working again.

Note: A normal car battery will perform this way (I realise the chemistry is different) - fully charged it is at 12.6v, when used to start the engine the voltage will drop. If it drops below 9.5v then the battery is duff.

Currently if one stops and waits when the scooter display has dropped to 2 blocks it will then gradually pick up to 4 or 5 and start working again for another 3 or 4 minutes. Since the damn thing is capable of considerable range it is not practical to see just how far it will go in this pissing about method.

This level of voltage recovery causes me doubt, but it could simply be that one cell is either being recharged or masked by the other 'good' cells.

I cannot work out how to meaningfully load test a single cell. Thoughts welcome.

I cannot find anybody who will investigate or repair my battery, though I can buy a new one (+/-£300) or buy an individual cell (+/-£10).

I could just go and buy a whole battery of course, but not only is that rather more than I really want to spend on it, I am not entirely convinced that the problem does not lie elsewhere – the BMS or controller for example.

So I am currently struck down with a bout of inertia-itis and dong nothing.

Your thoughts are welcome.