For testing purposes, I've tried the vacuum without a battery protection circuit.

The performance was good, but not the best. The battery capacity seems to be enough for ~10-15min, but the cheap "9900mAh" batteries are getting warm after ~30s.

The next try will be with 2x2 batteries (1+1||1+1), so this is the added 2nd battery holder:

I will measure the runtime with 4x2Ah cheap cells and 4x3Ah good cells.