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Scooba battery replacement

Replacing the battery pack for the Scooba.
The batterypack consists of 12 5/4A cells, in size equivalent to the more familiar 18650 size.

For 18650 we can also use Li-ion cells, to get the same voltage only 4 cells are required. Problem is that lithium cells need balancing to keep the individual cells charged equally.

For another project a test setup was made for a 3 cell balancer so this needs to be adapted for 4 cells and designed in such a way that it doesn't adds too much to discharging the battery.

I found a few references to similar projects:
Dismantle a scooba battery
Why not to use Li-ion
using Li-Ion cells for a scooba battery

A test setup for a Li-Ion balancer:

Li-ion 18650 3000mAh batteries were tried but they proved to be only capable of powering the scooba for a few minutes before the voltage dropped so low that scooba stopped with the charge message.
The batteries are not empty at that moment but not capable of delivering the required current. I even tried using 8 18650's but even then the delivered current was too little.

It would have been a nice fit:

Next try using some LiPo batteries, they should be able to deliver the current.
LiPo's I have succesfully used to replace the NiCd batteries in some cordless drills so the power is there.