Hi.
A 5 series cell battery pack for a tool had a bad cell replaced. Measured 4.1V, 3.63V, 3.88V, 0V, 3.7V. Of course the 0v replaced with one charged at 4.0V.
When placed on the charger after repair, shows as 'charged' state. Do the 'smart' chargers fill energy individually to the less-than-maximum-charged cells to balance them ending all at ~4.1V or that is not what 'balancing' does ?
After the next time I use such; will see if accepts charge, but still want to know the inner workings.

 

I have two Li-Ion chargers. One charge to 4.19V and the other charges to 4.21V. The battery cells work fine at each voltage.

 

Smart chargers only charge the battery pack. To balance the cells, there is usually a cell balancing circuit built into the battery pack.
If you don't have a balance circuit, you can balance the pack manually by charging each cell individually to the same voltage as the rest. Over time, Li-ion cells loose capacity, so it's far better to replace a cell in an old pack with a used cell that has about the same capacity as the rest of the cells.

 

A 5 series cell battery pack for a tool had a bad cell replaced. Measured 4.1V, 3.63V, 3.88V, 0V, 3.7V. Of course the 0v replaced with one charged at 4.0V.

It's not a good idea to mix and match batteries of different "conditions" in a battery pack. The battery that was at 3.63V is going to be the next one to die. With out an equalizing circuit, the stronger batteries will abuse the weaker ones.