OK, here's the beginning of another half-baked idea. One of the major issues with most BMS/cell balancing schemes is the amount of heat given off by the shunt resistors. Various other approaches have been tried to avoid this, like switched capacitor schemes or separate dc-dc converters for each cell.
The separate dc-dc coverters for each cell approach has been proven to work but is a bit bulky and expensive.
Switch capacitor charge pumping schemes work OK at very low currents, but don't work so well at 1 amp and have a hard time shuttling charge from one end of a pack to the other.
So here's my latest half-baked idea (the half baked ones appear late at night when I go into sleep deprivation mode):

If we put a bridge rectifier on each cell fed by a capacitor coupled bus line. The coupling capacitors will pass an AC signal, but block the DC present from the cell string. If the frequency is very high (over 100KHz?), then the coupling capacitors can be quite small. The coupling capacitors need to be rated for for the full string voltage.
The balance supply would be something like a typical 5v switching mode power supply, but we would take the output coming directly from the transformer, before the rectifier. I think most any switching power supply could be modified to work like this. A single voltage adjustment on the balance supply works for all cells. The balance supply need to supply enough to overcome the forward loss in the bridge diodes.
To the balance supply, the load would 'look' like a bunch of cells in parallel. Whichever cell had the lowest voltage would present the path of least resistance, so would take the most current. This balancing current could run simultaneously with the bulk charge current running through the battery string.
While perhaps not fancy, this topology looks to me like it should work (that usually gets me in trouble). This would need to be combined with a more conventional looking bulk charger and cell level voltage protection.
Now, where's my breadboard?
The separate dc-dc coverters for each cell approach has been proven to work but is a bit bulky and expensive.
Switch capacitor charge pumping schemes work OK at very low currents, but don't work so well at 1 amp and have a hard time shuttling charge from one end of a pack to the other.
So here's my latest half-baked idea (the half baked ones appear late at night when I go into sleep deprivation mode):

If we put a bridge rectifier on each cell fed by a capacitor coupled bus line. The coupling capacitors will pass an AC signal, but block the DC present from the cell string. If the frequency is very high (over 100KHz?), then the coupling capacitors can be quite small. The coupling capacitors need to be rated for for the full string voltage.
The balance supply would be something like a typical 5v switching mode power supply, but we would take the output coming directly from the transformer, before the rectifier. I think most any switching power supply could be modified to work like this. A single voltage adjustment on the balance supply works for all cells. The balance supply need to supply enough to overcome the forward loss in the bridge diodes.
To the balance supply, the load would 'look' like a bunch of cells in parallel. Whichever cell had the lowest voltage would present the path of least resistance, so would take the most current. This balancing current could run simultaneously with the bulk charge current running through the battery string.
While perhaps not fancy, this topology looks to me like it should work (that usually gets me in trouble). This would need to be combined with a more conventional looking bulk charger and cell level voltage protection.
Now, where's my breadboard?
