The Standard Rad battery is made up of 52 cells, with groups of 4 cells connected in parallel (to provide more current) and 13 groups connected in series (to provide more voltage).
Cells are in balance when all cells are at the same State Of Charge (SOC) and voltage, and out of balance when at a different SOC and voltage. The cells connected in parallel are always in balance, because they are all at the same voltage. The series-connected groups can get out of balance, with one or more groups having more or less SOC than the others.
It is desirable to have all cells at the same SOC to get maximum performance – run time – out of the battery as a whole.
The Standard Rad battery uses passive cell balancing. Passive balancing is performed only during the charge cycle, and when the charger is in Constant Voltage (CV) mode. It does so by bypassing charge current away from cells that are already at the desired SOC and into cells that are at a lesser SOC.
Looking at the attached figures, each group of 4 cells in parallel has a shunt resistor and a switch connected in parallel with the cells. When the switch is off, all charge current flows to and through the cells. When the switch is on, up to about 40 mA (0.04A) of current flows through the shunt resistors, and not to the cells.
When a depleted battery is first connected to the charger, the charger operates in Constant Current (CC) mode. About 2A of current flows to and through the groups of cells – about 0.5A to each cell – and none through the shunt resistors, because the shunt switches are off. In Figure 1, note that Cell Group 2 is at a lower SOC than the other cell groups.
As the cells are charged, their SOC and voltage increases. After (typically) 2-3 hours, the combined voltage of all cells approaches the charger float voltage of 54.6V, and the charge current decreases dramatically. Cell Group 2 has been charged along with the other groups, but remains at a lower SOC and voltage than the other cell groups. The Battery Management System (BMS) detects this voltage imbalance, and turns on the shunt resistor switches for all cell groups that have reached full SOC, bypassing current around the cells. It leaves the switch off for any cell group that is at a lower SOC, so that the current continues to flow into those cells. This stops the fully-charged cell groups from getting more charge current, and only the lesser-charged cell groups continue to get current. The BMS is balancing the cells. Because the balancing current is much smaller than the initial charge current, it can take several hours to bring the unbalanced cells up to the desired SOC.
The BMS will enable balancing any time the charger is in CV mode (green light) and any of the cell groups is at a lower SOC and voltage than the others. Balancing is enabled based on relative cell voltages, not an absolute voltage. There is nothing special about a 12 hour charge, other than 12 hours pretty much guarantees that the BMS will have had adequate time to balance. Once all cells are in balance, all the switches are turned on, and all low charge current bypasses the cells.