Hello, I happen to have purchased a BMS system which uses an Arduino to control balancing boards and display data on an LCD.
I can tell you that the balance boards contain the resistors and voltage monitoring which you would need.
Those boards then have ribbon cables running to the Arduino board which computes which cell is to be balanced.
The balancing boards can each manage 12 cells each.
If you like I can try to get you in contact with the person making them.
I do not however have the source code.