Balancing is required when 2 or more capacitors are connected in series to prevent one or more capacitor from an overvoltage condition. Ideal capacitors of equal value connected in series will have equal voltages applied to each capacitor. A major reason for mismatch of voltages is mismatch in leakage current. The capacitor with the lowest leakage current (Highest equivalent parallel resistance) will have more voltage applied. To avoid this problem you can connect resistors in parallel with each capacitor such that the resistor current is much larger than the leakage current. There are also active balancers that waste less power than the resistor method. Another reason for voltage mismatch is mismatch in capacitance. For two capacitors C1, C2 connected in series, and applied voltage V, the voltage across C1 will be V(C2/(C1 + C2)). If C1 and C2 are equal, then the voltages will be equal.