They have no overvoltage protection, the balance circuit is very limited in the ability to dissipate heat, only really there to handle tiny amounts of imbalance in the terminal voltage on the capacitors, not handle currents over around 50-100mA. If you want to use one, keep a 12V 55W headlamp ( preferably complete and in the housing) always across the capacitor, so as to provide some sort of load, as your bike charger should be able to at least supply 5A to keep it lit , and regulate the voltage to under 14V5 in doing so. Fail to do that and you get whatever is open circuit voltage of your charger, whatever that is, charging the capacitor till something, either on the bike or the capacitor bank, fails rather spectacularly.