Will directly cause 12v battery damage, serious can cause explosion or burning consequences. Because the connection is equivalent to a 24 volt battery and a 12 volt battery in parallel, the 24 volt battery will charge the 12 volt battery to form an internal circulation, burning the internal components of the battery. The termination charging voltage of the 12-volt battery is also called the limit voltage of 1.2 times the rated voltage, that is, 14.4 volts, so the charging voltage of the charger during the charging process can not exceed 14.4 volts, I hope this answer can help you.