Dear, the battery liquid will start to boil and able to start burning lamps because of the abnormal voltage.
The 12 volt lamps support up to 16 volts without problems, the subject, and much more expensive, is the battery ...
The bubbling of the battery is not that it is boiling, only that it is gassing a lot and the only thing that is achieved is to shorten the battery life and consume the electrolyte water.