Gas Generation:

A lead-acid battery cannot absorb all the energy from the charging source whenever the battery is nearing the completion of the charge. This excess energy dissociates water through way of electrolysis into oxygen and hydrogen.  Oxygen is generates through the positive plate, and hydrogen is produced through the negative plate.  This procedure is known as gassing.

Gassing is first remember when cell voltage reaches 2.30-2.35 volts per cell and increases as the charge progresses.  At full charge, the amount of hydrogen generates is about one cubic foot per cell for each 63 ampere-hours input.  An explosive mixture of hydrogen and oxygen can be readily produced if gassing occurs and the gases are prevents to collect.  It is necessary, thus, to ensure in which the area is well ventilated and in which it remains free of any open flames or spark- producing equipment.

As long as battery voltage is greater than 2.30 volts per cell, gassing will occur and cannot be prevented entirely.  To decrease the amount of gassing, charging voltages above 2.30 volts per cell should be minimized example for 13.8 volts for a 12 volt battery.