The voltage of the battery increases with the increase of storage power during charging. When the battery storage capacity reaches the saturated electrode material can not continue to charge, if continue to charge the electrolyte will be electrolytic, and in the anode to produce oxygen, hydrogen production in the cathode, so that inside the sealed battery caused internal pressure rise, can cause damage to the internal structure of the battery. Like this phenomenon is called overcharge.
To prevent the battery from being damaged, the capacity of the cathode is usually made larger than the anode capacity, so when the anode will reach saturation and produce oxygen when overcharge, while the cathode is not saturated without hydrogen, the oxygen produced by the anode diffuses to the cathode and then reacts with the metal cadmium that the charge produces to absorb the oxygen. and the velocity of the reaction is balanced with the velocity of the metal cadmium, therefore, the pressure rise of the battery can be avoided effectively. But if the charge current is too large (use fast charge) will lose the balance, the battery pressure over the Assembly will be the battery valve pushed away, hydrogen and oxygen will leak to the outside of the battery, Until the pressure is lowered the safety valve closes the battery before it is sealed again. But the gas leak has reduced the internal chemical materials, resulting in a shortening of battery life.