In the further development of lead batteries, care was taken to ensure that the liquid sulfuric acid would not cause any damage in the event of accidents or mishandling. It was bound. In the case of the lead-fleece battery, Absorbent Glass Mat( AGM), in a glass fiber fleece, and in the case of the gel battery, Sealed Lead Acid( SLA), the sulfuric acid has been bonded with silicic acid to form a gel.
The two battery types mentioned - the lead-fleece battery and the gel battery - are encapsulated so that they cannot leak, and are also equipped with a safety valve through which degassing takes place. This sealed design is called Valve Regulated Lead Acid (VRLA).
Valve Regulated Lead Acid (VRLA) does not adequately describe pressure equalization because recombination occurs within VRLA batteries through electrolysis between the oxygen generated at the positive electrode with the hydrogen generated at the negative plates. This recombination is over 99% and is aided by the relatively constant internal pressure. Water is generated from the recombination, which compensates for the loss of water. The pressure valve opens only when too much hydrogen is produced.
VRLA batteries are sealed lead-acid batteries whose battery cells cannot be refilled with water and which need to be vented of less gas than conventional lead-acid batteries. This has the advantage that such batteries can be operated even in poorly ventilated rooms.