A VRLA battery (valve-regulated lead–acid battery) more commonly known as a sealed battery is a lead–acid rechargeable battery. Because of their construction, VRLA batteries do not require regular addition of water to the cells, and vent less gas than flooded lead-acid batteries. The reduced venting is an advantage since they can be used in confined or poorly ventilated spaces. But sealing cells and preventing access to the electrolyte also has several considerable disadvantages as discussed below.
VRLA batteries are commonly further classified as:
- Absorbed glass mat (AGM) battery
- Gel battery ("gel cell")
An absorbed glass mat battery has the electrolyte absorbed in a fiber-glass mat separator. A gel cell has the electrolyte mixed with silica dust to form an immobilized gel.
While these batteries are often colloquially called sealed lead–acid batteries, they always include a safety pressure relief valve. As opposed to vented (also called flooded) batteries, a VRLA cannot spill its electrolyte if it is inverted. Because AGM VRLA batteries use much less electrolyte (battery acid) than traditional lead–acid batteries, they are sometimes called an "acid-starved" design.
The name "valve regulated" does not wholly describe the technology. These are really "recombinant" batteries, which means that the oxygen evolved at the positive plates will largely recombine with the hydrogen ready to evolve on the negative plates, creating water and preventing water loss. The valve is a safety feature in case the rate of hydrogen evolution becomes dangerously high. In flooded cells, the gases escape before they can recombine, so water must be periodically added.
Read more about VRLA Battery: Construction, History, Absorbed Glass Mat, Gel Battery, Applications, Comparison With Flooded Lead–acid Cells