A VRLA battery (valve-regulated lead–acid battery) is a type of low-maintenance lead–acid rechargeable battery. Because of their construction, VRLA batteries do not require regular addition of water to the cells.
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.
These batteries are often colloquially called sealed lead–acid batteries, but 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 turned upside down. Because AGM VRLA batteries use much less electrolyte (battery acid) than traditional lead–acid batteries, they are also occasionally referred to as 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 thehydrogen ready to evolve on the negative plates, creating water and so 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 have a chance to recombine, so water must be periodically added.
One result of this design is a much higher ratio of power to “floorspace” than large, flooded type battery systems; another is a high-rate power capacity, though of relatively short duration. As a result, VRLA batteries are frequently employed in UPS (uninterruptible power supply) or other high-rate application