Lead-acid cells consist of two plates of lead suspended in an electrolyte consisting of an aqueous sulphuric acid solution with a low concentration (typically 29–32%, by weight[3]). A lead-acid battery consists of multiple cells wired in series in a single container. The battery produces power by reducing (or oxidizing) the lead plates and turning them into lead-sulphur-oxide while simultaneously increasing the pH of the electrolyte. When the battery is charged, this reaction is reversed; the plates turn back into pure lead, and the acid is re-created. This cycle is not perfect - hydrogen gas often escapes the electrolyte before it can re-combine into water.
To prevent the flammable gas from building up to dangerous levels, venting is required, both in the battery and anything it is placed within. These vents generally mean that the electrolyte can spill out of the battery if it is tipped over, which presents a hazard during shipping and makes them unsuitable for many portable applications. Furthermore, the constant loss of hydrogen leads to a reduction of water in the electrolyte, which must be replaced by opening the battery and "topping off" the water. Many modern batteries include a small float behind a window to allow visual inspection of the electrolyte level, and pop-off caps for refilling it.
VRLA batteries attempt to avoid all of these problems by immobilizing the electrolyte. In doing so, the hydrogen is trapped near the plates, and is available for re-combining when the battery is re-charged. This dramatically reduces the water loss during repeated charge/discharge cycles, and generally makes them "maintenance free". It also reduces the amount of hydrogen build-up to the point that no venting is required, and the batteries can be completely sealed, which in turn allows them to be used in any orientation without fear of spilling.
During rapid recharging, the electrolyte may boil and pressurize the case, or gas build-up may be too rapid for recombination. These effects necessitate the "valve regulation". This is normally in the form of a one-way pop-off valve that only opens in the case of pressure build-up. Even if the valve pops, the immobilizing agent prevents rapid, or any, acid leakage. Most designs are so stable and contain so little electrolyte that they won't leak acid even if they are cut open.
The main downside to the VRLA design is that the immobilizing agent also immobilizes the chemical reactions creating power. For this reason, VRLAs have lower peak power ratings than conventional designs. This makes them less useful for roles like car starting batteries where usage patterns are brief high-current pulses (during starting) followed by long slow recharging cycles. VRLAs are mostly found in roles where the charge/recharge cycles are slower, such as power storage applications.