A maintenance-free lead-acid battery that may be used in any position was created by researchers in the middle of the 1970s. After moistened separators filled with liquid electrolyte have gelled, the enclosure has been closed. Ventilation is permitted during charge, discharge, and variations in air pressure through safety valves.
Two lead-acid systems were developed in response to various market demands: the smaller tiny sealed lead-acid (SLA), and the bigger valve-regulated lead-acid (VRLA). Each battery is comparable. Because no rechargeable battery can be completely sealed, engineers may claim that the term “sealed lead-acid” is misleading.
Both SLA and VRLA are built with a low over-voltage potential, unlike flooded lead-acid batteries, to prevent the battery from reaching its gas-generating potential during charge because excessive charging would result in gassing and water depletion. As a result, these batteries are never able to be fully charged. Lead-calcium is used in sealed lead-acid batteries rather than lead-antimony to lessen dry-out.
The lead-acid battery performs best at a temperature of 25°C (77°F). Longevity is decreased by increased temperature. As a general rule, the battery life is reduced in half for every 8°C (15°F) increase in temperature. When run at 33°C (92°F), a VRLA that would last 10 years at 25°C (77°F) would last 5 years. Keeping the same battery at a continuous desert temperature of 41°C (106°F) would cause it to stop after 2-3 years.
The sealed lead-acid battery is capable of discharging for 5 hours (0.2C) and 20 hours (0.05C). Due to fewer losses, longer discharge durations result in greater capacity measurements. Lead-acid batteries function well under heavy load currents.
The AGM employs absorbed glass mats between the plates and is a more recent style of sealed lead-acid battery. The plates are rigidly mounted, sealed, and require no maintenance to endure high shock and vibration. AGM batteries can recombine 99% of the oxygen and hydrogen, making them nearly universally recombinant. Water loss is almost nonexistent.
The charging voltage is the same as other lead-acid batteries. Even under heavy overload, hydrogen emissions are below the 4% required for aircraft and enclosed spaces. A low self-discharge of 1-3% per month enables long-term storage before recharging. AGM costs twice as much as a flooded version of the same capacity. For durability, German high-performance cars use AGM batteries in favor of flooded types.