Sealed Lead Batteries
With the increase in portable electronic and electric devices, the need for batteries has grown geometrically through the years, with no end in sight. Of these types of batteries, sealed lead batteries are the most common.
Sealed lead batteries were created around 1975 for the purpose of having a power source that was maintenance free while also being economical for both the manufacturer and consumer. Amongst features beyond the sealing of the lead electrolyte from spillage was the formation of special release valves to allow dangerous gas build-up to release safely. Further, these sealed lead batteries were often designed to be rechargeable (which is when most of the gasses would form in this type of battery).
Two basic types of lead acid batteries were created that survived the tumults of economic and consumer demands. These were the VRLA and SLA batteries. VRLA stands for valve regulated lead acid, whereas SLA is sealed lead acid. Both types are quite synonymous overall, with little difference between them.
One of the best features of these two sealed batteries, when they are rechargeable, includes an excess voltage safety mechanism which disallows the battery's explosive gas from becoming too great in quantity. This is effected by a depletion of full potential, should the battery become over-energized.
A great deal of effort has gone into this aspect of rechargeable batteries specifically. Not only do batteries become dangerous when they are overcharged, but on a more economic note, they lose their overall charge through time via the buildup of crystals or corrosion, which limit the amount of electrolyte storage ability.
While this is somewhat ideal for the makers of such batteries, in that it brings in further income once the recharged battery no longer will take a charge and must be replaced, there is a continual race to make this type of lead batteries more efficient and longer-lived. As well, in a time of speed being almost synonymous with efficiency, it should be noted that sealed lead batteries do not charge so quickly.
However, with lead, as opposed to lithium ion and other types of batteries, the damage and memory loss is next to nil. However, self discharge is still a fact of life, leading between 35 and 45% loss per year.