Valve regulated lead acid (VRLA) batteries are commonly referred to as sealed lead acid batteries, and are lead-acid based rechargeable batteries that require little to no maintenance. A common feature to this type of battery is as the name implies, they are always manufactured to be a sealed battery with a built in safety pressure relief valve. If the sealed lead acid battery were to ever be inverted or in any other position than upright, the battery is incapable of leaking and cannot spill its electrolyte (battery acid). Also, when the typical lead-acid based battery is recharged at a mild to high voltage, it will emit gases which can be considered dangerous if emitted in large quantities. Since the VRLA battery is sealed, when the battery is put on charge it relies on its built in pressure release valve to alleviate the pressure from the gases which build up inside the battery. The release valve emits small trace amounts of these gases and has enabled any VRLA battery to be placed in small units (such as battery backup systems) inside buildings or offices without worrying about harmful gasses being emitted. These batteries are further divided into two sub groups know as absorbed glass mat (AGM) batteries and the gel battery.
The AGM battery differs from other lead-acid batteries because it uses a glass mat separator to hold its electrolytes in place. These mats allow for the AGM battery to accumulate the highest frequency of acid available and never spill a drop. It was developed in the 1980’s by Concorde Aircraft battery, their intention was to create a warm weather, vibration resistant chemical alternative to the current expensive batteries being used in their fighter air-craft technology. Today the AGM is applied in motorcycles as well as many automobiles. They are also frequently used in remote sensors and cold environments due to their lack of free electrolyte they will never corrode or crack.
Pros to the AGM Battery:
- They have low internal resistance allowing then to be charged and discharged rapidly
- They are maintenance free
- Will not corrode their surroundings because the acid is trapped in the matting
- Handle vibration very well
Cons of the AGM battery:
- A tad expensive being nearly double the price of a flooded cell battery
- They don’t accept overcharging as it burns the electrolyte which is irreplaceable in the AGM model
Finally the Gel battery, sometimes called the gel cell, is unique due to its untilization of gelified electrolyte. The gel is created by sulfuric acid mixed with silica fume, causing it to become an immobile gel mass. The gel cells are almost extensively used in power wheelchairs because of their extensive deep-cycle capabilities and low acid output. Both types of VRLA batteries are widely used in aerobatic aircraft due to their knack for withstanding various flight altitudes and their large ambient temperature range.