Like most things these days, automotive batteries aren't what they used to be-but this is a good thing! That being said, the old "wet" or "flooded" style battery found in most vehicles is still widely used because of one thing-it's cheap to make. Most automobile owners don't care what's under their hood, engine included, as long as it starts when they turn the key. But for those who do care, you have a choice.
There are many names associated with batteries of all types, and while some are (or can be) applied to many of them, they aren't created equal. There are volumes that could be said about batteries-how they're made, and how they work-but we're just going to quickly outline the most popular types of automotive batteries in our market today.
* Flooded or Wet: This is the traditional automotive starting battery. It has a liquid electrolyte (lead acid) in it that's free to move in the cell's compartment inside. There is a lid(s) on the top of this battery to access the individual cells where distilled water is added as the battery dries out. Obviously, the chance for spills is highest with this style.
* Sealed or Maintenance-Free: This usually refers to a slightly modified flooded-style battery where there is no access to the cell compartments for filling. The internal structure is still basically the same as a flooded battery, but the manufacturer has ensured that a sufficient amount of acid is in the battery to sustain the battery's chemical reaction under normal use throughout its warranty period. Other types of lead acid batteries are also sealed, as explained below, and shouldn't be confused with a flooded type.
* VRLA: This stands for a Valve Regulated Lead Acid battery. This is also a sealed battery, but in this case the battery box is designed as a small pressure vessel with safety valves. Confining the oxygen and hydrogen gases, they react with each other and re-form into water. This is called recombination and is a smart way of all but eliminating fluid loss. It isn't possible to confine and recombine all the gas, but the loss is reduced dramatically. VRLA batteries are very sensitive to unstable current coming in through most any wall outlet. For this reason, traditional transformer battery chargers should not be used to charge a VRLA battery.
* AGM: An Absorbed Glass Mat (AGM) battery is in the VRLA family and keeps the acid in place by a separator paper that consists of a fiberglass mat operating like a sponge. These batteries can be constructed with extremely thin separators that keep the internal resistance low. This means you can leverage high power from a small volume, which makes it ideal as a starter battery. AGM has one drawback: the amount of acid is limited. All acid must be absorbed by the separator paper, and when the small amount of acid has been converted into lead sulphate, this signals that it's done for. To deal with this problem, AGM batteries often have a somewhat higher acid weight. This means an AGM battery can, and often should, be charged using a voltage that's a little higher with an AGM-specific charger.
AGM batteries have grown in popularity in the automotive aftermarket and with builders all over the world. Thanks to their diminutive size compared to a flooded battery and the AGM's ability to be mounted in any position, except upside down, they are a natural in the street rod and classic truck market since space is a premium in either one.
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