FYI, this from Ask.com:
AGM stands for absorption glass mat, so named because these batteries are made up of mats in which thin glass fibres are woven to increase the surface area. This allows the AGM battery to work like the flooded lead acid battery with the exception that the electrolyte in the AGM battery does not freely flood the plates but rather is held in the glass mats.
The thin glass fibres do not absorb the electrolyte nor are they affected by the acidic electrolyte but simply hold the electrolyte preventing it from spilling when the battery is tipped over. Although the AGM battery is built in a rectangular case that meets BCI battery codes the plates themselves can be just about any shape including flat, cupped or wound.
Advantages of an AGM Battery
There are several advantages to an AGM battery, which include:
•Unspillable. This means even if these batteries get tipped on the side or turned upside down the electrolyte is not going to spill out of an AGM battery. That makes this battery good for the environment and safer to handle.
•An AGM battery never needs watering. It is considered to be maintenance free. This makes these batteries ideal to use in situations where getting hold of water to water a battery may be an issue.
•AGM batteries don't give off any gas when being charged so there is less worry about having the right ventilation when charging an AGM battery.
•AGM batteries can withstand very low temperatures without freezing or cracking.
•AGM Batteries are highly vibration resistant due to their sandwich construction, making them the ideal battery to use in circumstances where vibration could otherwise be an issue.
Disadvantages of AGM Batteries
While AGM batteries have a lot of advantages that make them the ideal batteries to use in many circumstances, they do have a few disadvantages that should be noted:
•The cost of the AGM battery makes it a less attractive choice than the flooded lead acid battery for most every day uses.
•The deep cell AGM battery can only be discharged to 50% whereas the flooded lead acid deep cell battery can be discharged to 80%, which means that the flooded lead acid battery will run longer on a single charge.
•If an AGM battery is overcharged it will definitely shorten its life and could immediately kill the battery. So extra care must be taken when charging these batteries to ensure they are not overcharged.
Although an AGM battery does have some disadvantages, it is still a great alternative to flooded lead acid batteries in many situations.
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