An Absorbed Glass Mat ( AGM ) battery is a type of battery that can be used as a starter battery or a deep cycle battery, but is not as good at either use as batteries designed for a specific purpose. It is a good choice if you need one battery to do both jobs.
A starter battery delivers a sudden, quick burst of energy such as is needed to start a motor. These batteries have a high CCA (Cold Cranking Amps) rating. They lose their charge level fast and should be recharged as soon as possible. Typically these live a life of being at or near 100% charge, losing down to the 70% level (while cranking the engine) and then being quickly charged back up by the engine’s alternator. Starter batteries accept a charge fast. These batteries will run something such as a trolling motor, but not for as long as a deep cycle battery. You can use them as House Batteries in an RV, but they will discharge fast and wear out fast.
A deep cycle battery is made for long slow discharges, such as when running a golf car or as house batteries in an RV. They are often called Marine Batteries. They live a life of going from 100% charge slowly down to about 10%, then being slowly charged back up. It can take 24 hours to fully charge a deep cycle battery. In an emergency it can be used to crank the motor, but this will drain it fast.
The AGM battery falls in the middle. It works okay as a starting battery, but recharges slower than a high CCA starter battery. It can work as a house battery, but only works well down to about the 50% level, however an AGM battery charges faster than a true deep cycle battery. AGM batteries are sealed, no maintenance battery. It is about twice the cost of other batteries, but will also last longer.
All the electrolyte (acid) is contained in the glass mats. They cannot spill even if broken. There is no liquid to freeze and expand which makes them nearly immune to freezing damage.
Nearly all AGM batteries are “recombinant” which means the Oxygen and Hydrogen recombine inside the battery. The recombining is typically 99+% efficient, so almost no water is lost.
The charging voltages are the same as for any standard battery – no need for any special charging systems.
The internal resistance is extremely low so there is almost no heating of the battery even under heavy charge and discharge currents. Most AGM batteries have no charge or discharge current limits.