An AGM deep cycle marine battery is a type of battery that is designed for marine use and is built with an absorbed glass mat construction — so is sealed and maintenance free — and can withstand frequent discharge and charging cycles. They deliver energy over long periods of time plus they are spill proof.
Absorbed glass mat, or AGM, batteries are a type of valve regulated lead acid battery. The other main type of battery in this category is a gel cell battery, but AGM batteries are much more common. Their key feature is that they only contain a small amount of liquid — just enough to keep the glass mat wet. This is why they do not spill, and because they do not spill, they do not have to be kept in an upright position.
A deep cycle battery is one that is designed to be frequently discharged — it can be discharged to as low as 20 percent. They are different from starting batteries which are designed to give large amounts of energy in a short space of time. They therefore have several, thinner plates. In a deep cycle battery, the plates are thicker which means they are less likely to be damaged when being discharged.
Many AGM deep cycle marine batteries are also used as starting batteries though, while also delivering the other benefits.