New boaters and sailors have a lot to learn, and if you're among these newbies, you might already be getting frustrated with how complicated things can seem at first. Any time you find out that what you need is available in so many options, that can seem like the universe is conspiring to keep you confused. That's not the case, of course, and you just need to stop and study that particular item. Once you're familiar with that, you move on to learning about the next thing. One of the most basic parts of the boat that you need to learn about is the battery, and while it might seem like there are many different types, they can be divided into three groups.
First: The Category of Battery That the Boat Needs
Boats need two different types of batteries. One is the starter or cranking battery, and the other is a deep-cell battery. Starter batteries are used to start the boat, and deep-cell batteries are used to run the boat and everything on it. Sometimes you'll find dual-purpose batteries that let you do both with one physical battery, but these tend to work well only for boats that need less power. If you have a big boat, you'll want to have separate starter and deep-cell marine batteries.
Second: The Chemical Makeup of the Battery
Next, each battery has a chemical makeup. This is akin to electronics batteries being alkaline, carbon-zinc, lithium, and so on. For marine batteries, the choices are wet cell, gel cell, absorbed glass mat (AGM), and lithium. They differ in price, longevity, weight, maintenance needed, overheating risk, and more. One thing to remember is that, no matter what chemical makeup you choose, it's best to use the same type for each battery. So, if you have a starter and a deep-cell battery, both of those should have the same chemical makeup.
Third: The Physical Structure of the Battery
Next, the physical structure of the battery can differ. The main types here are flooded and sealed. Flooded batteries need refilling, while sealed batteries are sealed up so you can't open them. Sealed batteries have less of a risk of leaking battery acid.
When you get your boat and have to find batteries, you'll still have to decide which options you want, but at least now you know what the terms mean and which of those terms fit into which category. That should make shopping for marine batteries a lot easier.