How to Choose Marine Batteries

Many boats are equipped with two batteries, and one of them faces steep demands because of how boats are commonly used. In other words, one battery alone doesn't cut it when it comes to running a boat and using its electronic equipment while the engine is off.

Anchored offshore or tied up in a slip, you might run accessories that can quickly drain a battery, which is no good if the battery you use is the same one that also starts your boat. That’s why many boats need two separate marine batteries to operate at full capacity.

Starter and Deep Cycle Marine Batteries

The first is the starter battery, which works just like a car battery. It packs a lot of punch to turn the motor over and start it, and then the alternator takes over the boat’s electrical demands and recharges the starter motor.

The second is a deep-cycle marine battery that powers accessories (lights, audio equipment, etc.) while the motor is turned off. This battery is built differently from the starter battery, and gives you the power to use all of the features of your boat out on the water without jeopardizing the ability to start it back up.

What’s the Difference Between the Batteries on a Boat?

From the outside, the starter and deep-cycle batteries look almost identical, except that the deep-cycle might be slightly larger. However, you’ll find big differences inside that dictate how these batteries dole out power.

The starter battery has thin plates surrounded by an electrolyte, which allows the battery to produce more amps over a short period of time. On the deep-cycle battery, the plates are much thicker, which allows for a steady flow of amps over a longer period of time. This allows you to turn the boat off, drop anchor and run electronics such as audio equipment for a while out on the water. 

Are they interchangeable? Neither is suited to do the other’s job. The starter battery will drain in a hurry running accessories, and the deep-cycle battery probably doesn’t have the punch to turn the motor over and get it started.

How to Pick a Deep-Cycle Battery

First, you’ll need to choose the physical size of the battery, which is notated by “groups” like 34M, for example. Next, you need to settle on the amps. 

For example, a 100-amp battery can offer 10 amps of power for 10 hours, or give you the whole 100 amps for one hour. But if you only want to draw on half of those 100 amps, from a practical standpoint, a 100-amp battery gives you about 50 amps to work with.

Types of Marine Batteries

Lead-acid battery: Has a fluid electrolyte that needs to be topped off once in a while to keep the battery functioning properly. 

AGM battery: Has the electrolyte in fiberglass mats that sit between the plates in the battery. Doesn’t need to be topped off and won’t leak if it gets cracked. Can be mounted on its side if needed.

Gel battery: Has an electrolyte that’s like a jelly. Will leak if the case breaks.

Both the gel and AGM batteries cost a bit more than the traditional lead-acid battery, but they take a charge a little faster.