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Why Do Boats Have 2 Batteries?

Batteries Plus Blog - Power - by Bryan Veldboom - updated on 5/19/2022

two people riding in a blue boat on the water

Boats rely on different types of batteries for different applications. That's why they commonly have more than one battery onboard. In this article, we'll be taking a close look at your boat's battery requirements and what type of battery is best suited to each need.

What Type of Battery Does a Boat Use?

A boat requires batteries for two separate types of power needs.

Starting Load - Your boat requires a large, short-term burst of electricity in order to turn over the engine.

Running Load - Accessories like lights, trolling motors, depth finders and other onboard tech require their own source of power, which is often referred to as a "house battery." While the starting load only requires enough power to get the engine started, your boat's running load requires a battery capable of delivering smaller amounts of power for a much longer period of time.

What are the Different Marine Battery Types?

Now that you understand what boat batteries are used for, let's take a look at the different types of batteries you can use to fill those needs.

Starting Batteries - Starting batteries work by providing a large burst of power for a short period of time. This makes them ideal for starting your boat's engine, but makes them a poor choice for things like trolling motors and fish finders that require long-term power. A 12 volt battery should be enough for most boat engines, however, if you have a boat that is 15 feet or larger, you may require more than one starting battery.

Deep Cycle Batteries - These batteries feature a heavy charge and discharge cycle, which allows them to provide long-lasting power. Deep cycle batteries are typically used to power things like trolling motors, depth finders, GPS trackers, fish finders and other onboard tech. The number of house batteries you use will depend on how many additional applications you have on your boat and how much power they require.

Dual Purpose Batteries - Dual Purpose batteries are designed to work as both a starting and deep cycle battery in one. They provide the strong starting power needed to start your engine, with enough cycling capacity to power additional accessories.

How Do You Use a Dual Purpose Battery with Deep Cycle Batteries?

If you have a lot of applications on your boat, it's a good idea to pair a dual purpose battery together with several deep cycle batteries. Use the dual purpose battery to start your engine and run some of your electronics (graphs, depth finders). Then, use the deep cycle batteries to power your trolling motor. Since trolling motors typically have high voltage requirements, you'll most likely have to connect several deep cycle batteries in a series in order to provide the correct voltage.

How Do You Connect Two Marine Batteries in a Series?

So what happens if you need more starting or deep cycle power than a single battery can provide? In that case, you'll want to connect two or more batteries together in a series in order to increase their voltage. For instance, many trolling motors require 24 or 36 volts of power. To run one of these, you need to connect two or three 12 volt deep cycle batteries in a series to provide the proper voltage. In order to do this, connect the positive terminal of one battery to the negative terminal of another battery until the desired voltage is achieved.

When connecting batteries, it's crucial that all of the batteries share the same capacity, voltage, resistance, state of charge and chemistry. Don't mix and match different types of batteries, as this can damage them. Once the batteries are connected in this way, you will also need to use a charger that matches the total voltage of the batteries used in the series. For instance, if you have two 12V batteries linked together, you'll need a 24 volt charger. Three 12V batteries connected in a series will require a 36 volt charger.

Find Your Boating Essentials at Batteries Plus

Batteries Plus has a large selection of starting, deep cycle and dual purpose boat batteries available both online and in store. You'll also find boat battery chargers to help keep your battery maintained and battery boxes to keep your battery protected. Do you have a marine battery that isn't performing like it used to? Bring it in and have it tested for free.

If you have additional questions, you'll find additional answers in our Marine & Boat Battery Buying Guide or online blog. Some of our most popular boating topics include "Which Brand of Battery is Best for Your Boat?" and "How to Find the Right Battery Charger for Your Boat."

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