Having Two Boat Batteries: Is It Worth The Cost?

If you're going boating, you'll need to make sure you have enough battery power to get back to shore. Unlike cars, most boats have multiple batteries on board. The number of batteries you'll need depends on the size of your boat, how long you'll be out, and whether you have twin engines or a trolling motor. The minimum for many boats is two, even for small boats.

Most boats require two different kinds of power for proper operation: starting load and house load or running load. The starting load comes from the starting battery, which kickstarts the engines. The house load runs via a secondary battery and keeps the boat's electronic components running.

Having a second battery ensures you always have the power needed to get back to shore. It also extends how much time you can spend on the water.

Advantages of dual batteries

Having two batteries on a boat is highly recommended, and in some cases, it is even necessary. Here are some advantages of dual batteries:

Redundancy and Failure Protection

Dual batteries offer redundancy, which means that if one battery fails or drains, the other can be used as a backup to start the engine. This provides peace of mind and ensures that you won't be left stranded on the water.

Extended Time on the Water

Having two batteries extends how much time you can spend on the water. One battery can be dedicated to the starting load, which is used to kickstart the engines. The other battery, known as the house load or running load, can power the boat's electronic components, such as GPS, lighting, and other accessories. This setup ensures that you have enough power to get back to shore while still enjoying your time on the water.

Reserve Power for Accessories

If you plan to add accessories such as a powerful stereo system, cabin lighting, electric windlass, or other electronic gadgets, a second battery is a good idea. A single battery may not be able to handle the additional power requirements, especially if you plan to use these accessories for extended periods.

Efficient Charging

With two batteries, you can efficiently manage your power supply. While one battery is in use, the other can be charging, ensuring that you always have a backup power source ready when needed. This is especially useful if you plan to be out on the water for long periods or have twin engines, which typically require more batteries.

Space and Weight Savings

In some cases, having two smaller batteries can save space and weight compared to a single large battery. This can be advantageous, especially for smaller boats where space is limited.

Types of marine batteries

Marine batteries can be broadly categorized into three types: starting, deep cycle, and dual-purpose. Starting batteries, also known as cranking or boat cranking batteries, deliver a high-amperage current burst to fire up the engine. They are not suitable for running electronics, lights, or other accessories. Deep cycle batteries, on the other hand, provide steady power to the boat's electronics, lights, GPS, fish finders, and accessories throughout the trip. They are designed to handle longer discharges without compromising their lifespan. The third type, dual-purpose batteries, combine the functions of both starting and deep cycle batteries, saving space, weight, and cost. However, they drain quicker than deep cycle batteries and may not have sufficient power to crank larger engines.

Each of these battery types can be further classified based on their internal chemistry, including Flooded Lead Acid (FLA), Gel, Absorbed Glass Mat (AGM), and Lithium Iron Phosphate (LiFePO4). FLA batteries are the least expensive option, but they require regular maintenance, have a shorter lifespan, and are heavier and bulkier than other types. Gel batteries and AGMs are sealed and maintenance-free, addressing some of the drawbacks of FLA batteries. Lithium-ion batteries are newer to the market and offer significant improvements in terms of durability, weight, and lifespan, but they come with a higher price tag.

Battery capacity

The number of batteries needed depends on the size and type of your boat, as well as its electrical load. Smaller boats with minimal electrical demands may only require a single battery. However, most boats require at least two different types of batteries: a starting battery and a house or deep-cycle battery. The starting battery provides the high amperage needed to crank the engine, while the house battery powers the boat's electronic components, such as GPS, lighting, and trim tabs.

When selecting battery capacity, it is crucial to consider the starting load and the running load. The starting load refers to the power required to start the engine, while the running load is the electrical power needed to operate the boat's electronics and accessories. To determine the appropriate starting amperage, refer to your engine's owner's manual. For deep-cycle batteries, you need to calculate the anticipated amperage draw and estimate the period for which you will need that amperage.

Additionally, it is important to match battery chemistries between your banks. If you have a lead-acid starting battery, choose a deep-cycle battery of the same chemistry rather than opting for a gel or AGM battery with different charging requirements.

By considering the specific demands of your boat and selecting batteries with sufficient capacity, you can ensure reliable power for both starting and running your vessel.

Manual vs automatic switching

Manual vs. Automatic Switching for Boats with Multiple Batteries

Most boats require two types of power for proper operation: the starting load, which is the amount of power required to start the boat, and the house load, which is the amount of electrical power required to run the boat's electronic components. For this reason, boats typically have two types of batteries: the starting battery and the house battery.

A marine battery switch is a device that allows you to control the flow of electrical power between multiple batteries on a boat. It connects or disconnects the batteries in different configurations, such as parallel or series, to provide power to different systems or charge the batteries.

Manual Switching

Manual switches are reliable but require the user to remember to manually switch between batteries to keep all batteries charged. The most basic battery switch is a simple "On/Off" device designed to prevent a light or radio inadvertently left on when you leave the boat at the dock overnight from running down your batteries. It breaks the power flow to everything aboard and protects your batteries so that you can start up when you return.

Automatic Switching

Automatic switching uses devices such as voltage-sensitive relays (VSRs) or other devices that sense when a battery needs a charge and direct charging current from the alternator automatically. These can be built into the engine or may be a separate component. Such systems may simply have “on/off” switches rather than a four-position manual switch.

Automatic battery management solutions, such as the Blue Sea Systems' "Automatic Charging Relay" (ACR) or their "Add-A-Battery" system, eliminate the hassle and worry associated with manual battery management. They ensure your batteries are charged and ready without your intervention.

Battery installation and maintenance

The number of batteries you will need for your boat depends on the size of your boat, the power requirements, and the type of engine. Boats under 14 feet will only require one battery. Boats 15-23 feet, with a single-engine, will only require two batteries unless they have a trolling motor, which can require up to three more batteries. Boats 24 feet and above, with more than one engine, will require three or more batteries.

There are two basic types of 12-volt batteries: cranking or starting batteries, and deep-cycle batteries. Cranking batteries are designed to start your main engine, while deep-cycle batteries are used to power electrical accessories such as trolling motors, fish finders, and radios. Dual-purpose batteries, which combine the functions of both, are also available.

When installing batteries, ensure they are correctly mounted and secured in an acid-proof box or tray to contain spills and isolate them from physical harm. Positive terminals should be covered, and batteries should be robustly secured, with ventilation, and not installed underneath a battery charger or fuel-system fixture.

To maintain your marine battery, keep it properly charged. All batteries will self-discharge if left dormant, so it's important to periodically charge batteries that are not in use. Overcharging and undercharging will reduce the battery's lifespan and degrade its performance, so it's best to use a smart marine battery charger that will reduce the charge automatically when the battery is fully charged.

To avoid corrosion, frequently check the battery terminal connections to ensure they are snug and corrosion-free. Clean the terminals regularly using a mixture of baking soda and water. Keep the battery cool and out of direct sunlight, and secure it with a good battery box or tray to prevent it from banging around in rough water.

Frequently asked questions