Battery basics

Properly maintaining your batteries helps ensure your engine starts up easily and your other electronic accessories are properly powered when you are ready to head out for fun and adventure on the water.

What Is a marine battery?

Marine batteries are designed specifically for use on a boat, with heavier plates and a robust construction designed to withstand the vibration and pounding that can occur on board a powerboat. For this reason, marine batteries are usually more expensive than automobile batteries, which can tempt some boat owners to purchase an auto battery instead of one designed for marine use. Do not make that poor decision. A marine battery will last longer and will be more reliable than an auto battery in a boat.

Types of marine batteries

Marine starting battery

A marine starting battery provides a quick and powerful burst of energy over a short period of time to start the engine and is designed to be rapidly recharged by the engine’s alternator. A starting battery should not be used to power a trolling motor or other onboard accessories. Consult your Mercury Marine® operation and maintenance manual before replacing the marine starting battery in your boat. The new battery should meet the specification for battery type, marine cranking amps (MCA) and minimum reserve capacity. Your Mercury® authorised dealer can help you select the correct starting battery for your engine.

Marine deep-cycle batteries

Marine deep-cycle batteries are designed to discharge slowly over a longer period of time and to withstand several hundred charging and discharging cycles. If your boat is rigged with more than one battery, deep-cycle batteries are the right choice for powering an electric trolling motor and other battery-powered accessories such as audio systems, a windlass, depth finders and other appliances.

Marine dual purpose batteries

Marine dual purpose batteries combine the performance of a starting and a deep-cycle battery. While it is not as efficient as separate batteries, a dual-purpose battery is a good choice for a smaller boat that may not have space for two batteries.

Avoid battery trouble

Follow these basic tips to avoid the most common battery issues:

Battery tray

Each battery should be secured within a battery tray. The battery tray base should be screwed or bolted to the boat and have either a rigid bracket or a locking strap to secure the battery to the base. A loose battery can cause a short if it is banging around in rough water. A securely mounted battery will also last longer.

Non-conductive cover

Install a non-conductive cover over the positive battery terminal if one was not installed by the boat builder, even if the battery is in a covered box. The cover prevents sparks, arcing and possible explosion if a metal tool or other object contacts the terminal during maintenance.

Battery terminal

Frequently check the battery terminal connections to make sure they are tight fitting and free of corrosion. Marine batteries have both automotive-type lugs and threaded terminals. Always use the threaded terminals and replace the wing nuts often found on those terminals with nylon locking nuts, which are much less likely to vibrate loose.

High-efficiency prop

If you use the boat infrequently, use a maintenance-type battery charger to keep the battery fully charged between outings.

Offseason storage

Battery maintenance during the offseason is critical for a long battery life. Allowing a battery to remain in a fully discharged state for long periods of time causes it to become sulphated, in which case it will need to be replaced. Batteries naturally lose some charge when stored, so even if the battery is fully charged when you put the boat away in the autumn, by spring it could have become significantly discharged.

Follow these basic tips when preparing to store your boat:

Before offseason storage, completely charge all batteries and disconnect the terminals so nothing can drain them.
If there is power available at your storage site, connect the batteries to a battery maintainer/charger during the offseason to keep your batteries fully charged.
If your boat has an onboard charger, it will maintain the batteries. Leave the charger connected to the batteries.
If there is no power at your storage site, remove the batteries from the boat and store them where they can be connected to a battery maintainer/charger. Store batteries off the floor to help keep them at a stable temperature.

When should a boat battery be replaced?

Expect to get four to five seasons of life from a conventional lead-acid marine starting battery that has been well-maintained and not subjected to excessive vibration or impact in rough water by being properly secured in the boat.

Load testing

A load test is the best way to gauge the health of a battery that has seen a few seasons of use. Battery load testing entails measuring the amperage produced by a fully charged battery. You can have a battery load tested at a Mercury authorised dealer, an auto repair shop and many auto parts retailers.

Do not wait to discover your boat battery has reached the end of its life at the launch ramp, or even worse, at the far end of the lake or 20 miles offshore. It is better to replace an ageing battery than to lose a day on the water.

Fuses and circuit breakers

The electrical circuits powering your engine and the rest of your boat are protected by fuses and circuit breakers. A fuse is designed to fail in the event of a power surge, a short-circuit to earth or any other electrical mishap. Fuses are amp-rated so that the fuse will fail before the wires and equipment it is protecting are damaged. Fuses also help prevent electrical fires. A circuit breaker accomplishes the same protection by simply interrupting the current flow.

Replacement

In the event a fuse triggering on your engine or boat, you will want to replace it with spares you carry on board. The best way to determine which fuses your boat requires is to consult the owner’s manuals for your engine and your boat. Make an inventory list of all the fuses your boat requires, from the engine to the various aftermarket components on board, such as the main fuse from the battery, trolling motor, chart plotter, fish finder and galley appliances. Note both the amperage rating and the type of fuse. If you are not sure which fuses your boat requires, the pros at your local Mercury authorised dealer can help you figure out exactly what you need.

Circuit breakers are usually located at the helm of smaller boats and on the master electrical panel of larger boats. Circuit breakers can be reset so no spares are required.

Fuse safety tips

Never bypass a blown fuse with wire, foil or anything else, and only replace with a fuse of the same type and amp rating. Using the wrong type of fuse can cause serious damage to your electrical system and components.

When replacing or inspecting a fuse, take care that you do not short out the circuit by using a metal tool such as a screwdriver. Instead, use a plastic fuse tool to remove the fuse.

If you keep blowing fuses in the same circuit, take your boat to your dealer for troubleshooting. This can be a sign of a chafed wire or another unsafe issue that should be handled by a trained technician.

Next topics

Maintenance: What's in it for you?

Boating is an activity that fuels pride of ownership, and for many owners a strong do-it-yourself attitude. Knowing you have covered the basics of maintenance gives you confidence that your boat is in tip-top shape every time you leave the dock.

Fuel basics

Using the correct fuel in your Mercury Marine® engine and properly maintaining your fuel system can help ensure many seasons of reliable service. For more guidance on fuel requirements for your engine, see the fuel section of your Mercury® operation and maintenance manual.

Preventing corrosion

Your engine lives and works in the water, where it is constantly exposed to elements that can cause corrosion.