Boat Battery Basics - Construction, Applications, Wiring Setups, & Maintenance

Andrew McDonald
6 hours ago
4 min read

It can be argued that the single most important piece of equipment on board a modern boat is the vessel’s battery. Without batteries, the engine won’t start, the lights won’t shine, the GPS won’t power up and the bilge pump won’t keep you safe.

Because of their importance, marine batteries come in many shapes, styles and set-ups and can be broken down in a few ways:

1) Battery Construction

Lithium (Li), Lithium Iron Phosphate (LiFePO4), and AGM marine batteries

a) Lead Acid - traditional marine and auto applications - lead plates are immersed in a solution. When the battery is charged, the solution and lead plates undergo a chemical reaction to allow the battery to work. These batteries are vented to allow gases (a result of the chemical reaction) to escape.

b) AGM - Newer technology, where the lead plates and solution are replaced by absorbed glass mat. These batteries are heavier than lead-acid, but have no vents - meaning that they are viewed as ‘safer' in marine applications.

c) Lithium, in turn divided into two types, Lithium Ion (Li) and Lithium Iron Phosphate (LiFePO4) -- For marine applications, only Lithium Ion should be considered. These batteries have many advantages with respect to lifespan, output and charging speed when compared to Lead-acid and AGM. They require little maintenance and are quite light-weight. They are often quite expensive in comparison.

2) Battery Applications

Starting or Cranking Batteries: These are meant to push out high amperage in a short period of time in order to start an engine
House or Deep Cycle Batteries: These are meant to push out low amperage over a long period of time, and be charged/discharged many times over and over again. Ideal to run electronics onboard: lights, GPS, stereo, VHF, etc.

3) Battery Set-up

Series vs. Parallel / Photo - Delong Energy

Series - multiple batteries are connected to increasing the amperage, but not the voltage. Think in terms of: 1 battery lets my stereo play for 5 hours. 2 batteries lets my stereo play for 10 hours.
Parallel - multiple batteries are connected to increase voltage, but not the amperage. Think in terms of running something bigger: Running a blow dryer vs a single light bulb. Or a fridge rather than a radio. Larger appliances can require higher voltage to run: Windlasses, bow thrusters, trolling motors, gyro stabilizers and HVAC require more batteries in parallel to work
Series-Parallel - When batteries are connected to increase both voltage and amperage. Think in terms of running a trolling motor for longer

In each of these scenarios, batteries should be disconnected, fully charged and stored in a cool/dry place. Terminals should be kept clean -- sandpaper can be used to clean dirt and corrosion. Likewise, terminal connections should be kept clean and tight. Any broken terminal connections should be replaced with properly sized ring terminals.

AGM and Lithium batteries are often sold as ‘maintenance free.' What this really means is that there is no venting or internal solution to maintain, as in lead-acid batteries. Maintenance for lead-acid batteries is fairly straightforward. The cap is removed (using appropriate eye and hand protection) and a flashlight used to peer inside to check fluid levels. If the fluid is below the level of the lead plates, it should be topped-up with distilled water. Once topped up, the battery should be charged.

In my experience, the greatest volume of issues surrounding batteries and on-board power management is the connection point between the battery and the wiring. Any corrosion build up or loose connection will prevent power from the battery getting to the appliance. Think in terms of a garden hose: if there is a kink in the hose, the correct water pressure won’t reach the end. Remove all the electrical ‘kinks’ to get the best result -- correctly size ring terminals, use nuts on battery posts and torque them to spec, use the correct gauge wire for the application, and stack the ring terminals in such a way as to get the most surface area contact between the battery and the ring.

4) Battery Chargers

As batteries increase in sophistication, so do battery chargers. When selecting a charger, ensure it is a marine type: designed for humid environments with changing temperatures, and that it is ignition-protected (suitable for placement in an engine room). Make sure that it matches the battery construction and application. A charger for a lead-acid battery will be a danger to use with a lithium-ion battery, and vice-versa. Similarly, the battery settings should be adjusted for use on a deep cycle vs starting battery, and batteries connected in series vs parallel.

With spring coming soon, now is a great time to check on your batteries and to get prepped for launch. Here’s a short list of tips:

Check the physical appearance of the batteries: any battery with casing damage or bulging should be replaced
Test your batteries before connecting them using a multimeter or load tester. They should be a minimum of 12.6Volts, and should test well under load. In the case of lithium-ion batteries, check the internal fluid levels
Check all your connection points: clean terminals and rings, replace any damaged wiring or terminals. Check the integrity of the rubber protection boots and battery tray or box
Make sure that the battery is clearly labelled with positive and negative terminals, and plan out which connection is meant to go where - keeping in mind series vs parallel connections and start vs house applications
Check to ensure that every wire connection has overcurrent protection (fuses or breakers
Once connections are made, torque the battery terminal nuts to spec
Ensure that the battery is secure in its tray or box, that the lids are secure, that there is adequate ventilation, and any rubber terminal boots are in place.
Check the settings on your charger and ensure that they are optimized for your set-up