Wiring A Boat Battery Charger: A Step-By-Step Guide

Installing a battery charger on your boat is a great way to keep your boat's battery in tip-top shape. It's important to select a quality marine-grade charger that meets industry standards and is suitable for your boat's battery type. When installing the charger, you'll need to consider its location, ensuring it's well-ventilated and not too close to the batteries, and follow the manufacturer's instructions for wiring the AC and DC connections safely and securely.

Choosing the right battery charger

• Battery Type: Determine whether you have an AGM, flooded, or gel battery. This information will help you choose a charger that is compatible with your battery type.
• Battery Capacity: Consider the capacity of your battery, usually measured in ampere-hours (Ah). The charger's amperage output should typically be around 10-20% of the battery's capacity.
• Input Voltage: Marine battery input voltage is typically either 120 or 230 VAC. Match the charger's voltage to the output voltage of your battery. If you have multiple batteries in a series setup, the voltage is added together, and you'll need a charger with the corresponding voltage.
• Charging Speed: The amp rating of the charger determines how quickly your batteries will recharge. A higher amp rating means faster recharging. Consider your usage patterns and how quickly you need to recharge your batteries.
• Outdoor Temperatures: The ambient temperature of your operating environment can impact the type of charger you need. If you plan to use your boat in extremely hot or cold temperatures, adjust your charging voltage accordingly.
• Number of Batteries: If you have multiple batteries, look for a charger that can handle the number of batteries you need to recharge. The term "bank" refers to a single connection from the battery to the charger. So, a six-bank charger can simultaneously charge six batteries.
• Safety and Protection: Look for chargers with safety features such as overcurrent protection and ignition protection. This is especially important in the marine environment to prevent electrical issues and ensure your safety.
• Smart Charging: Smart chargers use multistage charging (bulk, absorption, and float stages) to optimise the charging process and protect the longevity of your battery. They are a worthwhile investment to maintain battery health and enhance safety.
• Temperature Compensation: Consider a charger with on-battery temperature sensing, as battery charging voltage should be adjusted based on temperature. This is especially important in warmer climates to prevent overheating and optimise charging.
• Ease of Use: Look for chargers with user-friendly features such as remote displays, custom charging profiles, and easy installation. This will make it more convenient to monitor and use your charger.
• Warranty and Customer Support: Choose a charger with a good warranty and a manufacturer known for excellent customer service and support. This will give you peace of mind and help you resolve any issues that may arise.

Selecting the correct location

• Ventilation and Moisture Protection: Choose a cool, dry area with good ventilation to prevent overheating and moisture-related issues. Avoid mounting the charger in the engine space, as it tends to stay warm even after the engine is turned off. If possible, select a location higher in the boat, as it usually provides better ventilation and protection from the corrosive humidity of the bilge.
• Proximity to Batteries: The charger should be installed as close to the batteries as practical. Shorter leads result in reduced installation costs, decreased voltage drop, and improved charger performance over time. However, avoid mounting the charger directly above the batteries. Batteries, especially liquid electrolyte types, release corrosive gases during charging, which can damage the charger if placed too close.
• Safety and Ignition Protection: Battery chargers installed on gasoline-powered vessels must be ignition-protected and comply with relevant UL specifications. Additionally, ensure that the charger is not mounted in the battery compartment or engine compartment. These areas can expose the charger to corrosive battery gases or high temperatures, respectively.
• Accessibility and Convenience: Opt for a location that allows easy access to the charger for monitoring and maintenance. If installing in a deck or cockpit locker, place it as high as possible and away from the hatch to minimise exposure to water. If the storage area has poor ventilation, improve airflow by avoiding clutter and consider adding additional locker vents.
• Weight and Mounting: Consider the weight of the charger when selecting a mounting method. For heavier chargers, through-bolting is recommended, while screws can be used for lighter chargers if mounted on a thick bulkhead or structure. Always use marine-grade stainless steel for mounting hardware.

Mounting the charger

When mounting the charger, it is important to choose a location that is cool, dry, and well-ventilated, with adequate breathing room to prevent overheating. Higher locations are preferable to protect the charger from the corrosive humidity of the bilge. Avoid mounting the charger in the engine compartment or directly above the batteries, as the gases emitted by the batteries during charging can damage the charger. Instead, mount the charger as close to the batteries as possible without placing it directly above them.

When selecting a mounting method, consider the weight of the charger. For smaller chargers, screws are usually sufficient, while heavier chargers should be through-bolted for added security. Use marine-grade stainless steel for all mounting hardware to prevent corrosion. Ensure that the charger is securely fastened and will not move around during the boat's operation. Follow the manufacturer's instructions for specific mounting guidelines.

Additionally, pay attention to the wiring connections. The DC wiring should be sized according to the manufacturer's recommendations, taking into account the distance between the charger and the battery. Use marine-grade butt splices with heat-shrink tubing for wire-to-wire connections. Avoid using wire nuts or electrical tape, as they can come loose over time. If the charger's DC wire terminates in spring clamps or alligator clips, replace them with proper marine-grade ring terminals.

Installing AC wiring

When installing a battery charger, the AC wiring can be as simple as plugging the charger into an extension cord and running that to an outlet in your garage. However, if you want a permanent installation, there are two main options:

• If the charger is the only AC device on the boat, install a compact shore power inlet that accepts a standard straight blade extension cord. Cut off the charger's AC plug and connect it to the AC inlet.
• If the boat has an AC distribution panel, supply the charger from one of the circuit breakers in the panel. Before working in the distribution panel, ensure all AC sources are off, including inverters and generators. There may be a circuit breaker designated for the battery charger; if not, pick an unused breaker of 15-20 amps for the new charger circuit. Run an appropriate marine-grade, three-conductor AC cable between the panel and the charger.

When installing AC wiring, it is important to use marine-grade, multi-stranded, three-conductor AC wiring. Do not use residential-type solid-strand wire.

For the connection to the AC distribution panel, the black wire will connect to the circuit breaker, and the white and green wires will connect to the AC neutral and ground, respectively.

To ensure safety, always unplug the shore power cord and turn off the main breaker inside the boat before working on the AC system.

When making AC connections, use crimp-type connections rather than wire nuts. For extra protection, apply tape on top of the connections or use butt splices with adhesive-lined heat shrink tubing.

Finally, ensure that all AC wiring is securely fastened along its length and does not bind the panel when opened.

Installing DC wiring

The DC wiring is a very critical part of a charger's performance. Most manufacturers want to see a maximum voltage drop of between 1% and 3%. Voltage drop is determined by the amperage flowing through the cable over the round-trip length of the circuit. This means you add the full length of the negative and positive wires, plus the max amperage that will flow, to determine your voltage drop.

The DC cables should be connected to the charger as directed by the manufacturer. Wire nuts, common in household applications, must not be used. If a wire-to-wire connection must be made, it should be done using a butt splice, preferably one with adhesive heat-shrink tubing and secured inside the wiring compartment on the charger.

The wires are terminated with crimp-on ring terminals. Before crimping, slip a length of heat-shrink tubing down to cover the joint, then warm it using a hot-air gun or hair dryer to make the connection waterproof.

Use a hot-air gun or hair dryer if possible, to warm heat-shrink tubing. A small portable torch or even a match works, but tends to leave soot marks, which look unsightly.

The heat-shrink tubing also supplies some measure of strain relief to the wire. Either use one of the less expensive all-in-one crimp-on tools or invest in the ratchet-type double crimpers, which do a far better job.

Cables must be adequately supported, at least every 18 inches. Plastic clips and cable ties can be used, but other types are available.

It is critical to use the manufacturer's suggested wire size between the battery and the charger. This will have a disproportionately large impact on how well your battery charger works. The wire size required will be based on the distance from the charger to the batteries; longer runs require proportionally larger wire. If the wire is too small, the resulting voltage drop can cause the battery to be chronically undercharged and increase charging time.

It is also important that any wires are properly supported using saddle clips and wire ties. Finally, double-check all connections before plugging in the AC power.

Frequently asked questions