Select & Install a Battery Charger

Battery performance and life will be improved by an onboard charger.

 Maintaining a boat’s batteries is crucial, both for performance and increased service life. Keeping them properly charged can be an issue, however, particularly for vessels that see limited or sporadic use. A great way to monitor and promote good battery health is to install a modern “smart” battery charger. But how does one select and install the right charger? Read on to find out.

Marine-grade batteries aren’t cheap and can easily be destroyed by improper charging, so the last place you want to skimp is during charger selection. Always go with a quality marinegrade unit, one built to ABYC (American Boat & Yacht Council) and UL standards. Look for smart chargers that provide numerous charging options and features, such as the ability to select between the different types of battery technology: wet cell, AGM (absorbed glass mat), gel, etc.

Unlike old resonance chargers from the past — the ones that hum like something out of a mad scientist’s laboratory — many modern chargers are “fully potted,” a term that means their electronics are completely encapsulated in resin, which makes the unit waterproof and significantly reduces the chance of corrosion.

Waterproof chargers can be used on any craft but are particularly useful for installations aboard boats that are often wet by nature, such as personal watercraft and small fishing boats.

As uncontrolled combustion is not our friend, battery chargers installed on gasoline-powered vessels must be labeled as being ignition protected and conform to appropriate UL specifications, which include testing in an explosive environment.

Regardless of which battery charger you choose, stay away from el cheapo automotive chargers found at the local Big Mart. They’re not designed for marine use and can cause a multitude of problems, from stray current corrosion to shock hazards.

As to size and output, always consult the manufacturer’s instructions when selecting a charger, but a general rule of thumb is to choose one with an output that’s at least 10 percent of your boat’s battery (or battery bank) capacity. For example, a 300Ah (amp hour) battery or bank needs a 30-amp charger. If you can’t find an exact match based on the 10 percent formula, a charger with a little more output is a better than one that’s too wimpy.

Figuring out where to mount a charger will be an exercise in compromise, so be sure to follow all manufacturer instructions when selecting a mounting location. The best option is a cool, dry area with adequate ventilation. A higher location is generally better than a lower one, since a higher spot typically provides better ventilation and protection from the corrosive humidity of the bilge. Avoid high-temperature locations such as the engine compartment, if possible, though doing so can be difficult on smaller boats with limited options.

If you are considering installing the charger in a deck or cockpit locker, locate it as high as possible in a dry corner of the locker away from the hatch, to reduce its exposure to water drips and spray.

Many of the storage areas where battery chargers are installed have marginal ventilation at best. If that’s the case with your installation, help the charger breathe easier by not cocooning it with life jackets, boat covers and other air-blocking items. Adding additional locker vents that will increase air flow may be a good option.

Chargers should be installed as close to the battery or bank as is practical. Shorter leads mean less installation cost, less voltage drop — discussed in a moment — and increased charger performance over the life of the unit.

That being said, the charger should not be mounted directly over the battery or bank. Batteries, particularly liquid electrolyte or wet-cell types, can produce corrosive gases such as sulfur dioxide while they charge. Such gases can quickly damage a charger if it is located directly above them. A charging lead acid battery produces hydrogen, which is not toxic, but at high concentrations it is highly explosive (think Hindenburg) — yet another reason the charger should be ignition protected and installed in a well-ventilated area.

As should be clear, the battery charger will likely wind up in an out-of-the-way location where it can’t easily be monitored. To address this out-of-sight, out-of-mind dilemma, many new chargers offer a remote display option, which is a great feature to look for during charger selection. Remote displays can provide a wealth of information; however, even the most basic units will provide all you really need: AC input voltage, DC output voltage, battery temperatures (if a sensor is installed), total charge time, current charge cycle info and more.

Installation of a remote display is pretty much a plug-and-play affair—typically four mounting screws for the display and a length of cable with phone jack–style connections at each end.

With a suitable spot chosen, it’s time to mount the charger. For smaller chargers being mounted to a thick bulkhead or structure, screws are an acceptable option. If the charger weighs more than a couple of pounds, it should be through-bolted with suitably sized bolts, nuts and washers. Regardless of the mounting option, always use marine-grade stainless steel for all mounting hardware.

Connecting the charger involves the installation of both AC (to power the charger) and DC wiring (between the charger and battery). DC wiring should be sized according to the manufacturer’s recommendations, which will be based on the distance between the charger and the battery. This measurement is round-trip length (i.e., the full length of the negative and positive wires added together).

The longer the wire run, the larger the wire diameter needed to offset loss due to voltage drop — a loss of power resulting from the use of wire that’s too small for the run. Using smaller diameter wire than called for can decrease charge voltage at the battery and slow the charge rate way before the battery is fully charged, increasing charge time or actually preventing the battery from becoming fully charged.


Wire runs should be routed as directly as possible and given support and chafe protection as needed. DC wiring connections should be made using marine-grade waterproof ring terminals or butt connections. These are the ones that have a short length of heat-shrink tubing to protect the termination once the connector is crimped and the heat shrink is activated.

If a wire-to-wire connection is required, always use a butt splice or connector. Never make connections using residential wire nuts or electrical tape, because both will eventually fall off due to vibration or age and leave exposed, energized wires.

When making the connection from a charge cable to a battery post, use closed barrel terminals where possible, sealing each with a small length of heat shrink to protect them against corrosion. Place the charge cable terminals on top of the primary battery cable terminals, rather than beneath. Doing so keeps the terminal where it can easily be monitored for corrosion and raises it slightly above the top of the battery, which should prevent spilled electrolyte from wicking up the wire and causing problems.

If the charger’s DC wire terminates in spring clamps or alligator clips (to make the battery connection), cut them off and replace them with proper marine-grade ring terminals. Finally, spray or coat all connections with a corrosion inhibitor, such as Boeshield T-9, for added protection.

On smaller craft without a permanently installed AC system, hooking up the AC side can be as simple as plugging the battery charger into a suitable extension cord. For a more permanent install where the charger is the only AC-powered item on board, a grounded AC power inlet that accepts a standard extension cord plug, such as the Marinco 15A Battery Charger Inlet, is an option.

If the boat already has an AC system installed, plugging the charger into a convenient outlet may be an option, if one is nearby. If not, you’ll want to power it from the main AC distribution panel using marine-grade, multistranded three-conductor AC wiring (no residential-type solidstrand wire, please). Pick an unused circuit breaker of the amperage called for by the charger manufacturer — typically 15 to 20 amps — and connect per the instructions.

Practice safe wiring by making sure the AC shore-power cord is unplugged at the dock, the main AC circuit breaker is off and that all AC power sources, including inverters and generators, are off and disconnected before starting any work. To wire safely, start from the AC side of things and work toward the battery, making the connections at the battery the final step. This is a safety strategy that minimizes exposure to energized circuits — DC power from the battery, for example — assuming, of course, power to the AC system is turned off as discussed previously.

When all the connections have been made, and with the AC power off, make the final charger connections to the battery and verify that the installation is complete. Review the entire installation. Check that all the connections are tight, all wiring is properly supported (with appropriate chafe protection) and that everything is installed per the charger manufacturer’s instructions.

But wait! Don’t turn anything on just yet. First, check that the battery charger is set for the boat’s battery type. Next, measure the voltage of your battery with a multimeter prior to the charge cycle, which provides a good starting reference point and will be handy later if battery issues develop.

Now’s the time to plug in the shore-power cord, energize the main breaker and trip the appropriate branch circuit breaker to power up the battery charger. Once the battery charger is powered up, check the battery voltage level. It should be greater than the reference voltage from earlier; in fact, it should begin to rise as soon as you power up the charger.

Don’t simply power up the charger, call it good because nothing caught fire and then head home for your favorite Barcalounger and a cold beverage. Once the battery charger is powered up and running, stay on board to observe at least the first few hours of operation. You should ideally hang around long enough to verify that when the batteries are fully charged, the charger transitions from the initial charging mode to a maintenance level float charge.

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