Install a Battery Charger

Author: Frank Lanier
Do you lust after the convenience that a permanently installed battery charger offers, but you lack the intestinal fortitude to tackle the job yourself? Follow these tips and you’ll soon be tripping the light fantastic toward battery charging nirvana!BW-01-2015-DIY-01

Selecting a Charger
Marine-grade batteries aren’t cheap and can easily be destroyed by improper charging, meaning the last place you want to skimp is on a charger. Choose a quality marine-grade unit, ideally one built to ABYC (American Boat and Yacht) and UL standards. Look for smart chargers that provide numerous charging options and features, such as the ability to select between different types of battery technologies (wet cell, AGM, gel).

If a charger is going to be used on a gasoline-powered vessel, it should be labeled as being ignition protected. For PWCs and other open-type craft, you’ll want to choose a sealed, waterproof charger. Regardless of what you choose, stay away from the el cheapo automotive chargers down at the local XYZ-Mart. They’re not designed for marine use and can cause a multitude of problems, from stray current corrosion to shock hazards.

You’ll want to consult the manufacturer’s instructions when sizing your charger, but a general rule of thumb is to choose one with an output that’s at least 10 percent of your battery (or bank) capacity. For example, if you have a 300 Ah (amp hour) battery or bank, you want a 30-amp charger. If you can’t find an exact match based on that simple formula, a charger with a little more output is a better choice than one that’s too wimpy.

Choosing a Location
Figuring out where to mount your charger can be an exercise in compromise, so make sure you follow all manufacturer instructions when selecting a spot. The best option is a cool, dry area with adequate ventilation. Higher up is generally better due to better ventilation and protection from the corrosive humidity of the bilge. You’ll want to avoid high-temperature locations such as your engine compartment, if possible, although it can be difficult on smaller boats with limited choices.

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

Finally, chargers should ideally be as close to the battery or bank as practical — shorter leads mean less installation cost, less voltage drop (which we’ll discuss in a moment) and increased charger performance over the life of the unit.

One place they should not be mounted isdirectly over the batteries. Batteries (particularly wet-cell types) produce corrosive gases while charging, which can quickly damage a charger above them.

Mounting the Charger
Once you’ve picked a suitable spot, 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, however, it should be through-bolted. Regardless, make sure you use marine-grade stainless steel for all mounting hardware.

Getting Wired
Next, you need to connect the charger, which involves installation of both AC (to power the charger) and DC wiring (between the charger and the battery).

DC wiring should be sized according to the manufacturer’s recommendations, which is 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-diameter wire you’ll need 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 recommended can both decrease charge voltage seen at the battery and increase charging time.

Install a Battery Charger

Wires should be routed as directly as possible and provided with support and chafe protection as needed. Once the wire runs are in place, connect them to the battery charger as per the owner’s manual. Wire-to-wire connections should be made using marine-grade butt splices, preferably those with heat-shrink tubing. Never use wire nuts or electrical tape joints, both of which will eventually fall off, leaving exposed, energized connections. If the charger DC wire terminates in spring clamps or alligator clips (to make the battery connection), cut them off and replace with proper, marine-grade ring terminals.

For smaller boats or watercraft 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, another option is installation of a grounded AC power inlet (such as the Marinco 15A Battery Charger Inlet), which accepts a standard extension-cord plug.


If your boat already has an AC system installed, plugging the charger in a convenient outlet (if one’s nearby) may be an option. 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 solid-strand wire please). Pick an unused circuit breaker of the amperage called for by the charger manufacturer (typically 15 to 20 amps) and connect as per the instructions.

Practice safe wiring by making sure all AC power sources (including inverters and generators) are OFF and disconnected before starting any work! When all the connections have been made (and with the AC power off), make the final charger connections to the battery and verify the installation is complete. Once that’s done, all that’s left is energizing the AC circuit, powering up the charger and basking in the glow of success!

Get a Charge

When the realization hits that you need a battery charger, don’t be intimidated by the installation process. After you’ve chosen a location — one with ventilation and away from humidity — the installation should look neat and organized (like in the top photo below), with wire runs kept as short as possible and the proper gauge used. Avoid the engine space if possible, and try to use an existing AC outlet to power the charger.


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