Installing A Marine Battery Charger

By Mark Corke

A proper charger is the answer for keeping your boat battery topped up. Here's how to install one.

Illustration of the installation of a marine battery charger with parts and connections labeledIllustration: ©2015 Mirto Art Studios

Arriving at the ramp to find that the boat batteries are dead ruins your day before it's begun. Adding an onboard battery charger is a fairly simple project, and it ensures that your batteries remain in tip-top shape between trips or while the boat is stored for long periods, such as over the winter.

All battery chargers aren't created equal, and although it might be tempting to use one of the cheap automotive brands, which can often be identified by alligator-type battery clamps and a low price, don't do it. Although they offer a swift boost to the battery, they work by pushing out a large charging current, which diminishes over time as the batteries accept their charge. One of the reasons these battery chargers are so cheap is that they have unsophisticated internal circuitry. With such chargers, it's possible to overcharge batteries. The consequence is called "boiling dry": the high current makes the electrolyte bubble to such a degree that excessive hydrogen gas is produced, which reduces the electrolyte to unacceptably low levels and ruins the batteries. A marine battery charger, with more sophisticated internal circuitry than the cheaper automotive types, is designed to be connected permanently to the battery bank; even if it's switched on for several weeks, it won't harm the batteries, dropping the charge if needed to avoid slow cooking of the battery. Many marine battery chargers offer specific charge modes for the different battery types (lead-acid, gel, and AGM). Better chargers often offer temperature sensors, too, because a hot battery is less able to accept a charge than a cooler one.

Installing a charger isn't too complicated, and a competent DIYer with an average toolbox should be able to complete the job easily in a weekend. The instructions here are for a small center-console that isn't equipped with shore power. If you have a larger vessel or you already have shore power, the installation will be similar, but you can connect the charger directly to a spare 115-volt breaker on the AC panel.

  • Photo of marking the spot with pencil for 20-amp socket
    Photo of drilling the hole for the 20-amp socket
  • Photo of inserting socket into drilled hole
    Photo of crimping wires
  • Photo of connecting wires to socket
    Photo of plugging in an extension cord

1–6. Some chargers come with an AC lead already connected to the charger. While it's possible simply to tuck this out of the way and plug it into an extension cord when you need to use the charger, it's not the best way. For a neat and tidy job, add a 20-amp socket to the outside of the console as shown; now a regular extension cord can be connected to keep the batteries charged.

Start by deciding where you want the battery charger installed. Ideally, the closer to the batteries, the better, but it's a bad idea to install the charger directly above the batteries, where escaping gas can compromise it. Wire runs should be kept as short as possible and use the correct gauge, to avoid voltage drop. The instructions that come with the charger should give some guidance if you're unsure.

It's essential to have some form of circuit protection between the charger and the battery. Most chargers have internal overcurrent protection for the DC side built into the charger itself, but this leaves the cabling between the charger and battery unprotected. A short circuit in the wires between the charger and battery could be catastrophic, so a fuse should be installed in the positive lead as close as possible to the battery. The American Boat & Yacht Council suggests that this be no farther than seven inches from the battery's positive terminal. This isn't always possible in practice, but the fuse should be as close to the battery as you can reasonably mount it.

  • Photo of mounting the battery charger
    Photo of a 25-amp terminal mounted fuse

1. Mount the charger using self-tapping screws or, better yet, stainless-steel nuts and bolts in a dry, well-ventilated location. We mounted our charger under the center-console, keeping it away from the compass because mounting it closer than a couple of feet is likely to cause some compass deviation.

2. The charger I installed has a maximum output of 20 amps, so the fuse that I chose was the next size up, a 25-amp terminal-mounted fuse.

Photo of heat-shrink tubing

3. 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. 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.

  • Photo of supporting cables with plastic clips
    Photo of final battery charging setup

4. All cables must be adequately supported, at least every 18 inches. I used plastic clips and cable ties but other types are available.

5. And you're done! You're charging set up should look all nice and neat like the one above when finished.

With the DC connections made, turn your attentions to the AC side. The charger that I was using didn't have a lead connected, so I had to add one. Use the wire size recommended in the installation instructions that come with the charger, not just any old wire that you happen to have lying around. It's 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. 

— Published: Fall 2015


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