Charging Boat Batteries With a Portable Generator

The best way to charge boat batteries with a portable generator is to plug your boat’s shore power cord into the generator’s 120V outlet and let your onboard battery charger do the work. Don’t use the generator’s built-in 12V outlet. It’s far too slow and isn’t designed to charge a battery bank properly.

We carried a Honda generator for years aboard Barefoot Gal, our catamaran in the Florida Keys and the Bahamas. Here’s what we learned about charging boat batteries with a generator: how we hooked it up, how fast it charged, and how to run it safely at anchor.

Why Run a Generator When You Have Solar?

We lived mostly on solar power. But in December and January, solar panels just can’t keep up: the days are short and the sun sits low in the sky. A string of cloudy days does the same thing any time of year. We’d run out of room to add more panels.

Running the diesel engine just to charge batteries isn’t a good answer either. Diesels are happiest working under a real load, not idling at anchor for an hour or two. A small, fuel-efficient portable generator filled the gap for us.

Solar, wind, and generators each have pros and cons, and most cruisers end up using a mix. I compare all three in how to make electricity when your boat is at anchor.

How to Connect a Portable Generator to Your Boat

You’ll use the same shore power cord you plug in at the marina. Your shore power cord almost certainly needs an adapter to fit the generator’s household-style outlets. For a boat with 30-amp shore power, that’s a 15-amp male to 30-amp female adapter (Amazon). Because we also ran our watermaker off the generator, we kept a spare adapter aboard. Dropping one overboard is a real possibility.

Here’s the order we followed every time:

1. Turn off the shore power main breaker on your AC panel. Also switch off big loads you don’t need, like the water heater, so they don’t overload the generator.
2. Start the generator with nothing plugged in and let it run for a minute to warm up.
3. Plug in the shore power cord, with the adapter at the generator end and the other end in your boat’s shore power inlet.
4. Turn on the shore power main and the battery charger breaker.
5. Check your battery monitor to make sure amps are flowing into the batteries.

To shut down, reverse the steps: charger and main breaker off, unplug, then stop the generator.

Why Not Use the Generator’s 12V Outlet?

Boaters ask this all the time. The Honda EU2200i and most similar generators have a 12V DC outlet, but it puts out only 8.3 amps. That would take all day to make a dent in a cruising boat’s battery bank.

It’s also unregulated, so it doesn’t taper off as the batteries fill the way a real charger does. That can overcharge and damage your batteries. On the Honda, you can’t use the 12V outlet and the 120V outlets at the same time, either. Save the 12V outlet for an emergency boost to a single dead battery.

How Fast Will a Generator Charge Your Batteries?

Three things decide how many amps actually go into your batteries.

Your battery charger’s size. The charger is the funnel. A 40-amp charger will never put more than 40 amps into your batteries, no matter how big the generator is, and in real life it’s usually less. With a small charger, the generator just loafs along.

The generator’s size. A 2,000-watt class generator like the Honda EU2200i can run a battery charger only up to a point. It’s rated for 1,800 watts of continuous power, and a charger loses some power turning AC into DC. In real life, that works out to roughly 75 to 100 amps of charging at most on a 12V boat.

We saw both limits. Our original 75-amp charger put about 60 amps into the batteries in real life. When we upgraded to a 120-amp charger, we got about 75 amps. The bigger charger helped, but the Honda couldn’t feed it anywhere near its full 120.

Your battery type and how full the batteries are.

• Lead-acid and AGM batteries take full amps only until they’re about 80% full. After that, they accept charge more and more slowly. Running a generator to top them off completely burns a lot of fuel for very little gain.
• Lithium (LiFePO4) batteries take full amps almost all the way to full. When we switched to lithium batteries, we ran the generator far less, because none of its output was wasted.

One quirk to watch for: some chargers put out full amps for only a set time, then ramp down on their own. Our 75-amp charger cut back after 30 minutes. Our workaround was to switch the shore power off, wait about 30 seconds, and switch it back on for another 30 minutes of full charging.

Let Solar Finish the Job

If you have solar panels, run the generator early in the morning, when the batteries are at their lowest and will take the most amps. Let the generator do the heavy lifting, then let solar top off the batteries during the day. Watch your battery monitor to see when the charging amps start to drop, and that’s your cue to shut the generator down.

If you’re not sure your charger and battery bank are sized right for how you live, calculate your boat’s power requirements before you buy a bigger charger.

Choosing a Portable Generator for a Boat

Most cruisers who carry a portable generator have a Honda EU2200i (Amazon) or its predecessor, the EU2000i. Hondas are quiet, reliable, and easy to find parts and service for almost anywhere. The downside is price. A few lower-cost brands have caught on with cruisers, and some report good results, but it’s still too early to know how well they hold up.

Shopping, you’ll find generators with more power for less money. The big differences are noise and size. Avoid a construction-style (open-frame) generator. The noise is truly obnoxious, and you won’t make friends in the anchorage. Look for an “inverter” generator instead. We ran both the EU2000i and the EU2200i, and the 2200 was noticeably quieter.

The EU2200i weighs about 47 pounds, so think about where you’ll stow it between uses. Besides the battery charger, it can run a watermaker or power tools.

How long does a tank of gas last? The EU2200i’s tank holds just under a gallon. Honda rates it at 3.2 hours at full load and up to 8.1 hours at a light load with Eco-Throttle on. Running a battery charger hard sits toward the short end of that range. As the batteries fill and the charger eases off, Eco-Throttle slows the engine and the tank lasts longer.

These small portables aren’t the same as built-in marine generators, which put out more power and are built for the marine environment from the ground up. But plenty of cruising boats get by with a portable, and we did too.

Wondering about battery “solar generators” (portable power stations)? Those are a different thing entirely. Here’s why I’d choose solar panels over a solar generator on a boat.

Dual-Fuel Generators: A Propane Option

Most cruisers already carry gasoline for the dinghy outboard. But if your boat cooks with propane and you have an electric dinghy motor, a dual-fuel generator that runs on either gasoline or propane is worth a look.

A reader, Cabot, uses the Champion Dual Fuel Generator (Amazon) on propane. He found that propane gives somewhat less power than gasoline, but the tradeoff was worth it since he didn’t need gasoline for anything else. He also likes that propane doesn’t go bad and keeps the carburetor clean. A 20-pound tank runs his generator about 24 hours.

There are also propane conversion kits for the Honda (Amazon), though they aren’t cheap. I haven’t used the Champion or a conversion kit myself, but propane makes real sense on a boat that’s already set up for it.

Running a Generator Safely Aboard

A portable generator on a boat needs some care. Here’s what mattered most to us:

• Carbon monoxide is the big risk. Point the exhaust away from hatches, ports, and the companionway. Don’t count on the wind to carry it away, because air swirls around a boat at anchor.
• Have working CO alarms below. This is non-negotiable.
• Pick your spot and your weather. We ran ours on Barefoot Gal’s front deck, sitting on a rubber mat, and only in calm weather. When we were done, it went right back into storage.
• Keep it dry. Don’t run it in the rain or in spray.
• Refuel only when it’s cool, and store gasoline in proper cans in a ventilated spot on deck, never below.

You may hear that a portable generator turns on the reverse polarity light on some boats’ AC panels. That’s because inverter generators like the Honda don’t connect neutral and ground the way shore power does. Ours never did that on Barefoot Gal. If yours does, or if your boat has an ELCI main breaker that keeps tripping on generator power, talk to a marine electrician rather than rewiring plugs yourself.

Ready to Learn More About Life Aboard?

Charging your batteries is one of the everyday skills that comes with living on a boat. If you want a clear, practical overview of what liveaboard life actually involves (boat systems, anchoring, provisioning, handling unexpected situations, and more), my course, The Basics of Living on a Boat, covers 31 topics that most people don’t realize they need to know until they’re already out there.

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