Charging Boat Batteries With a Portable Generator

Charging boat batteries with your portable generator? Can you do it? Should you do it?

Many cruising boats carry a small generator aboard, most often a Honda 2200 (Amazon) or its predecessor, the Honda 2000. The Hondas are quiet and long-lasting. Plus you can find parts and service shops easily. The biggest problem with them is that they are pricy. A few copycat brands are starting to emerge. And some cruising friends are reporting good results although they admit that it’s too soon to really know the longevity. 

These are not the same as built-in generators. They generally produce more power but are also larger. They are truly built for marine conditions, which the small generators are not. However, many boats do use the small generators – us included. 

Choosing a Portable Generator

If you start looking at portable generators, you’ll see that there are many with more power for less money than the Honda 2200 (or the copycats). The big differences are the noise level and size. You do not want what’s called a construction generator – the noise is just obnoxious. And it will drive you and everyone else in the anchorage nuts. The Honda 2200 is the quietest I know of.

We previously had a Honda 2000 and switched to the 2200 when we inherited one from a friend who died last spring. The 2200 is noticeably quieter than the 2000! 

These generators weigh about 40 pounds. And you can use them to charge batteries or run a watermaker or power tools. (You can also run power tools off your house batteries with an inverter.)

Charging Boat Batteries – How To

The Honda 2200 and most copycats come with a 12V plug and I often get asked if that’s the best way to charge boat batteries. No, it’s not. That outlet only puts out 8.3 amps – it will take all day to charge your batteries! 

Instead, connect the generator’s 110v outlet (what looks like a standard household outlet) to your boat’s shorepower cord and use your shorepower charger to charge the batteries. This way, you’ll be able to charge much faster – most likely you’ll put in about 80-90% of the amperage your shore power charger is rated for. 

Most likely, your shorepower cord will need an adapter to connect to the generator. Boats generally have either a 30 amp or 50 amp shorepower cord (and those with a 50 amp usually have a built-in generator), while the generator has a 15-amp outlet. You need a 15 amp male to 30 (or 50) amp female adapter, such as this one (Amazon). Because we use our generator to also run our watermaker, we always carry a spare adapter just in case we drop one overboard (we never have but there is always that risk). 

Charging Speed – What to Expect

Readers often ask why the generator isn’t charging the batteries as quickly as they expect. The speed of charging is governed by the lesser of the generator’s output or the size of the shorepower charger and by how full the batteries already are.  

The generator size usually isn’t the limiting factor. The first culprit is an undersized shorepower charger. If you have, for example, a 30- or 40-amp charger, that’s the most the generator will be able to put into your batteries. Actually, due to inherent losses in the system, you’ll get somewhat less.  

You’ll put in far more with a larger charger. We have a 120-amp charger and typically put in about 75 amps, so charging takes much less time and uses less gas. 

Additionally, how much charge your batteries will accept is governed by how full they already are, particularly with lead-acid or AGM batteries. The closer they are to being fully charged, the less charge they’ll accept. So, if you also have solar power – assuming that the solar panels put out less than the wattage of the generator, which is virtually always the case – it’s best to run the generator for a limited time early in the day when your batteries are at their lowest and then let the solar panels finish the job.  

Portable Generator Safety

Finally, you have to carefully consider the safety aspects of using a small generator on a boat. Carbon monoxide poisoning is a real risk if the exhaust gets funneled back into the boat. You absolutely must make sure that the exhaust is directed away from the boat’s interior. I recommend having a CO alarm. Additionally, it shouldn’t be left uncovered or run in the rain. And you’ll have to carry more gasoline to power it. 

We live primarily on solar power. However, in winter and periods of prolonged cloudiness, it’s just not quite enough and we really don’t have room to add more. Running our diesel engine just to charge the batteries is hard on it (diesels like to be run with a heavier load on them than is required for just battery charging). So charging boat batteries with a small fuel-efficient generator just makes sense. 

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