Batteries are the heart of your RV electrical system. They keep the lights on, the control boards in many appliances operating, and turn the pump over so you can take a shower. We may not think much about them, particularly when hooked up to shore power, with a converter keeping up with the low-voltage needs. But step away from shore utilities and head out for a session of boondocking, and you’ll soon know if your batteries are staying happy.
Even for those RVers equipped with solar or wind power, there are times when we need a bit more of a boost than our alternative energy system can provide. It may be that nature lets us down, or that we’ve simply been using more battery power than we can easily replenish. And for those who don’t have solar power, if you get away from the utility grid for very long in the boonies, you too will soon be whimpering, “More power! Please—more power!”
Is it feasibly to recharge house (chassis) batteries with a generator? It depends on the generator, and how you go about it. Many portable generators have a “12-volt” output, and yes, this can be hooked up to the house battery system. The issue is the output. If the generator offers a low-ball output (we’ve seen some that offer a 3-amp output), you’ll be running through a lot of generator fuel to catch up on your power needs.
Other portable generators offer a greater low-voltage output, perhaps pumping out 8 or 10 amps. Here’s where a bit of caution needs to be exercised. Check your generator manual to determine if this is a regulated, tapered output, or simply an unregulated “constant amperage” output. This is important. When batteries are low, a big fat bulk charge is not a problem, but when batteries get close to their capacity, they can be overcharged.
If you’re not sure what kind of charging your generator is offering, then when you are in a charging cycle, it’s important to regularly shut off the generator and test the voltage of the batteries. You will want to ensure that you’re not overcharging by using a digital voltmeter—the old “needle on a scale” analog meter just can’t tell you what you need to know.
To avoid being fooled by the surface charge that’s present on a freshly charged battery, you’ll need to stick a good, heavy load on it for a minute or two, shut off the load, and then test the voltage. A fully charged, 12-volt “flooded acid” battery reads 12.6 volts. If you’re close to that, it shouldn’t be a problem to stop charging. Batteries are very receptive to charging when they are deeply discharged, but the closer they get to “full,” the harder it is to pump current into them. Some RVers figure once they’ve hit the 12.5-and something mark, they’re good enough to go.
Even so, if your battery bank is fairly low, even a single, group 27 battery with a 105 amp-hour capacity, down to say 50 percent of charge, can easily take many hours for a small output generator to make up for. That’s a lot of noisy fuel usage. What about using a portable generator coupled to a high-output battery charger? A smarter, more fuel-efficient move, but again, a few cautions are in order.
Many portable generators, particularly the “contractor” style, do not produce “clean” electricity. A few spikes here and there, a lot of electrical “noise,” can raise Cain with sensitive converter-chargers. If using a portable generator, the purest (and safest) electricity available will come from a pure sine-wave system. Check out Honda or Yamaha portable inverter generators as an example of something that won’t eat your electronics. If your rig has a built-in “genset” like an Onan or similar, your electricity should be clean and fully edible for a fancy (expensive) converter-charger.
How much muscle does your portable generator need to run a high-output battery charger? Simply speaking, a 300-watt generator can handle most 10-amp chargers; step up to a 600-watt generator to power a 30-amp charger. What about coupling a portable generator with your RV’s existing battery converter? Unless you’ve got one of the sophisticated converter-chargers mentioned earlier, the output from the converter is so low as to be almost useless.
The usual precautions apply: When fiddling around with even low-voltage systems, it’s always best to remove your jewelry, including rings and watches. Turning your wedding band into an electrical conductor (while having it on your finger) can be an event you’ll never forget.
Russ and Tiña De Maris are authors of RV Boondocking Basics—A Guide to Living Without Hookups, which covers a full range of dry camping topics. Visit icanrv.com for more information.
Hi there. Thanks for the great post.
I’ve been trying to figure out what size portable generator I need to power a battery charger to charge my 100 amp house battery. I would like to get a small light-weight generator (1000 watt or under) but I am having trouble finding out how much wattage it takes to run a battery charger. (I don’t want to buy a generator that is too small.) I would like to get a good, quality charger because I live in Alaska where my solar panel isn’t very effective during winter months.
In your post above, you write:
“How much muscle does your portable generator need to run a high-output battery charger? Simply speaking, a 300-watt generator can handle most 10-amp chargers; step up to a 600-watt generator to power a 30-amp charger.”
Based on that statement, it seems the 1000 watt (really 900 watt) generator would work fine. How does one determine the wattage draw of a battery charger? (I’ve looked at many of the spec sheets online and they don’t seem to provide that info. Is there a formula you use for your above statement?
Any advice would be much appreciated.
Penelope Smith says
This is some really good information about generators. It is good to know that a bunch of different type of generators that have different outputs. It does seem like a good idea to have an idea of what you want to use the generator for before you get it.