For a long time I've been repeating this:
"Generators are really inefficient at charging batteries"
But why?
People may think I say this because I'm biased in favor of solar. Actually, I've been saying this because I've seen so many people disappointed to find that after running a 2000-watt generator for hours, their batteries still aren't fully charged – and they don't understand why.
Because the reason for this is pretty technical, I've never taken the time to explain it. But recently I decided it's time to give it a go.
If you're not interested a deep dive into electrical efficiency kilowatt hours, and other geeky topics, here's the short answer: A lot of power is lost between the generator, inverter, converter, and batteries. In the end, more than 90% of the energy is wasted along the way. Yes, 90%. And batteries charge slowly, so that's why it takes a lot longer than most people think.
Even if you're pretty knowledgeable about generators and chargers, you might be surprised at how truly inefficient it is to try to charge batteries with a generator. Let's dig in.
First, the generator
A generator's job is to turn gasoline into electricity. It's not a very efficient process, to say the least. Typically most of the gasoline turns into heat, noise, and vibration instead of becoming useful electricity. (This inefficiency is the nature of all gasoline engines, not just generators.)
Let's look at the Honda eu2200i generator, which is very popular in RV circles. It can run for anywhere from 3.2 hours up to 8.1 hours on a tankful of gas, which is 0.95 gallon.
That amount of gas contains about 31.73 kiloWatt-hours (kWh) of energy1,2. But you don't actually get that much electricity out of it, due to inefficiencies in the engine and the inverter. In the case of the Honda, you get up to 5.76 kWh total if you run it at full load3, which is about 18% efficient.
That's pretty poor, and it gets even worse if you engage the Honda's Eco-Throttle and let the generator run with a load of less than 450 watts. At that rate, the Honda will run longer but produce only a total of 3.65 kWh from a full tank4, which is about 11.5 % efficient.
So, ironically, Eco-Throttle doesn't make the generator more efficient, it just makes it run longer when you don't have a heavy load on it. You actually get less total energy out of a tank of gas if you run it in Eco-Throttle mode.
The thing is, you don't actually have a choice about whether or not to run it in Eco-Throttle mode. Turning off Eco-Throttle will allow the generator to run faster, but the charging time will be exactly the same. The generator might even run out of gas before the batteries reach a 90% charge.
That's because the batteries will accept only a limited amount of power when recharging. In the case of Lead-Acid batteries, a pair such as those typically installed in a travel trailer will charge at a much slower rate than the rated output of Eco-Throttle mode.
Even fast-charging LFP (lithium) batteries have the same problem because, like all batteries, their charge rate slows down as they get full. We'll talk more about that further down.
Simply put: Running the generator at higher speed, or using a bigger generator, will accomplish nothing in terms of adding more charge to the battery – except use up your gas more quickly.
In any case, for a gallon of gas and eight hours of running a generator, you get roughly 50 cents worth of electricity.
Second, the power conversions
Maybe you don't care about the cost of gas versus the cost of electricity, because your primary concern is getting power to your batteries while you're camping, far away from a plug. Fair enough.
But you might care about the energy that's wasted by the process of multiple power conversions.
- A generator makes DC (direct current) power.
- It then turns the DC power into AC power using an inverter. This process usually turns about 15-20% of the energy into heat.
- The RV's power converter turns it back into DC to charge the batteries. This wastes at least 20% more.5
Finally, the batteries
With a "55 amp" converter/charger like the commonly-installed WFCO 9855-AD, you could theoretically charge the batteries at up to 660 watts. But in practice, you won't get anywhere near that rate.
That's mostly because the batteries themselves won't let it happen. A typical pair of lead-acid batteries will charge at less than 200 watts. LFP batteries are faster, but even they will hit 500+ watts for only brief periods of time, before quickly slowing to 250 watts or less. (The actual rate depends many factors such as temperature, depth of discharge, and the available output of your RV power converter.6,7)
This is the big bottleneck. It doesn't matter that you've got a 2,000 watt generator. Most of the power available can't get into the batteries. So any excess power is just wasted.
It takes how long to charge the batteries?
Let's say we've been camping a couple of days and now it's time to charge up our pair of 100-amp-hour LFP batteries from 20% to 80%. That's a total of 120 amp-hours we want to gain, or 1.536 kWh.
(We need to get 1.92 kWh from the generator just to overcome the losses of the RV's power converter! Yikes.)
Since batteries charge slowly, we have to run the Honda generator in Eco Throttle mode, to avoid running out of gas. If the batteries charged at a constant rate, the generator would need to run for over six hours, and burn most of the gas in the tank.
In reality, since batteries don't charge at a constant rate, it will take even longer.
The bottom line
So this is where my oft-repeated statement comes from: "Generators aren't efficient at charging batteries." But to be fair, it's not entirely the generator's fault. Between the generator engine, inverter, RV power converter, and slow battery charge rate, only about 9% of the energy in the gasoline reaches the batteries—at best.
The real disappointment for generator owners is the slow recharge rate. Lots of people will fire up the generator for two hours "to charge the batteries" but don't realize how little effect this has.
In ideal conditions you might get 300 Wh into a pair of lead-acid batteries after two hours. An electric refrigerator alone will use more than that in a day. So a daily 2-hour recharge session with a generator is a losing proposition for most people.
That explains why so often people find themselves out of power in the morning, even after running the generator for a couple of hours every day.
Advice for generator owners
1. Don't try to get the batteries to 100%
The generator is most effective when the batteries are deeply discharged. Remember, the rate of charge slows drastically as the batteries get fuller. The last 10% can take hours.
Your best option with a generator is to try to get the batteries to no more than 80% each day, and stop there. If you can, coordinate your charging session to overlap with other electrical needs, like microwave, coffee pot, or air conditioner. This will reduce wasted gas.
2. Get an amp-hour meter
Don't be fooled by the factory-installed voltmeter. It doesn't measure the level of charge in the batteries – it only measures the volts in the electrical system, which is pretty poor way of guessing at the batteries state of charge.
An amp-hour meter is really the only accurate way to measure how much battery charge you have. This is true no matter what method you use to charge them, and no matter what type of batteries you have.
3. Switch to solar for battery charging
Solar panels are a much more efficient way to recharge your batteries. They are the perfect way to deliver the kind of slow, multi-hour charge that batteries need. Keep your generator for the air conditioner if you need it, and use solar to keep the batteries topped up.
You don't have to invest a lot of money to do this. Our Portable Solar Kit is designed for exactly this purpose.
If you have a power station, you can plug your solar panels into it, too.
As I've often said, the best use of a generator is for large power needs, rather than for charging batteries. Get a generator if you must power an air conditioner for hours or days—that's the kind of thing they are well-suited to do.
For most other purposes, a couple of solar panels and/or portable power station will be all you need.
20 comments
B. Frey
Hello Rich
Would it be more efficient to attach alligator clips directly to the batteries for charging? I’m not sure where the alligator clips would plug into on the generator side though.
Rich Luhr
Hi B Frey. Good question. The answer is no, because even if your generator has a 12-volt output, it isn’t a battery charger. To be a battery charger, it needs to output different voltages as the batteries charge (which means it has to sense the state of charge of the batteries), AND it needs to be designed for your battery chemistry (either lead-acid or lithium-iron-phosphate).
M.R.Murray
Hi Rich,
If you run your generator 3 hours per day while charging batteries you would also be running some appliances at the same time thereby reducing 3 hours of battery run time. I find using both solar and a generator a great combination.
M.A.S.
What about AGM batteries? Propane generators? Plugging a smart battery charger directly into the generator and charging the batteries with that?
I agree with solar, but some of us live and camp in areas with thick forests and cloudy skies. We’ve had people with new airstreams leave rallies due to their 12V fridge draining the batteries on the first night due to rain and tree cover.
Rich Luhr
M.A.S.: Whether you have a propane or gas generator makes very little difference, the charging will still be slow and inefficient.
An AGM battery is a lead-acid battery. It may charge a little faster than a traditional “wet cell” lead-acid battery but it will still be slow.
If you can’t use solar, a portable power station is what we’d recommend to extend your battery capacity at the lowest possible cost.
Plugging a battery charger into one of the generator’s AC outlet would not improve things. You’d just be replacing the charger in your Airstream with a nearly identical one, with the same poor results.
Rui DeSousa
It could be the converter setting and the converter charging profile. I had issues with my batteries only sometimes charging and also losing a charge while pugged in. My converter was set to AMG batteries from the factory while having lithium batteries. I replace the convert with a pair of Victron Energy Blue Smart IP22 converters. They can to either 15 or 30 amps each. I have one set at 30A and other set to 15A for total of 45A of power/charging and that replaced the standard 50A converter. 60A was too much with the factor 300w solar for the batteries to handle as it would cut out due to over amperage. Now I have no problem charging the batteries off the generator within a few hours to a full charge and that also includes charging a Jackery 1000Wh at the same time using a suitcase 3kw generator. I would recommend replacing the converter and of course solar.
Rich Luhr
Rui, you are correct that a converter set for lead-acid batteries (which includes AGM) will not properly charge lithium batteries. I’m glad you figured out the problem in your system.
However, the problem for all owners remains as I stated it. Your dual converter/chargers can charge at up to 45 amps (about 550 watts) but most of the time a pair of lithium batteries won’t accept a charge at even half of that rate. That’s why it still takes “a few hours” to charge them. There’s nothing that can be done to improve that, including switching to a new converter.
One good thing you’re doing is using the generator to charge a 1000Wh Jackery portable power station at the same time. This makes for a better use of the generator’s output, since most of the time it’s producing far more power than the batteries can use.
William McChesney
Rich – thanks for all that info – really appreciate your explanation- I didn’t realize just how much loss there was in all that voltage conversion.
Rob Kirkpatrick
Rich,
I recently bought a 3.2 kWh Sorein Power Station from Airgear that I have not had the opportunity to use. I still plan to carry, at least for a while, my Yamaha 2000 W generator. I am wondering, would it be better to plug my Airstream trailer, with two 100 Ah batteries, into the power station and the power station into the generator or plug each separately into the generator? Thanks for the great products and information you provide. Rob K
Rich Luhr
Hi Rob
The best use would be to plug the Airstream into the power station for most situations, and pull out the generator only when you need to run the air conditioner. Otherwise, you shouldn’t need it.
When time comes to recharge the power station, you can find an outlet at a campground, charge via a portable solar panel, or via a DC-DC charger on your truck.
You can use the generator to recharge the Sorein Power Station, but it’s just as inefficient as trying to recharge your Airstream from a generator, so I’d save that for situations where you have no other option.
In that case, you’d plug the Sorein into the generator for as long as you want to recharge, then put the generator away again. This way you’re using the generator as minimally as possible, which will keep your campsite nice and quiet, and free from fumes.
GBD
I have been unhappy with the inefficiencies of a Gen with attempting to charge my 400aH of BB Batts, with the current Airstream OEM Converter. It too was set wrong at the factory and has been adjusted for Lithium, but still slow up the charge uptake. I had 400aH of BB Batts in my previous RV but outfitted with a 3000 W Victron Inverter/Charger. I was able to pull about 47 Amps from Gen to Inverter/Charger to my batteries, and able to charge them from 55% SOC to 94% SOC with this setup, inside of an hour and fifteen minutes. Would you recommend that I go back to the Victron Multiplus so I don’t waste time, fuel and generator run time?
Rich Luhr
GBD: Yes, with a lithium battery bank of that size, the Airstream converter is likely the bottleneck. It was chosen by Airstream to charge a pair of batteries, not four. Switching to a more capable charger/converter may improve your charge times.
Synergy1281
Rich, am I thinking correctly that the inefficiency you are describing for generators generally doesn’t apply to a DC/DC charger powered from the tow vehicle’s alternator? The alternator produces AC current, with diodes that somehow convert that to DC, so the opposite of a generator that produces DC current and uses an inverter to change that to AC (and then a converter in the rig that changes it back to DC)? There still is a conversion, AC to DC, but only one conversion. Is that a more efficient conversion? Or is the additional load on the tow vehicle engine (and associated fuel consumption) negligible because the motor already is under such a load propelling the vehicle?
Rich Luhr
Synergy1281: You’re on the right track. Yes, a DC-DC converter installed on the truck is a more efficient way to charge the trailer batteries, at least if you do it while you’re driving (not idling).
It’s more efficient because the power undergoes fewer conversions, as you noted, since the DC power connection is directly to the trailer batteries, rather than going through the trailer’s power converter/charger. But keep in mind that the speed of the charge is not going to be any better, due to the limitations of the batteries themselves.
So the best scenario for using a DC-DC charger on a truck is when you’re on a long drive. As you noted, the decrease in fuel economy at highway speed is negligible.
Darren C.
If I parallel two Honda 2200 generators would I get a faster charge than just using one?
Bluebeak
Hi Rich,
Thank you for the great info! So, if I understand this correctly, if you had (2) 50 Amp 12 VDC DC to DC converters for a potential total of 100 Amp battery, or 1200 watts, charging capability, you would still be limited by the battery charging characteristics of LFP batteries of about 250 watts or less? Meaning, installing more than (1) 50 Amp 12 VDC to DC converter would pointless, as well as, be a waste of time and money? Thank you!
Ron
I have a Trade Wind with 810 amp hours of lithium and 600 watts of solar. Will I get some benefit from a generator or would I be better off adding an external movable solar? I believe the most I can add is another 100 watts. I have a Renogy suitcase, but I think that is 210 and won’t work with current setup. Thanks
Rich Luhr
Darren C: You could have 50 generators connected to the trailer, and the charge rate would be exactly the same. That’s one of the key points of this blog.
Rich Luhr
Bluebeak: You are correct. The only way you could use 1200 W of charging power would be to vastly increase the size of the battery bank. In other words, if you had 6-8 LFP batteries in the trailer, you might be able to take full advantage of 1200 W of charging power. Otherwise you’re just wasting money.
Rich Luhr
Ron: the first question I would ask would be whether you really need additional charging capacity at all. The battery bank on your Trade Wind is equivalent to over 10,000 Wh of energy. It is so big that unless you run the air air-conditioning for hours, you will probably never use all of it. Are you really going to need more, in any reasonable camping situation?
Yes, the 600 W rooftop solar is inadequate for charging a battery bank of that massive size. but adding another 100 or 200 W is not going to make a significant difference here.
You would be hard-pressed to set up a solar array that can keep up with a 1600 W air conditioner all day long. In that case, if the battery bank really isn’t large enough for you, you would need to look at a generator.
I would stop and evaluate your actual electrical needs before putting another penny into anything. For more information, I do have a blog specifically about the Trade Wind trailer, which you can find on the Air Gear website.