This is something that's been kicking around in my head. I've been researching alternators and i've seen 2 that have an output of 370 amps at 1200 rpm. If my math is correct then that is 4440 watts @ 12 vdc. Putting this in an inverter should give me 37 amps @ 120 vac.I think there will be a loss of maybe 10% but I think I could power my trailer including A/C. Thoughts?

Okay I'll bite. How is this more efficient than an inverter generator? What more would be gained?

Are you taking into account the amount of gas that engine is going to use to run that alternator, as compared to the gas that a smaller generator will use?Keep in mind, your numbers suggest that you'll have to run that alternator at 1200 rpms. What does that mean the engine has to be turning? Are you going to sit there with your foot on the gas pedal to bring the RPMs up from their idle speed?This might be fine, while driving down the road, but when you stop, you'll want something else.Also, how much power does your TV require for operation? These newer units are using that alternator for more than the battery.....so just tooling down the road, will have the various ignition and fuel systems drawing from the battery too.

Why do you want to power your trailer A/C while you are driving? Hopefully, no one is back there.Now, if you want to use that alternator to keep your batteries charged while you are driving you should be able to set that up without issue.

After we caught the RVing bug by trying out a TT first, we realized a motorhome fit our style of traveling much better. One of the biggest detractions from the TT was the inability to cool it while on the road. We love to pull off and fix a meal, take a break or what ever. How about stopping for a bathroom break? This is all very uncomfortable when the temperatures are in the 90s with high humidity. Although we had the ability, we never once took the gennie and busted it out while traveling. It's too much of a hassle, specially when your destination is a full-hookup campground. I think the best part of the OPs plan is to keep the TT cool while moving down the road. Makes sense to want to do that to me. I will leave the details of whether this will work or not to the experts.

Powering trailer with an alternator | Good Sam Community

39 Replies

westend
Explorer
Sep 02, 2013
Hybridhunter wrote:
So I just spent my first weekend camping where I am back to using the alternator in my F150 for power. The alternator is 220 or 230 amps, (not sure), and the engine is a 3.7 V6, with EPS. With no power steering pump, and small displacement, it idles very efficiently. I have ordered a WIFI OBD plug in, and I will report actual fuel used at a later date. It was able to run my 6000btu AC effortlessly.

I used fairly cheapo #8 AWG booster cables 12' long to connect the vehicle to one of my 2 batteries in the rear of my TT. From there, it goes #2 AWG hard wired to my 1500 watt inverter 12' away. The lowest voltage I saw at the truck was 13.4v when the cooling fans cycled. It idled near silently while powering my trailer. Running a 60-70 amp load off a 220 amp alternator shouldn't appreciably shorten its life IMO.

Everyone loves to say how inefficient this type of setup is, but my genset used .4 gph when powering the AC. An inverter gen would be more efficient. But I think spending $1K to save on fuel is donkey economics.

I really like the quietness, no extra fuel to haul, no genset to haul, no other campers bothered.
Thanks for posting to show that a 6K AC can be run off the inverter and batteries.
I think you may be mistaken on the output of the alternator. More common are 100-130 amp alternators in Ford pickups. I may be behind the curve, though, as I don't have much experience with newer Ford's.
Bottom line is that it's working for 'ya and that is the most important.
mena661
Explorer
Sep 02, 2013
Hybridhunter wrote:
So I just spent my first weekend camping where I am back to using the alternator in my F150 for power. The alternator is 220 or 230 amps, (not sure), and the engine is a 3.7 V6, with EPS. With no power steering pump, and small displacement, it idles very efficiently. I have ordered a WIFI OBD plug in, and I will report actual fuel used at a later date. It was able to run my 6000btu AC effortlessly.
Great info! Thanks.
pnichols
Explorer II
Sep 02, 2013
Hybridhunter wrote:
So I just spent my first weekend camping where I am back to using the alternator in my F150 for power. The alternator is 220 or 230 amps, (not sure), and the engine is a 3.7 V6, with EPS. With no power steering pump, and small displacement, it idles very efficiently. I have ordered a WIFI OBD plug in, and I will report actual fuel used at a later date. It was able to run my 6000btu AC effortlessly.

I used fairly cheapo #8 AWG booster cables 12' long to connect the vehicle to one of my 2 batteries in the rear of my TT. From there, it goes #2 AWG hard wired to my 1500 watt inverter 12' away. The lowest voltage I saw at the truck was 13.4v when the cooling fans cycled. It idled near silently while powering my trailer. Running a 60-70 amp load off a 220 amp alternator shouldn't appreciably shorten its life IMO.

Everyone loves to say how inefficient this type of setup is, but my genset used .4 gph when powering the AC. An inverter gen would be more efficient. But I think spending $1K to save on fuel is donkey economics.

I really like the quietness, no extra fuel to haul, no genset to haul, no other campers bothered.

Thanks for your clear and very reasonable post!!

We're not afraid to idle our motorhome's V10 main engine as needed to charge our coach batteries and cool the motorhome when, for instance, eating lunch in crowded parking lots in scorching weather from the dash A/C. It idles very quietly - quieter from the outside than any small portable generator - and vibration-free through the floors inside the coach area.

BTW .... going back the the OP's first post .... you can install special large alternators on the main engine without having to use them at the RPM levels above idle. Just install an alternator with large enough amperage capacity such that at engine idle speeds it still is putting out the power you need. For instance, my Ford's V10 130 amp stock engine alternator - according to Ford's published RPM versus amperage curves - still puts out 60-70 amps with the V10 idling. This is enough for our needs, however what this means is that a huge alternator - say for example a 400 amp at 1300 RPM alternator - might put out in the neighborhood of 250-300 amps with the engine only idling. This is how I would size a large engine alternator for special use in an RV - get the size you need so the engine can be left at only idle RPM.
Hybridhunter
Explorer
Sep 02, 2013
So I just spent my first weekend camping where I am back to using the alternator in my F150 for power. The alternator is 220 or 230 amps, (not sure), and the engine is a 3.7 V6, with EPS. With no power steering pump, and small displacement, it idles very efficiently. I have ordered a WIFI OBD plug in, and I will report actual fuel used at a later date. It was able to run my 6000btu AC effortlessly.

I used fairly cheapo #8 AWG booster cables 12' long to connect the vehicle to one of my 2 batteries in the rear of my TT. From there, it goes #2 AWG hard wired to my 1500 watt inverter 12' away. The lowest voltage I saw at the truck was 13.4v when the cooling fans cycled. It idled near silently while powering my trailer. Running a 60-70 amp load off a 220 amp alternator shouldn't appreciably shorten its life IMO.

Everyone loves to say how inefficient this type of setup is, but my genset used .4 gph when powering the AC. An inverter gen would be more efficient. But I think spending $1K to save on fuel is donkey economics.

I really like the quietness, no extra fuel to haul, no genset to haul, no other campers bothered.
Huntindog
Explorer
Aug 17, 2013
MEXICOWANDERER wrote:
As a designer and owner of such a critter (385 amperes at 28 volts) the idea is valid. At the expense of weight a suitable engine can operate at say 1,600 RPM under FULL LOAD 24/7 364 (one day in hours for maintenance). Try operating a Honda for a solid year and then shut it off and yank the cord. The engine will spin for a half minute cause it ain't got no compression (worn out).There have been reports here of Hondas going over 15000 hours.
I haven't kept track of mine, but they get a LOT of use. Don't know just how many thousands I have on them, but it for sure is multiple.

Rule of life.....nothing's perfect. Almost everything has strong and weak points.

It is VITAL to control alternator voltage PRECISELY with a system like this. Within a few hundredths of a volt. Adjustable voltage to allow the inverter to see an exact predetermined voltage to achieve output voltage. Frequency will be correct but I highly suspect the internals of a MAGNUM inverter to be of slightly higher quality than the inverter of Honda generator. I use a diesel prime mover and it operates at 1,300 RPM.
Hybridhunter
Explorer
Aug 17, 2013
I previously used my Tacoma alternator to charge our batteries, either via the 7 pin while travelling, (marginal), or via booster cables, or the built in 400 watt inverter to run a battery charger.

Charging via the booster cables worked well, to a point. The truck was able to sustain enough wattage to run our window AC through our inverter. The weakness of this setup was the 13.3ish volts that the Tacoma like to settle at, it would belt out the amps, when the inverter was drawing power, or the batteries were low. With independant control of the voltage, it would have worked better.

The Tacoma's inverter was good for running a good quality charger, as the charger would ramp up the voltage to over 15 volts to rapidly finish up the last 15% of the charge. Nowhere near as good a system for more than a couple batteries, the advantage was in the ability to "steal" power from the Tacomas battery to charge the trailer when sitting. Also, totally silent, and as soon as we go somewhere, the truck battery would recharge itself.

It seems that folks like to poopoo using the alternator, but the fact is the Tacoma would run on about .4 gallons an hour, while puting out anywhere from 15-80 or so amps. I now have a small genset, and it's a cheap pain in the ass. We haul an ATV, an extra water tank, and all the usual camping stuff. The generator is an extra 70#, plus jerry can, plus the space. There is no way I would ever be able to amortize the cost of a Honda 2000,(which is a slick machine) over just idling our current truck, a 3.7L F150. And I'm not sure why anyone posted that vehicles "don't like to idle for extended periods". Not an issue with a modern engine. The F150 uses around .3 GPH, and I plan to go back to using setup.
And guess what, you can run a truck, anytime, anywhere, no extra fuel can, maintenance, or noise. Our setup was a godsend in the "no generators" loop at the grand canyon. Oh yeah, the new F150 comes with a beast of an alternator, over 150 amps.
MEXICOWANDERER
Explorer
Aug 16, 2013
As a designer and owner of such a critter (385 amperes at 28 volts) the idea is valid. At the expense of weight a suitable engine can operate at say 1,600 RPM under FULL LOAD 24/7 364 (one day in hours for maintenance). Try operating a Honda for a solid year and then shut it off and yank the cord. The engine will spin for a half minute cause it ain't got no compression (worn out).

Rule of life.....nothing's perfect. Almost everything has strong and weak points.

It is VITAL to control alternator voltage PRECISELY with a system like this. Within a few hundredths of a volt. Adjustable voltage to allow the inverter to see an exact predetermined voltage to achieve output voltage. Frequency will be correct but I highly suspect the internals of a MAGNUM inverter to be of slightly higher quality than the inverter of Honda generator. I use a diesel prime mover and it operates at 1,300 RPM.
mlts22
Explorer II
Aug 16, 2013
Roadtrek has the option on the CS Adventurous and the E-Trek for what appears to be an additional alternator plus an inverter. They call it an "engine generator". It is a nice thing to have so one can run the roof A/C while driving, but when parked, I'd far rather put hours of wear and tear on a generator than the vehicle's engine. A new generator can cost me $4000, a new Sprinter engine can be 20k+, and a clogged DPF can cost $3200 to replace.
christopherglen
Explorer
Aug 16, 2013
Some people have built gas engine/alternator contraptions to charge batteries at high speed.
subcamper
Explorer II
Aug 16, 2013
Is that 370 amp rating a peak (short time only) or continuous rating?

You could probably get the 1200rpm at idle by sizing the pulley correctly.

Automobile engines generally don't like idling for long periods of time. Also, with no airflow through the engine compartment (other than the electric cooling fan for the radiator when it cycles on) at idle, the alternator will be hard pressed to avoid overheating. It may have been designed for high current output while the vehicle is moving.

Some trucks had optional dual alternator setups, usually for specialty use such as ambulance conversions.

Personally, I would rather wear out a $500 generator than a $5K-$10K engine (and all its accessories) in a truck.

Steve

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Powering trailer with an alternator

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