Friday, June 2, 2023


RVelectricity – Updates on Hughes Autoformer, Dometic 12-volt fridge, SoftStartRV on a solar panel

By Mike Sokol

Dear Readers,
I’ve been working on several unrelated testing projects over the last few weeks, but don’t have enough new data on any single item to warrant a full article. So I’m going to break this article up into a three-fer with a brief update on each project.

Hughes Autoformer experiment

There’s been a ton of interest in my experiment with the Hughes Autoformer to determine how useful it might or might not be in reducing air conditioner overheating during low voltage conditions, and to determine if my findings would be useful in getting the NEC to reverse their decision to ban the the use of voltage-boosting transformers in campgrounds.

Read my Hughes Autoformer testing posts Part 1 and Part 2 where I go into the logic of what I planned to measure, and a little bit of raw data from earlier in the week where I actually connected the Hughes Autoformer to my Dometic Penguin II with the stock starting capacitor, and saw these voltage/current readings.

Here’s my basic bench setup, with the meters in the upper half of the photo measuring the pedestal voltage and current (supplied by a 3 kW VARIAC), and the bottom half showing the voltage and current required by the air conditioner. Yes, I need to get four matched meters because the calibration is a little bit off, and I’ve already asked my Southwire and Fluke colleagues to provide factory calibrated meters for these tests. Cool stuff, eh?

In the first photo I’m supplying 120 volts AC and the air conditioner is drawing around 14.2 amps of current (or 13.7, depending on which meter you believe).

In the next photo you’ll note that I’ve reduced the pedestal voltage to 114 volts so you can see the booster transformer action in the Hughes Autoformer. From my observations, the relay in the Hughes Autoformer kicks in the boost winding around 115 volts or so.

Here it’s stepping up the 114 volts from the pedestal to 122.5 volts feeding the RV (and air conditioner). And while it does boost the voltage powering the RV, at normal voltages it also increases the amperage draw on the pedestal up to 15.9 amperes side following Ohm’s law.

But that’s not the entire story. There just might be some advantages to a booster transformer for the air conditioner when the pedestal voltage gets very low (below 100 volts). In this picture it boosts the incoming 100 volts up to 107.5 volts. And the 15.27 amps of pedestal current is a little less than an air conditioner running at that reduced voltage. Yes, I have LOTS more pictures at 1 volt intervals, and I need to get all my meters calibrated to the same test voltages and currents.

So I’ll leave you to ponder what this all means for another week while I gather additional data and spend some time crunching the numbers. There’s no clear-cut yea or nay on the Hughes Autoformer technology just yet. But I’m fascinated by all the variables that are creating this interplay of readings I’m measuring for this project.

Dometic 12-volt DC Danfoss compressor experiment

Yes, my Danfoss 12-volt refrigerator experiment is in a full test cycle, with my first demonstration showing it would operate for 48 hours from a fully charged 100 amp-hr Briter Products Lithium Battery. Read that first impressions article HERE, along with my May article using a Vitrifrigo 12-volt DC refrigerator with a Danfoss compressor for a similar test.

In real-world applications where the kids are opening the door dozens of times a day this could still mean at least 24 hours of run time from a single 100-amp-hr Lithium battery, which is way beyond the 8 to 10 hours of operation that I’ve measured for a residential refrigerator using a 120-volt AC compressor and an inverter to power it.

I promised some actual current draw numbers, so here you’ll see that the run current for this refrigerator s around 6 amperes at 13.2 volts, which works out to 78 watts of power while the compressor is running. On a residential refrigerator which draws perhaps 4 amperes of current at 120 volts, that would require around 40 amperes of current at 12 volts DC from your RV storage battery. Yikes!

The Danfoss 12-volt DC compressor is at the heart of this high efficiency, so I’m really impressed with how well this technology works. Again, you’ll have to wait a while for my full report with all the details, but I’m encouraged by the initial numbers. Personally, I would never install a 120-volt AC residential refrigerator in an RV, but one of these purpose-built RV refrigerators with a Danfoss 12-volt DC compressor sure is tempting.

SoftStartRV and solar panels

Last but certainly not least, I’ve now begun testing regarding using solar panels to power an RV air conditioner. And it does look like in order to do this with a 2,000-watt inverter I’m going to need to reduce compressor starting surge considerably.

So, in addition to talking VMAX out of one of their storage batteries optimized for solar panel charging (yes, it has its own built-in solar charge controller), I’ll be testing this with traditional Lithium batteries and a REDARC® DC to DC charger with solar panel input.

To reduce the compressor surge current to where a 2,000 watt inverter can reliably start the compressor,  I’m also installing a SoftStartRV™ controller into a 15kBTU Dometic Penguin II air conditioner for this test. As many of you have found, this technology allows you to reliably start a rooftop air conditioner on on a 2,000-watt generator or inverter. See the graph below.

And even if you do install a 3,000-watt inverter for your RV, the SoftStartRV™ controller will reduce the compressor starting surge from 52 amps down to 24 amps, which allows you to run more electrical gadgets at the same time without manual load shedding.

This technology  also useful on a 30-amp campground pedestal that could already be loaded to the max. I think that reducing your air conditioner starting surge current is just being a good neighbor in a campground that’s struggling to supply enough power to all the other power-hungry RVs out there.

Once again I’ll use the CarGenerator™ Hybrid Inverter for the initial experiment, but since Xantrex provided me with a Freedom XC-2000 pure-sine inverter I’ll also include that inverter for this test in the next few weeks.

And I’m setting this up with 200 watts of Xantrex solar panels for a start, and I’m asking a few other manufacturers for loaner products as well. But for now, 200 watts is a good start to gather recharging data.

Again, everything I’m doing in my Funkstown Skunkworks lab is scalable, so if you want (and can afford) to install 2,000 watts of solar panels, then just multiply or divide my results by 10 to find capacity or charging time. Yes, it’s really pretty simple once you understand ALL of the variables.

This is sort of bleeding edge technology for now, but all the numbers I’m gathering are relevant no matter what new battery or inverter technology comes along. And I’m also going to do a test on a mini-split air conditioner soon which has an inverter compressor that shouldn’t have any startup surge current at all. But if you’re one of the 10+ million RVers out there with a conventional rooftop air conditioner, then you’ll want to follow along very closely.

That’s it for now. Yes, my Funkstown Skunkworks lab is really hummin’, so stay tuned for more data and reports.

Let’s play safe out there….

Mike Sokol is an electrical and professional sound expert with 50+ years in the industry. His excellent book RV Electrical Safety is available at For more info on Mike’s qualifications as an electrical expert, click here.

For information on how to support RVelectricity and No~Shock~Zone articles, seminars and videos, please click the I Like Mike Campaign.



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Cal Wing
2 years ago

Hi Mike,
I’ve used surge protectors and battery backup power supplies for all of my computers since moving into our house in 1998 because our neighbors warned us about the dirty power issues and frequent outages.
I just put an 1998 Onan 6.5kw generator with only 100 hours on the clock in my 2014 fifth wheel.
In order to provide clean power to the pc, tv’s, microwave, ect. while running the generator can’t I plug in the same type of battery backup power supply in line to clean up the generator power? I could plug one to supply the televisions and another that supplies the microwave and computer station.
Wouldn’t this be a much less expensive route for clean generator power than buying an inverter generator that’s doing basically the same thing?
Now, tell me the error in my thinking 🧐😂

Larry H Lee
2 years ago
Reply to  Cal Wing

My Onan diesel powered 8000 watt generator has the inverter built in and puts out clean sine-wave electricity. It was made in 2013.

2 years ago

Replacment cooling units with 12 volt Danfoss compressors are now available. I had to replace my cooling unit on my Norcold 1210 three years ago with an Dutch Air absorption model but would definitely have opted for the 12 volt compressor model if they had been available at the time. Cost less too.

2 years ago

Hey Mike thanks so much for all your work.

Is there any chance you could do a short paragraph summarizing your findings each time more oriented to us dummies who have to be reminded every time what Amps are and just want to know if this thing is good or bad?

I have tremendous respect for you but I just get lost so quickly. Thanks

2 years ago

Outstanding Mike.

Do you see ANYTHING that would technically justify Autoformers being banned for safety reasons?

I have a 50 AMP rig and doubt if I ever use more than 25 AMPS.

Dave Helgeson
2 years ago

Mike, Love the term “manual load shedding”! I will have to add that to my vocabulary.

2 years ago

Looking forward to the mini-split testing. I think with the efficiency and quietness of those units, they may be the wave of the future someday. I’ve read of a few conversions with good results. Thanks for all that you do!

2 years ago

Funkstown Skunkworks. Love it. Thank you for all of your research.

Thomas D
2 years ago

Make that at least 2.
I noticed you have 4 meters going for the test. But they are all different. Maybe if you had all 4 the same brand and model. I know thats asking a lot of suppliers. I’ve always dismissed the “tenth of” digital is so sensitive.

mike henrich
2 years ago

Love this research. There is a typo in the following paragraph….

“Here it’s stepping up the 114 volts from the pedestal to 112.5 volts feeding the RV (and air conditioner). And while it does boost the voltage feeding the RV, at normal voltages it also increases the amperage draw on the pedestal up to 15.9 amperes side following Ohm’s law.”

Probably should be 122.5 volts. Curious to know why the amperage would raise with higher voltage.

Last edited 2 years ago by mike henrich

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