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Old 03-10-2020, 02:21 PM   #1
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Boondocking with a compressor fridge

I have years of experience camping with no hookups in a small travel trailer with an absorption fridge. I use about 10 amp hours each day with no furnace or fan use and maybe a few more Ahs for the fridge controls when on and quite a bit when I run the furnace which is rarely. I also have twenty years of experience (2 of them full time) boating with a compressor fridge.

I am considering an Interstate RV with a compressor fridge. My Ah budget ex fridge will probably be higher than my trailer- TV watching when decent cell data is available and just the base loads from all of those electronics in a new Interstate. So call it 30 Ahs.
Finding the amperage draw of a compressor fridge is difficult. Anyone know the specific make and model in the new Interstate? But my boating experience indicates that they will draw between 3 and 6 amps when on and run at a cycle time of 40% to 100%, the latter being when it is 95 inside. So lets call it 4.5 amps at a 60% cycle time or a total amp hour draw over 24 hours of 65 amp hours. That means that our use with no hookups in a mild climate will total about 100 Ah which is about what we used on our boat.

So how do you supply that 100 Ahs and replace it daily?

Not sure exactly what comes from the factory, but I have learned (I think) that there is room for two 6V 220 Ah batteries which when wired in series gives 220 Ahs at 12V. Since you don't want to run lead acid batteries below 50% that means you will have to charge daily and replace 100 Ahs. I am going to ignore Li batteries for the moment.

Many newer Interstates come with 200 watt of solar panels. These panels can be expected to produce 60-80 Ahs on a full sunny day. So between cloudy days and the difference between 60-80 and 100 you will need to do more charging. So for sake of further discussion lets assume you need to replace 60 Ahs daily on average beyond the solar contribution. And of course that depends on parking in sunny campsites which we rarely do.

So 60 Ahs in a sunny campsite and 100 Ahs in a shady one needs to come from the generator/converter.

The OEM converter is rather small, 45 amps typically and may be wired with minimum wire size which will produce quite a voltage drop. So lets assume that you upgrade the converter to 75 amps and with large wire to keep the voltage drop down to a few tenths of a volt.

You will be charging for about .8 hours with solar or 1.2 without using the generator. The generator uses about .4 gph at half load which is all you will pull running just the converter. So you will use 0.3 gallons or .5 gallons of LPG.

You can improve these values somewhat by having a larger Li bank and/or a larger bank of solar panels. 300 watts of solar and 200 Ah of Li (which doesn't have the depth of discharge restriction or the tail end restriction like lead acid) will probably cover your use entirely with minimal genset running time. Even with no solar or in a deep shade spot the ability of Li to charge up near 100% quickly only adds less than a half hour of genset running time and a few tenths of a gallon of LPG.

So what are my plans. I think I will start with the OEM 200 watts and upgrade the batteries to Battleborn LiFePO4 with the upgraded converter so I can discharge further but also recharge fully in a reasonable time.

I will have to watch my propane usage and refill the 15 gallon tank at least every two weeks, maybe more if we are using the stove significantly and are in shade.

OK, I realize that was a lot of data and math. I would welcome others experiences with managing a compressor fridge in an Interstatoe.

How does the propane usage running the genset to replace 100 Ahs stack up against an absorption fridge? Compressor fridges are pretty efficient but propane gensets are horrible. My guess is that it takes much more propane to run the genset to power the compressor fridge than to use it directly in an absorption fridge. This is balanced by the fact that the compressor fridge can use solar power efficiently where the absorption fridge cannot (well it can but it uses a huge amount of DC amps).

David
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Old 03-10-2020, 03:12 PM   #2
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Also, can anyone tell us how many amps the OEM chassis alternator will charge a well discharged coach battery bank at moderate, ie 1,500 rpms.


My TV will only charge my trailer at about 10-15 amps due to small wiring from the TV's alternator all the way back to the trailer's house battery. The charging circuit is rated at 30 amps. The small wiring is intentional to limit the current. I would expect that an integrated chassis/coach alternator/battery system would not have these limitations.



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Old 03-10-2020, 03:23 PM   #3
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Duh! What is an absorption fridge? Want is a compressor fridge?
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Old 03-10-2020, 03:36 PM   #4
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Duh! What is an absorption fridge? Want is a compressor fridge?
Absorption fridge runs on propane or electric. Compressor fridge runs on electric only. There are other differences, of course, but that is the most important distinction for this case.
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Old 03-10-2020, 03:36 PM   #5
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Duh! What is an absorption fridge? Want is a compressor fridge?
Absorption fridge is the ammonia based RV fridge that operates on electricity or propane

Compressor is what you find in homes and works on electricity only

On some RVs, Airstream is using compressor refrigerators. While these have a much lower power usage than the ones in homes, it will run the battery(s) down over time.
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Old 03-10-2020, 04:15 PM   #6
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I converted my absorption frig (krefft) to compressor with a kit from IsoTherm.

Last fall I did a consumption test. The details are on post #64, page 4 of our thread. Here's the numbers I got.

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The frig was empty and with temps in the low 60's it was not an extreme test, but gives me a baseline to compare to as it gets hot this summer.
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Old 03-10-2020, 04:53 PM   #7
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I don't understand all of your data items, but one item should be simple: cummulative KWh. So for 12.5 hours the fridge/freezer used .91 KWhr or 910 watt hours.

I don't understand the approximately 50% power factor. Were you measuring at the compressor's terminals? If it is a Danfoss, it is an inverter compressor and runs at 200 or so Hz AC. So should I multiply 910 WHrs by .535 and divide by 12 to get DC amp hours used.

If I do this I get 910*.535/12= 40 amp hours, which is in the higher end of what I would expect. But could easily be explained by your near freezing fridge temps.

Thanks for the data.

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Old 03-10-2020, 05:22 PM   #8
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Originally Posted by DavidEM View Post
How does the propane usage running the genset to replace 100 Ahs stack up against an absorption fridge? Compressor fridges are pretty efficient but propane gensets are horrible. My guess is that it takes much more propane to run the genset to power the compressor fridge than to use it directly in an absorption fridge. This is balanced by the fact that the compressor fridge can use solar power efficiently where the absorption fridge cannot (well it can but it uses a huge amount of DC amps).
My Interstate has a 3.1cf Nova Kool refrigerator/freezer that runs on either 12vDC or 120vAC. The way it's hooked up, if there's ANY source of 120vAC (generator, inverter, or shore power) it will run on 120vAC. Only if the breaker is off or no source of 120vAC is present will the fridge run on 12vDC.

Why is this important? Because the fridge itself includes a converter to turn 120vAC to 12vDC. Which means that if you're running on the inverter, you're turning 12vDC to 120vAC through the inverter, and the fridge is converting the 120vAC back to 12vDC. Which is very wasteful.

For boondocking in an Interstate, you may consider unplugging the 120vAC cord for the fridge, thereby forcing it to run on 12vDC all the time. The amount of electricity you save might be small, but when boondocking, every little bit helps.
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Old 03-10-2020, 08:31 PM   #9
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Why is this important? Because the fridge itself includes a converter to turn 120vAC to 12vDC. Which means that if you're running on the inverter, you're turning 12vDC to 120vAC through the inverter, and the fridge is converting the 120vAC back to 12vDC. Which is very wasteful.

For boondocking in an Interstate, you may consider unplugging the 120vAC cord for the fridge, thereby forcing it to run on 12vDC all the time. The amount of electricity you save might be small, but when boondocking, every little bit helps.
My 2018 runs only on 12v. No AC input. That is precisely why I have set up my Goal Zero 1000 to supply 12v directly to the 12v fuse string. If I hooked it up to output 120v like a lot of folks are doing, there would be a conversion to 120v AC in the Goal Zero and a conversion back to 12v in the inverter/charger.

Conversions are lossy. I can see some conversion loss in the Goal Zero display when I turn on the inverter, even without a load. Down conversions from 120v AC to 12v DC may be as poor as 75% efficient. Using 120v from a Goal Zero is just throwing power away.

I can monitor the amount of power I am using from the Goal Zero and my 12v hookup is very efficient.
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Old 03-10-2020, 10:17 PM   #10
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David the frig was running on shore power. The control system defaults to 120v when present. The control steps that voltage down to 12 volts. The data is pulled at the 120v plugin.
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Old 03-11-2020, 04:56 AM   #11
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David the frig was running on shore power. The control system defaults to 120v when present. The control steps that voltage down to 12 volts. The data is pulled at the 120v plugin.


Ahhh! So that .535 power factor is all from the inverter. Ok so inverters are about 80-90% efficient (on an RMS power basis) so the actual DC amphours you are using for the compressor fridge is higher- 910*.85/12 = 64 in 12 1/2 hours. That is pretty high.


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Old 03-11-2020, 06:33 AM   #12
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Originally Posted by DavidEM View Post
I have years of experience camping with no hookups in a small travel trailer with an absorption fridge. ...

I am considering an Interstate RV with a compressor fridge. ...

So how do you supply that 100 Ahs and replace it daily?

Not sure exactly what comes from the factory, but I have learned (I think) that there is room for two 6V 220 Ah batteries which when wired in series gives 220 Ahs at 12V. ..... I am going to ignore Li batteries for the moment.

Many newer Interstates come with 200 watt of solar panels. ....

David
Short, non-mathematical version:

Most people who are serious about boondocking upgrade their electrical systems. Husband and I have a 2007 Interstate which we upgraded with 300 AH lithium, 300 watt solar, and 2,000 watt inverter, and a Vitrifrigo marine refrigerator (Danfoss compressor - see installation description here) which we installed after TWO absorption fridges failed on us. We actually installed four-way charging capability (solar, generator, alternator, and shore), but solar takes care of the daily burden which is dominated by the Vitrifrigo. Which is really neat actually - it's a "set it and forget it" thing. I can go off-grid for four weeks at a time, and barely glance at our BMS.

Cross-posting this recent chart that I made for Class B Forums, just so you can see the types of systems used by serious boondockers:

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Old 03-11-2020, 08:17 AM   #13
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That is a very interesting chart. I presume it is a mix of compressor and absorption refrigerator users.

I was particularly interested in the zero or low solar guys. Most used the chassis alternator as their primary charging source although one outlier was PJ2016 who used his generator for the majority of charging. That must mean that most of those moved quite a bit, probably every other day or so to keep up with mostly the chassis alternator.

Do you know if any of them used a Sterling or similar battery to battery type charger to enhance the standard alternator's output. I think you would need to do that to get significant amps out of your chassis alternator.

David
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Old 03-11-2020, 01:01 PM   #14
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Some fresh amperage readings

As I posted earlier, I have the 40-amp output adapter from my Goal Zero 1000 tied backward through heavy wires to previously unused fuse 15 to the input bus for 12v fuses. This allows me to feed all 12v users in the coach without conversion losses.

This morning, I took a series of readings with the GZ turned on and the master switch at the door turned off. Here are some of my readings:
  • Fridge and freezer compressors both running, nothing else - 10.5 amps
  • Fridge only, no lights - 3.6 amps
  • Fridge compressor only and propane switch on - 4.3 amps
  • Fridge compressor, propane, and all lights - 7.9 amps
  • Fridge and all lights - 7.2 amps
  • Fridge compressor and water pump - 5.3 amps
  • WH gas on - no change
  • Fridge compressor + USB fast charger to my S10 phone - 4.7 amps
  • Add 3 LED reading lights - no change

We do not plan to use the freezer during extended periods without hookups.

The AC plug is for the front TV. If I want to use the TV without hookups, an 18" extension cord will hook it to the Goal Zero.
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Old 03-11-2020, 03:38 PM   #15
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Extra 12v outputs

While I was running wires, I added a set of two Powerpole 15a outputs above the desk in my GT. I have the mini-PC mounted in that area and I will feed 12v direct to the PC rather than use the bulky plug-in 12v charger under the desk.

I sourced current from the wires serving the light in the wardrobe.

Here is the Powerpole outlet wired and ready to be installed.


I'll cut a length off an old 12v charger to plug into the PC and put Powerpole connectors on the other end.
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Old 03-12-2020, 05:29 AM   #16
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That is a very interesting chart. I presume it is a mix of compressor and absorption refrigerator users.

I was particularly interested in the zero or low solar guys. Most used the chassis alternator as their primary charging source ....

Do you know if any of them used a Sterling or similar battery to battery type charger to enhance the standard alternator's output. I think you would need to do that to get significant amps out of your chassis alternator.

David
I'm shortening and flattening this response because I'm including references to previous content regarding the questions asked.

YES, many people rely on alternators, but there are hazards with doing that, one of which is clutch pulley failures that can disable your entire van. Been there, done that.

There's a lot of resistance in the #vanlife community to this idea that a "properly" configured alternator-based charging system can torpedo a perfectly good 200 A alternator. If you doubt that conclusion, search for a 5:55 minute YouTube video by Advanced RV titled 'alternator improvements'. ARV changed their alternator design a few years ago after their clients experienced failure after failure after failure of their "properly" configured systems.

YES, those of us with "properly" designed electrical systems use the Sterling device. You can see it in the image at the end of this post. It is NOT the magic bullet that it is widely reported to be - it does not protect against overwearing and grossly-premature failure of every alternator component. I have encountered some posters (not necessarily on this forum) who were SO SURE that I could not crash an alternator if a Sterling was present that I had to send them pics of the thing as installed, with the label visible confirming that it was the correct Sterling model, before they'd even begin to believe me.

Here below is a general diagram of our DIY retrofit, and here is a longer system description (there's also my husband's lithium thread, but it's massive and contains a lot of sidebar):

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Old 03-12-2020, 06:14 AM   #17
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I think one of the points you are making is that a Sterling regulator can put increased stress on the stock alternator which leads to premature failure and I certainly agree with that.


Could you mitigate that problem by using Sterling's alternator temperature probe which cuts back output to maintain a reasonable alternator temp. I suspect that if you keep the alternator down to 180F or so it will last much longer. It is temperature that kills alternators due to sustained high output.



David
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Old 03-12-2020, 09:25 AM   #18
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I think one of the points you are making is that a Sterling regulator can put increased stress on the stock alternator which leads to premature failure and I certainly agree with that.

David

It might not be a bad idea to carry a spare alternator in that situation. If you had the ability to change it yourself that would be a real plus.

Dan
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Old 03-13-2020, 04:48 AM   #19
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InterBlog:

I think one of the points you are making is that a Sterling regulator can put increased stress on the stock alternator which leads to premature failure and I certainly agree with that.

Could you mitigate that problem by using Sterling's alternator temperature probe which cuts back output to maintain a reasonable alternator temp. I suspect that if you keep the alternator down to 180F or so it will last much longer. It is temperature that kills alternators due to sustained high output.

David
It's not the Sterling per se. It's the nature of the charging profile which causes too many start-stops.

My husband was in the process of building a logic circuit to mitigate that, but all I really wanted is the simplest possible switch plus a companion SOC read-out in the cab so that I can turn the alternator charging on and off at will for an hour here or there without having to pull over to the side of the road and walk to the control panel in the rear of the van.

But that idea offended my husband, who is an engineer, and who wanted to fully automate our system so that we would never have to manually intervene, or even look at it.

So he was in the process of building an additional circuit, too much effort for too little return IMO when my eyeballs can perform that same function at a tiny fraction of the time and energy input.

The circuit never got finished because it was OBE - overcome by events.

I just leave off alternator charging for the most part. I might activate it two or three times a year when a situation calls for it.
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Old 03-14-2020, 05:24 AM   #20
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It might not be a bad idea to carry a spare alternator in that situation. If you had the ability to change it yourself that would be a real plus.

Dan
There are two issues with that suggestion:

(1) Alternators large enough to be fit for this application cost $475 and up. It would be a nice touch if we could solve the problem without that kind of expensive redundancy.

(2) A 200 amp alternator is heavy - about 17 pounds - and it also takes up valuable cargo space. For those of us with older Sprinter 2500 vans, we are already bumping up against GVWR. Every item taken on board is scrutinized in terms of its weight cost-benefit ratio. It would be nice not to have to lug a large second alternator around.

That being said, my husband and I do have two functioning 200 A Bosch alternators, although we don't typically carry both. We had to get the second because the first failed while I was en route from Houston to Nova Scotia the year before last. We had no capacity to repair the clutch pulley on the road, especially given that we didn't know at that time what the problem was - we just knew that the alternator was failing. So we bought a replacement from Million Mile Sprinter in Philadelphia, where I had diverted while en route.

Incidentally, for those of you who are reading this and who are MMS fans, Joel and his wife just announced the birth of their 13th child (some are adopted).
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