Residential fridge powered while towing?

[JBill9694]

Hi John,

There seems to be an "assumption" by Norcold that there will be enough battery reserve to accomplish this. You might want to do some basic calculations to determine how long you can expect your existing battery (bank?) power to last.

howson Howard or TucsonJim Jim can probably provide the best estimate of what the fridge will require in Ah, from the research that they have done.

Rob

It has been a while since I've done any electrical calculations so some help would be appreciated, but first some ancillary information: Through an unplanned experiment (convertor breaker was accidently thrown to the off position) we know that we can run off our two 12-volt Interstate wet batteries for roughly 48 hours before the refer starts beeping. This was in a parked situation with normal opening and shutting of the doors to the refer as well as use of the lights and other features powered directly from the batteries or through the inverter. We typically limit or travel to 7 hours or less and hook up to land power at night. I'm not particularly worried about draining the batteries down during travel or the batteries not having time to recharge before we take off again.

With that said, the installation manual for the refer does not list the AC draw, but it is on a 15 amp protected circuit which may power other outlets. For DC, the installation manual says to not use more than a 6 amp fuse. The wiring diagram in the owner's manual calls out a 5 amp fuse on the DC line and an 8 amp fuse on the AC line. So you sparky types can chime in here.

As for operating on propane, that gets into whether or not to travel with the bottles open which seems to be a completely different thread (rabbit hole) to go down. I'll just leave it at I'm not comfortable doing that.

[ncitro]

It has been a while since I've done any electrical calculations so some help would be appreciated, but first some ancillary information: Through an unplanned experiment (convertor breaker was accidently thrown to the off position) we know that we can run off our two 12-volt Interstate wet batteries for roughly 48 hours before the refer starts beeping. This was in a parked situation with normal opening and shutting of the doors to the refer as well as use of the lights and other features powered directly from the batteries or through the inverter. We typically limit or travel to 7 hours or less and hook up to land power at night. I'm not particularly worried about draining the batteries down during travel or the batteries not having time to recharge before we take off again.

With that said, the installation manual for the refer does not list the AC draw, but it is on a 15 amp protected circuit which may power other outlets. For DC, the installation manual says to not use more than a 6 amp fuse. The wiring diagram in the owner's manual calls out a 5 amp fuse on the DC line and an 8 amp fuse on the AC line. So you sparky types can chime in here.

As for operating on propane, that gets into whether or not to travel with the bottles open which seems to be a completely different thread (rabbit hole) to go down. I'll just leave it at I'm not comfortable doing that.

The fridge uses very little DC power when run on AC or propane, basically just to control the logic board, the gas valve, and the igniter. If everything was off in the rig except the fridge and you were operating on propane I would expect it to run for a very long time with no issue. If you are looking to operate the fridge off of propane while driving that opens up a whole other can of worms. This is the rabbit hole howson has gone down. As you stated earlier a figure of 11x use for the inverter to make AC from DC is a good estimate. Lets figure the fridge uses 10 amps to make the math easy (get yourself a kill a watt device that goes between the fridge plug and the outlet if you want to know for sure). 10 Amps AC would equate to 110 amps DC. Its been a while since I had 12v lead acid batteries, but if you have two I think they run around 105 amp hour each. You do not want to deplete them past 50%, so you have about 100 amp hours usable battery. The fridge will have a "duty cycle" which basically means it does not run all the time. It cycles on and off as the temperature rises in the fridge. A good estimate here is 50%, so you could estimate you will use 55 amps of DC per hour. That means with two 105 amp hour batteries you can run the fridge for about two hours. You will get 10 amps per hour charging from the truck, so maybe an extra ten or fifteen minutes of use. And you are going to begin to see the issue of running an absorption fridge off of an inverter, and why the charging we get out of our trucks seems so paltry.

Your options are either; add a insane amount of batteries, increase the charging from the truck (@howson has a write up on this), or add solar panels (becomes an issue in rain or clouds). I went with option 3, but also switched to 4 lithium batteries to give me enough power for a day of towing in rain or clouds. None of these options are cheap, obviously the cheapest option is to run it on propane, but if you are not comfortable with that then you are left with the other options.

[howson]

It has been a while since I've done any electrical calculations so some help would be appreciated, but first some ancillary information: Through an unplanned experiment (convertor breaker was accidently thrown to the off position) we know that we can run off our two 12-volt Interstate wet batteries for roughly 48 hours before the refer starts beeping. This was in a parked situation with normal opening and shutting of the doors to the refer as well as use of the lights and other features powered directly from the batteries or through the inverter. We typically limit or travel to 7 hours or less and hook up to land power at night. I'm not particularly worried about draining the batteries down during travel or the batteries not having time to recharge before we take off again.

With that said, the installation manual for the refer does not list the AC draw, but it is on a 15 amp protected circuit which may power other outlets. For DC, the installation manual says to not use more than a 6 amp fuse. The wiring diagram in the owner's manual calls out a 5 amp fuse on the DC line and an 8 amp fuse on the AC line. So you sparky types can chime in here.

As for operating on propane, that gets into whether or not to travel with the bottles open which seems to be a completely different thread (rabbit hole) to go down. I'll just leave it at I'm not comfortable doing that.

Edit: ncitro beat me in responding! The info below was after his but we are in complete agreement.

John,
To ensure we're on the same page, can you confirm you have a Norcold 2118 refrigerator?

IF it is the Norcold and you went "48 hours before the refer starts beeping" the only way that happened is 1) the refrigerator's heating element was powered from 120vAC or 2) the refrigerator was cooled using propane.

Forgive me if you know this already, but it is critical to understand how your refrigerator is powered. There are three power inputs:

1) 12vDC (from battery or Converter): The only items powered by 12vDC are the control circuit, display, and fans (and possibly other minor items).
2) 120vAC (from shore power or an inverter): This is one of two options to power the "power sucker" in the refrigerator--the heating element.
3) Propane: This is the other way to activate (heat) the absorption process in the refrigerator.

There's no chance--none--that two wet cell batteries lasted 48 hours powering the 120vAC heating element of an RV refrigerator through an inverter in a normal use scenario. (I'm sure someone can dream up an extreme outlier.)

I have four 100aH Battle Born batteries. I've tracked consumption from my Dometic 12cuft RV refrigerator over many, many days of towing. The maximum I could get was a little over 10 hours based on using an average of 30aH. (That means "30 amps per hour consumed from the 400 total I started with in the batteries". So 400/30=13 hours, but I won't run my batteries to 0. Think of it like using 30 gallons of water per hour from a tank that holds 400 gallons.) My spreadsheet is below.

Based on your experience I expect you'll probably initially reject this response, but I urge you to read through this info again before doing so. I'm absolutely certain in the "48 hour" situation you mentioned in your post that the refrigerator's heating element was powered by 120vAC. (I bet your microwave and TV worked, right? Then you were on shore power!)

Fire away with questions. I've spent countless hours on the issue of powering an RV refrigerator on 120vAC while towing.

Howard

[JBill9694]

Thanks for the input, howson and ncitro. Howard is correct that we were docked when we had our unplanned experiment, so my previous post revealed my ignorance of how the Norcold 2118 operates. Hopefully I'm not the only one and others will benefit from the information that you have provided.

We parked until early May so I have some time to review the information before I have to make a decision.

[ncitro]

Thanks for the input, howson and ncitro. Howard is correct that we were docked when we had our unplanned experiment, so my previous post revealed my ignorance of how the Norcold 2118 operates. Hopefully I'm not the only one and others will benefit from the information that you have provided.

We parked until early May so I have some time to review the information before I have to make a decision.

Yes, I reread your post now and realize that if you were plugged in, but the converter was off then the fridge would be running its DC circuits off the batteries (very little draw), but the heating element in the fridge (the BIG draw) would be running off of the AC shore power. That explains your long battery life.

[Cate&Rob]

Interesting collaborative discussion on this.

Something that I found interesting in the data posted by Howard howson in post 16 . . . dividing the Ah used numbers by the total hours numbers results in about 30Ah consumed per hour.

Doing a little digging on the Norcold (Thetford) Model 2118 parts list finds electric heater PN 630811 which is a 300W 120V heating element. (This s a common size heating element even for smaller fridges BTW) 300W requires 2.5A at 120VAC. Through an inverter, this is going to draw about 30A at 12VDC . . . or, about what Howard's fridge is drawing continuously. This means that the fridge is cooling on a 100% duty cycle. Not the 50% duty cycle that we often assume for these calculations.

Howard . . . you thoughts ??

Rob

[howson]

Howard . . . you thoughts ??

The insulation in these RV refrigerators is less than ideal. That's what I think.

Also keep in mind the conditions in my RV while traveling--no air conditioning on so the inside of the RV was probably hotter than the outside ambient temperature. This surely didn't help the refrigerator's duty cycle.

JBill9694 & Bill M
Given what I know now from actual hands-on experience, if the goal is to power the refrigerator with 120vAC while towing, the Renogy 40A DC-DC Charger connected to a 2000W inverter and two Battle Born (or equivalent) batteries is the ticket.

Converting to a residential refrigerator will also extend your tow time, though you'll lose the ability to run on propane.

If boondocking is desired, scale up the number of batteries until you run out of wallet (!) or storage space in your trailer. I'd also go with a 3000W inverter, too, and AMSolar's Smart Phase Selector. Then consider adding solar.

I've posted lots of information on these topics. Best to start with the diagram in this thread: https://gdrvowners.com/forum/operati...lts-f-350-mods. There's also link in that thread to most of the other threads related to this topic.

Please don't hesitate to ask questions. There are many folks here who've helped me over the years get to this point!

Howard

[Cate&Rob]

The insulation in these RV refrigerators is less than ideal. That's what I think.

Also keep in mind the conditions in my RV while traveling--no air conditioning on so the inside of the RV was probably hotter than the outside ambient temperature. This surely didn't help the refrigerator's duty cycle.

Hi Howard,

I completely agree with both of these considerations. I can confirm that a similar size, 120VAC only, residential compressor fridge in my boat runs about a 50% duty cycle when fully cooled down but at summer ambients. I believe that this is because it is much better insulated than the fridge on the RV.

The point of my earlier calculation/observation is that the data so far would indicate that the battery capacity to run a standard RV adsorption fridge on 120VAC through an inverter should be calculated at a 100% fridge duty cycle because of less efficient energy use and because of poor insulation. The battery capacity to run a residential compressor fridge can be reduced towards a 50% duty cycle . . . because of both the energy efficiency of the compressor and the fridge likely has better door sealing and insulation.

Rob

[ncitro]

The insulation in these RV refrigerators is less than ideal. That's what I think.

Also keep in mind the conditions in my RV while traveling--no air conditioning on so the inside of the RV was probably hotter than the outside ambient temperature. This surely didn't help the refrigerator's duty cycle.

Howard

Understatement of the year. I could not believe how light the Dometic four door unit I removed was. I would say there was hardly any insulation to speak of.

[Cate&Rob]

Understatement of the year. I could not believe how light the Dometic four door unit I removed was. I would say there was hardly any insulation to speak of.

Hi Neil,

I'm not convinced that weight is that much different between RV adsorption and residential compressor fridges. What does seem to change is outside vs inside dimensions. To our collective point that residential fridges have thicker walls and doors. Insulation doesn't weigh much, but it takes space. The online specs for a Norcold 2118 include a unit weight of about 230 lbs. (Slightly different weights with different door coverings). An 18 cu ft Frigidaire (random comparison residential fridge) is 210 lbs. The point being that if someone were to replace their adsorption fridge with a same external size compressor fridge they would probably give up interior volume but weight would be about the same.

Rob

[wygieman]

I just finished the conversion of my Dometic 1350 4-door refrigerator to use the JC Refrigeration DC compressor unit. The insulation was actually better than I expected. The side and top panels were vacuum panels which is actually actually as good as it gets I think. The back is 2.5” of foam and cooling plates for over half of it. I did take the opportunity to fill the empty space in the cabinet with blue board insulation foam. 3” on top, 2 “ on the outboard side and 1” on the inside. The rv units also dump the heat from the refrigeration outside through the back vents while the residential units dumps the heat into the rv. The refer draws 8.5 amps dc while running which can be supplied by the truck on the road.

[ncitro]

Hi Neil,

I'm not convinced that weight is that much different between RV adsorption and residential compressor fridges. What does seem to change is outside vs inside dimensions. To our collective point that residential fridges have thicker walls and doors. Insulation doesn't weigh much, but it takes space. The online specs for a Norcold 2118 include a unit weight of about 230 lbs. (Slightly different weights with different door coverings). An 18 cu ft Frigidaire (random comparison residential fridge) is 210 lbs. The point being that if someone were to replace their adsorption fridge with a same external size compressor fridge they would probably give up interior volume but weight would be about the same.

Rob

That’s an interesting thought. My residential was quite a bit heavier, but it also was larger, and had more door shelving, which maybe made up the different. Walls were definitely thicker as you say.

[howson]

I just finished the conversion of my Dometic 1350 4-door refrigerator to use the JC Refrigeration DC compressor unit. The insulation was actually better than I expected. The side and top panels were vacuum panels which is actually actually as good as it gets I think. The back is 2.5” of foam and cooling plates for over half of it. I did take the opportunity to fill the empty space in the cabinet with blue board insulation foam. 3” on top, 2 “ on the outboard side and 1” on the inside. The rv units also dump the heat from the refrigeration outside through the back vents while the residential units dumps the heat into the rv. The refer draws 8.5 amps dc while running which can be supplied by the truck on the road.

Did you document the conversion and process? There's quite a few folks that would like to see a thread like that posted here. (Please--start a new thread if you have the info!)

[wygieman]

I haven’t posted anything on the installation yet but I can. I only finished a couple of weeks ago and wanted to get some more experience with it. I can post an overview of the process and answer questions if that is of interest.

[howson]

I haven’t posted anything on the installation yet but I can. I only finished a couple of weeks ago and wanted to get some more experience with it. I can post an overview of the process and answer questions if that is of interest.

Absolutely! I know it's a lot of work to fashion a mod thread, but when you get a chance I'm positive there will be a lot of interest. Again, please make it a new thread so it doesn't conflict with this one, OK? (TIA!)