Dometic 2852

[JPalky]

I would like to power my fifth wheel Dometic refrigerator (DM2852) while driving, but don't want to use LP gas.  An inverter is the solution!  I would like to use a portable power station (600w / 1800 surge), by plugging it into a 12v cigarette adapter located within the trailer, while also plugging the 120v power cord from the refrigerator into the inverter.  My thought is, while hooked up to the tow vehicle, the RV (12v )battery would be receiving a charge from the tow vehicle, the inverter would be receiving a charge from the RV battery, and the refrigerator would be receiving power from the inverter.  This would allow me to travel several hours with the refrigerator running while not depleting the inverter battery.  I believe the refrigerator uses approx. 325 running watts with an approx. start up surge of 975 watts.  Am I on the right track with my thoughts?

Dan

[Daveh]

It certainly can be done. I think you will overdraw the cigarette lighter circuit and blow the fuse. The cigarette sockets  usually are rated for 15 amps or less. You will be pulling about 25 amps with a much higher surge. Do you have the battery pack size to support this?

[Kirk W]

55 minutes ago, Daveh said:

The cigarette sockets  usually are rated for 15 amps or less.

The other thing to look at is the size of wire that supplies 12V power to the battery from the tow vehicle. Most of the wires as small enough that they have a significant voltage drop when you attempt to supply too much current. The typical RV refrigerator will draw between 2a & 3a on 120V which requires roughly 10 times that amount from the 12V system. If your wire size from the tow vehicle limits you to 15a it will mean that you will run the RV battery down. 

[D&J]

Your refrigerator control's already runs on 12V but the 325 watt heater runs off of 120V AC so all you have to do is get it plugged into the inverter and the refrigerator controls will recognize that the heater has 120V and run off the heater. 

You can see where the 120V heater plugs into a receptical in the back of the unit.

Denny

Edited April 29, 2021 by D&J

[oldjohnt]

JP, I like to power my RV fridge with 120 VAC when possible to preserve propane, plus I feel safer not using propane when driving. So lets do a bit of engineering and look at the numbers.

1) For an easy example lets just use 300 Watts (2.5 Amps) at 120 Volts to power the fridge (when its drawing power of course). To get that via an Inverter I use around 11 to 1 to account for inefficiency and heat losses, so the battery would have to supply around  11 x 2.5 or 27.5 amps which equals 330 Watts.

2) Some of those lighter duty 12 Volt Power Outlet cigarette lighter type are only rated at 100 or 200 watts but you need like 350 Watts to power the fridge via an Inverter I WOULD SUGGEST A HARD WIRED INVERTER INSTEAD OF ANY CIGARETTE LIGHER UNITS !!!!!!!!!!!!!!! Something like a 400 Watt (or bigger to allow for expansion and other uses) MINIMUM  Pure Sine Wave hard wired is what I recommend.  

3) So when driving lets use 6 hours, if your power source or battery or whatever energy storage device you use needed to run the fridge constantly (even though sure subject to temp etc it may not run constantly) that's 27.5 Amps for 6 hours or 165 Amp Hours at 12 Volts of energy you would use, and then when you stop you need to recharge it !!!!!!!!! Sooooooo without any recharging while driving 6 hours be sure and have a big enough energy storage unit (a battery) to run the fridge without draining the battery over the recommended levels (50% for Lead Acid or 70% to 80% or more for Lithium) .  

4) POWERING OR RECHARGING WHILE DRIVING: One thing you could do is use the engines alternator to supply 27.5 amps back to the RV battery to keep up with the power the fridge uses via the 400 Inverter. How you gonna do that???? I would suggest a DC to DC Smart regulated type of Charger say a 30 amp or more that uses the engines alternator to supply (Smart regulated) charge back to a trailer battery without possibly overloading the  alternator. As long as your alternator is sufficient rated and the DC to DC is suited to smart charge your house battery, that's one method to power the fridge without the need for an aux power unit which is basically just another battery.

5) NOTE If you need to run 30 or more amps from the truck back to the trailer Line Voltage Drop becomes an issue and it depends on current, wire size and distance sooooooo you may be looking at 8 Gauge Minimum  or even 6 Gauge and THIS IS NOT something you want to do using the standard 7 pin round RV plug. This is more like high current say 50 amp connectors.  

6) If you had an Inverter at the truck and could run 120 VAC back NOW THAT TAKES MUCH LESS (only 1/11 as much)  CURRENT (still needs its own system not the RV plug) but I would prefer an Inverter in the trailer myself 

7) Sure you could supply lesser charge say only 10 to 15 amps back and not need such big equipment so long as you dont discharge the trailer battery too much but you still need to recharge. if you prefer that approach I can run the numbers let me know.....

DISCLAIMER there are other methods to do this and I dont have time to cover each and every possibility and the above contains some NON PERFECT NON EXACT numbers and approximations, This is ONLY  guideline noooooooo warranty and not allllllllll out there 

 

 John T  Too darn long and retired electrical engineer and rusty so no warranty

[JPalky]

Thank you everyone for the detailed responses.

I was trying to figure out the cheapest and easiest way to keep my fridge running while traveling. I realize installing a large enough inverter into the trailer with adding the necessary batteries to power it, would be the most optimal and efficient way, but there is a much larger cost associated with that, compared to purchasing a portable power inverter.  I guess I'll suck it up and install an inverter.

Again, I really appreciate all of the input. 

Safe travels,

Dan    

[Chad Heiser]

A 600 watt power station only has 600 watts of total power available.  It will only run your fridge for a short time.  Even assuming no line or heat losses, you are looking at less than 2 hrs of run time for the fridge before the power station is completely dead.  A cigarette lighter plug in for the power station will not provide enough power to keep the power station charged with that kind of draw on it.  As John T stated, your best bet is a dedicated, hardwired inverter.  Even with that, you will need a significant battery bank to supply the power needed to keep your RV fridge running on 120 volt for any significant amount of time.  The tow vehicle will not be able to provide enough current through the standard 7 pin connector to keep up with that usage either.  As it seems you have discovered, there is definitely more needed than just a small powerstation to do what you would like to do.  Simply running the fridge on LP while traveling would be the most cost effective method as no modifications are needed and the system was designed to do just that.  Your only cost would be buying propane.

[oldjohnt]

1 hour ago, JPalky said:

I was trying to figure out the cheapest and easiest way to keep my fridge running while traveling. I realize installing a large enough inverter into the trailer with adding the necessary batteries to power it, would be the most optimal and efficient way, but there is a much larger cost associated with that, compared to purchasing a portable power inverter.  I guess I'll suck it up and install an inverter.

FIRST purchasing a "portable power inverter" is fine and yes they work OTHER THEN how much energy does its internal battery store ?? Is it enough to run the fridge very long (probably NOT much)  

JP, a power station is nothing more then a battery but coupled with say USB or 12 VDC outputs plus a built in Inverter so you have 120 VAC also HOWEVER its the total Amp Hours of energy storage you need to power a fridge for x amount if time and many of them just aren't all that big as far as Amp Hours of energy storage or the Inverters power rating. Again to run that fridge via battery and inverter power you're gonna be drawing 27.5 Amps from your house batteries or power station battery and 6 hours at 27.5 Amps = 165 Amp Hours.

Soooooooooo your idea to install an Inverter (I recommend at least  400 Watts) sounds good to me, just remember you need a way to keep those batteries charged, such as Solar or a DC to DC Charger powered by the engines alternator or running an auxiliary genset. Of course the CHEAPEST way to run it and NOT have to buy anything is to use propane but that defeats the whole purpose of your question. 

PS to give you an idea, if your trailer had say four 6 Volt Deep Cycle lead Acid Trojan Gold cart batteries wired in series/parallel, that's 450 Amp Hours of which 225 is useable, and that could provide the 165 Amp Hours needed to power the fridge for 6 hours, but they need recharged. Again a 30 Amp DC to DC charger (powered by engines alternator) would be a reasonable method to maintain charge when driving ???????   To do what you want you're gonna need a 400 Watt Inverter and either adequate battery capacity or a method to keep your batteries charged PERIOD.

You're asking good questions, that's great, Im confident you will figure this out

Best wishes

John T   

[Kirk W]

2 hours ago, JPalky said:

I was trying to figure out the cheapest and easiest way to keep my fridge running while traveling.

Now you probably realize the reason that so many people operate the RV refrigerator on propane while traveling. There is some additional risk, but it is pretty small. The other thing to consider is that an RV refrigerator will stay cold enough for preservation of both frozen and chilled food for as long as most of us travel, especially if you do not open the door at all until you arrive at your destination. That may be your best solution. 

[D&J]

4 hours ago, JPalky said:

Thank you everyone for the detailed responses.

I was trying to figure out the cheapest and easiest way to keep my fridge running while traveling. 

The easiest and cheapest is let it run on propane, we started Rving in 1978 and we have always run ours on propane when traveling. We do run a inverter for our chest freezer when traveling but that draws less than the cartridge heater in your rv refrigerator. The inverter is located above the batteries and a I installed a dedicated receptical for the freezer, the truck keeps the batteries charged when on the road.

Denny

[hemsteadc]

Propane is already there and real easy.  12v socket will not work.  I don't think the refer heaters have any startup surge.