Adding 12v heating element to RM2510 Fridge

[DougG]

So, I was poking around my Dometic 2510 fridge today and noticed that dispite being the 2 way model it has the receptacle on the flue for the 12v element next to the 120v element. Has anyone added a 12v element to their 2 way fridge?

I am not looking to restart the great debate around running propane while driving, to each their own, but for me running propane while driving simply isn’t the best practice.

We tend to do long days on the road, starting out around 9am, ending the day around 7pm, but with many short stops to eat, take pictures, etc. My though would be to run the element direct from an ignition switched power source so it only runs when the car is on. It wouldn’t have a thermostat at all, but it doesn’t seem that there is much concern of the fridge getting to cold on 12v.

Ok, open the flood gates, tell me why it won’t work.

[Brian B-P]

Originally Posted by DougG

... My though would be to run the element direct from an ignition switched power source so it only runs when the car is on.

That appears to be common practice in Europe, where the standard tug to trailer electrical connection includes a separate circuit for this purpose. On the other hand, I don't know that a separate circuit is necessary, since a good tow vehicle connection would only be powered when the engine is running (or at least the ignition is on) anyway; the two-circuit system was probably designed for trailers of an earlier era which didn't have their own batteries.

If you want to ensure that the refrigerator never runs on 12V when the battery is the only source of power, you can use a voltage-controlled (or voltage-sensing) relay, which would switch on when typical charging voltage is present (such as from a running engine or the converter) and off in the voltage range of just a battery (even fully charged). This is an example (with much higher current capacity than required; it's just the first one I found):
http://https://www.amazon.ca/Baoblaz...69263042&psc=1

[rubicon327]

Just brainstorming here. You don't mention if you already have solar or an inverter. Instead of messing with the fridge could you operate it on 120V AC by wiring to an inverter? The addition of a dc to dc charger like a Redarc or equivalent could be wired up from the tow vehicle to keep the trailer battery up. Solar would help too if you have it. I know there is some inefficiency with an inverter but it might be worth thinking about. Santiago did a dc to dc charger to run a DC compressor based Novakool fridge. He didn't have solar and an inverter wouldn't apply to his design, but it is the same principle. He didn't have the trailer very long but some of his early reports indicate it worked well. I'm sure the electrical gurus will jump in shortly...

[DougG]

Originally Posted by rubicon327

Just brainstorming here. You don't mention if you already have solar or an inverter. Instead of messing with the fridge could you operate it on 120V AC by wiring to an inverter? The addition of a dc to dc charger like a Redarc or equivalent could be wired up from the tow vehicle to keep the trailer battery up. Solar would help too if you have it. I know there is some inefficiency with an inverter but it might be worth thinking about.

Thank you for your thoughts. This was actually what I was thinking of doing until I saw the slot for the 12v element on the fridge. I don’t currently have an inverter, but no matter what I end up doing I plan to add a Dc-dc charger. I already have 4 gauge wire run to an Anderson plug at the back of my tow vehicle, ignition switched with an 80amp continuous duty solenoid. If this line ties into a dc-dc charger then it will also serve as a convenient source of ignition power at the front of the trailer. So, at the end of the day, I guess I am just looking for a way to avoid buying an inverter.

[cpaharley2008]

Does your tow vehicle have a specific power line from the front to the rear hitch? Have you measured the output at the Anderson plug in the truck bed?

[DougG]

Originally Posted by cpaharley2008

Does your tow vehicle have a specific power line from the front to the rear hitch? Have you measured the output at the Anderson plug in the truck bed?

Yes, there is a specific power line. I am not sure what you mean by measuring output. With no load it will be 13.6ish volts when the truck is running, but will of course drop when a load is applied due to the voltage drop that plagues long 12v wire runs.

[NuthatchBC]

We have a 2011 19 with the Dometic RM2510 fridge and I don't see anything that looks like a place for a 12vdc heating element. Would you please post a photo of what you're referring to?

Doug

[DougG]

Originally Posted by NuthatchBC

We have a 2011 19 with the Dometic RM2510 fridge and I don't see anything that looks like a place for a 12vdc heating element. Would you please post a photo of what you're referring to?

When looking at the back, on the right hand side, where the 120v element goes into the flue, there is a stamp in the sheet metal cover that says 110-240v where the 120v element goes in, and under it it says 12v. That is what encouraged me to investigate further. If you remove the sheet metal cover, inside there is a metal tube that the 120v element goes in, right next to it there is a second empty tube where the 12v element would go. I can’t get a good picture of those 2 tubes. But here are a couple photos in the sheet metal housing I am talking about.

[DougG]

So, I finally had some time to do a little more research and poking around on this issue. Turns out these manual domestic fridges came with the exact same switch assembly and thermostat regardless of weather they were sold as 2 way or 3 way. Just keep turning the knob passed “gas” and you will get to a blank spot as far as the knob will turn that is the 12v setting. Meaning that for all intents and purposes this should be able to function identically to a factory 3 way rm2510 All that is missing for 12v operation is the element and a bit of wiring. I ordered the element, Dometic part # 0173739012, and it fits in the slot perfect. I just have a bit or wiring to do to get it wrapped up. I will update with final results when done.

[DougG]

Couple photos of the 12v element.

[cpaharley2008]

Please report back on the 12v vs propane vs 120v operation efficiency operation of the refer...

[rubicon327]

Originally Posted by DougG

So, I finally had some time to do a little more research and poking around on this issue. Turns out these manual domestic fridges came with the exact same switch assembly and thermostat regardless of weather they were sold as 2 way or 3 way. Just keep turning the knob passed “gas” and you will get to a blank spot as far as the knob will turn that is the 12v setting. Meaning that for all intents and purposes this should be able to function identically to a factory 3 way rm2510 All that is missing for 12v operation is the element and a bit of wiring. I ordered the element, Dometic part # 0173739012, and it fits in the slot perfect. I just have a bit or wiring to do to get it wrapped up. I will update with final results when done.

Doug: I assume you saw in the install manual that the wiring must be dedicated to the heater element. Separate from existing igniter wiring. I know when Escape does it they take it to a dedicated slot in the power center. Element will draw almost 15 amps so 20 amp fuse recommended. Found the wiring and fuse recommendations in manual online. Also curious because in your picture it looks like you installed the element pointing up into the flue. In all the diagrams for 3-way fridges I have seen both elements point down. Is this model different?

Looking forward to your results. It will be interesting to see how this performs as it is 175W versus 325W for the 120V AC element. On edit: I see that 325W is for larger fridges. The 2510 AC element is 185W so on par with the DC element.

[DougG]

Originally Posted by rubicon327

Doug: I assume you saw in the install manual that the wiring must be dedicated to the heater element. Separate from existing igniter wiring. I know when Escape does it they take it to a dedicated slot in the power center. Element will draw almost 15 amps so 20 amp fuse recommended. Found the wiring and fuse recommendations in manual online. Also curious because in your picture it looks like you installed the element pointing up into the flue. In all the diagrams for 3-way fridges I have seen both elements point down. Is this model different?

Haha, it’s my photo posting skills that are upside down, not the element. Turn your screen upside down and it will make more sense.

As for the wiring. Yes, it will be a dedicated circuit run from the panel. That circuit will be relay controlled by a separated ignition switched circuit running from the tow vehicle to trailer though a separate Anderson plug. The plan is to also install a dc-dc charger that is also powered by this ignition switched circuit. 4awg right from battery to battery. And then also upgrading the battery to converter wiring to 4awg to minimize voltage drop. 6awg from converter to fridge. The point of all this heavy wiring being to reduce voltage drop and actually get 13+ volts to the fridge so it can actually cool appropriately. With the fridge circuit being ignition controlled, and the dc-dc charger providing the juice, I am hopeful that I will arrive at the camp site with fully charged batteries and cold food, and not have to run on propane going down the road.

[cpaharley2008]

Wow, a lot of forethought here, looks like you have addressed the "Seven P's" in your project. Prior, proper, planning, prevents, ####, poor, performance!

[75thRanger]

I used the DC function on my new fridge to see how it fared on a 600 mi roundtrip fishing trip.

I cooled the fridge on AC and switched to DC for travel. The DC kept it as cool as it found it after 7 hr drive.

I don't know if I will continue this but nice to know how effective it is.

[cpaharley2008]

Charles,
Please elaborate, what is your tow, solar, dual 6 batteries, outside temperature, refer setting?

[75thRanger]

Originally Posted by cpaharley2008

Charles,
Please elaborate, what is your tow, solar, dual 6 batteries, outside temperature, refer setting?

F250 Diesel, 190 Solar panel, Dual 6V Trojan AGM, about 75 outside, Fridge set to max during travel

[DougG]

So, I finally finished up this project installing both the DC fridge circuit, and a Redarc BCDC1240D DC-DC charger. I did a bit of testing, to see how much power this DC element is actually getting.
For reference, on my fridge, both the original AC element and the new DC element I installed claim to be 175w elements.

So to start with, I fully charged my batteries. I then hooked up the standard 7 pin trailer plug and fired up the truck, and then turned on the Fridge in DC. The trailer battery voltage quickly started dropping, I gave it about 10 minutes to stabilize and then took the following readings:
Battery Voltage: 12.15V
Voltage at fridge element: 11.42V
Current: 13.02A
Fridge element power: 149w

It is clear from this that the 7pin charge circuit doesn’t come anywhere near providing enough power to keep the batteries charged while running the fridge. Also, related to this, although 149w doesn’t seem terrible, I would bet this would go down as you drive and deplete the battery further.

So next I plugged in the DC-DC charger. I left the fridge running and once again gave it about 10 minutes to stabilize. At the start of the 10 minutes the DCDC was in bulk charge, but at the 10 minute mark was in absorption. I then took the following readings:

Battery Voltage: 14.8V
Voltage at fridge element: 13.82V
Current: 15.7A
Fridge element power: 217w

I also took a few measurements of what the DCDC was doing at this time:
DCDC input current: 41.9A
DCDC output current: 35.5A
Current into batteries: 18.2A
Current into trailer: 17.3A ( I had a few lights on as well as the fridge)

I then turned off the fridge, waited for the DCDC to enter float mode, and then turned fridge back on. I then took the following measurements:

Battery Voltage: 13.2V
Voltage at fridge element: 12.3V
Current: 14.02A
Fridge element power: 173w

So, what I can figure from all this is that running an absorption fridge on 12v likely gets a bad rap due to poor charging circuits. That running on DC with a DCDC charger should yield performance at least equal to running the fridge on AC. Finally, that with a DCDC charger I am defiantly, at least some of the time, putting much more power through the dc element than it was designed for. It will be interesting to see how long this element lasts before burning out.

Doug

[cpaharley2008]

I'm not familiar with the DC/DC adapter but it seems to increase your 12v output somehow and where it is getting this new energy is unknown, but as you say sometimes too much is just as bad as too little. Thanks for the information though, for others may want to pursue this approach.

[DougG]

Originally Posted by cpaharley2008

I'm not familiar with the DC/DC adapter but it seems to increase your 12v output somehow and where it is getting this new energy is unknown, but as you say sometimes too much is just as bad as too little. Thanks for the information though, for others may want to pursue this approach.

A DC-DC charger is very similar in function to a MPPT solar charger. It takes the 13-14v that the truck alternator is producing and steps it up to whatever voltage you set it at to match the charge voltage recommended by the battery manufacturer. It also compensates for the voltage drop on the long wire run from alternator to trailer batteries. There is no new energy produced, just an exchange of volts for amps. As you can see above, it was consuming 41 amps (at around 13v) from the truck alternator, and putting out 35amps at about 14.8v