Swap 12v for 120v Fridge Element? UPDATE 2

[BFL13]

EDIT--it turned out you can't swap them--they are not the same and it would fry things if you applied 120v to the 12v element
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UPDATE 25 Jan--12v is cooling down the freezer.
UPDATE 2, 26Jan

I found my 120v heating element that was not working to be rusty and it and the 12v element beside it are both rusted to the coil pipe (or whatever the assembly is supposed to be like ????)

I was poking with a screwdriver and it cracked open or made? a fault in the 120v heating element pipe that is rusty dark. the 12v element beside it looks identical but is shiny--maybe never used.

It would be easy to swap the wiring so that the 12v element's two wires went to the terminals on the board that the 120v element's wires go to now. Only thing would be to eliminate the 20 amp fuse on one of the 12v elements wires.

But--are these two elements the same so I can do that?

Photo shows the 120v on the left of the 12v element which is the shiny one to the right of the photo.

[MrWizard]

you really want to find out , how much it might cool

make pigtail to plug the 12v element into a cable that you connect directly to your batteries, just for testing

leave it connected for 12-24 hrs

[BFL13]

Mine did not maintain the temperature at all, even though it was "on". Within an hour after leaving it on 12v, the freezer temp had climbed back up to about -10C from the -20C it started at, and the 12v element had not come on yet. I don't know when the 12v came on later, but it was too little, too late to do anything to stop it getting right back up to ambient.

There was no frozen food in there to help keep it cold, as there would be while travelling.

There is a tiny bit of pink insulation for it inside the flue but I suspect most of the heat goes elsewhere instead of heating that cooling pipe beside it. The 120v heater right next to the 12v worked though, last year when it was still working. Beats me.

Also my wattages are low it seems, so that might be it.

[dougrainer]

BFL13 wrote:
So the results are in--the 12v worked overnight, but did not keep the freezer cold.

Yesterday I had the freezer down to - 20C on gas and switched over to 12v. (The freezer gets colder than that when camping--don't know what the deal is here--perhaps the fridge needs to be in a heated space like with a res fridge?)

12v did not come on right away and temp started back up. Left it, and this morning after 13 hours, the Kill-A-Watt is showing that the 12v heating element is drawing power at approx. 135w as per 13.6v from earlier testing.

Freezer temp is now +3C near ambient, so despite whatever the 12v has done, it was not enough. Even if it all did work better if the fridge were in a warmer place, the scenario for day-before-camping use would be with the RV furnace off, the rig at ambient.

So that was a bust. Probably why they don't even bother with 12v in newer RV fridges.

As I stated, the 12 volt is for in transit ONLY. To MAINTAIN the existing temp. In Transit, most use LP. 12 volt was never a popular option in the "old" days. The OEM's had to wire the refer as per the heavy gauge guidelines and very few wanted/would do that. 12 volt is still popular on B and B+ motorhomes, Doug

[BFL13]

MrWizard wrote:
Now you know
Btw, I don't think all this had anything to do with saving LP
It was all about you wanting to do something with the RV
Anything just so it involved the RV 🙂

Ha! Everyone needs a hobby. 🙂

[MrWizard]

Now you know
Btw, I don't think all this had anything to do with saving LP
It was all about you wanting to do something with the RV
Anything just so it involved the RV 🙂

[BFL13]

So the results are in--the 12v worked overnight, but did not keep the freezer cold.

Yesterday I had the freezer down to - 20C on gas and switched over to 12v. (The freezer gets colder than that when camping--don't know what the deal is here--perhaps the fridge needs to be in a heated space like with a res fridge?)

12v did not come on right away and temp started back up. Left it, and this morning after 13 hours, the Kill-A-Watt is showing that the 12v heating element is drawing power at approx. 135w as per 13.6v from earlier testing.

Freezer temp is now +3C near ambient, so despite whatever the 12v has done, it was not enough. Even if it all did work better if the fridge were in a warmer place, the scenario for day-before-camping use would be with the RV furnace off, the rig at ambient.

So that was a bust. Probably why they don't even bother with 12v in newer RV fridges.

[BFL13]

road-runner wrote:
On battery 12.8v/9.5a = 121.6w
On converter at 13.6v = 135.2w
On converter at 14.8v = 172.6w

There has to be some measurement error here. If you crank the numbers to get the element resistance, it's lower at 14.8 (1.27 ohms) than at 13.6 (1.37 ohms) That's opposite of reality. Also, for good data, the voltage needs to be measured at the element.
___________________________________________________________________
On paper, going from 12.8 to 13.6 volts should have increased power by about 21% to about 147 watts.

I don't see that happening. The element resistance at 12.8 volts is 1.35 ohms. 12.8 volts/1.35 ohms=9.48 amps*12.8 volts=121.3 watts. Using the same resistance at 13.6 volts: 13.6/1.35=10.1 amps*13.6 volts=137.4 watts, a 13.3% increase.

Don't bother trying to get it to be exact. It does show the increase in watts with higher voltage.

The converter ones were watts as shown on the Kill-A-Watt on the shore power line where mostly it was fridge, the batteries being "full" I had the fridge 12v off, took the watts, turned it on, took the watts, subtract the difference. Used the voltage from battery via Trimetric.

I think there must be some voltage drop. Not much to the fridge PC board, but perhaps more to the element from there. I can't measure DC voltage at the element or at its terminals on the PC board because they are covered with the connections' plastic covers.

I agree the numbers are ropey, but they do still tell the tale.

EDIT--another failure! Got the temp back down in the freezer using gas and went over to 12v. No 12v heater action. Turned fridge off wait two minutes, back on 12v, nothing. So I just left it that way and will see in the morning if it ever did come on.

I don't know how high the freezer temp has to get when on gas so it cycles back on, so I can't say if this is wrong that the 12v did ot come on. (the gas was not burning at that freezer temp, which indicates it was down to as far as it was going to get. The temp settings is at coldest, but it is kind of flakey how that works.)

Anyway the fridge works ok on gas and cycles properly while camping so it gets to stay in place for now.

[landyacht318]

Sounds like a job for a dedicated 18$ cheapowatt power supply set for 15.3v.

[road-runner]

On battery 12.8v/9.5a = 121.6w
On converter at 13.6v = 135.2w
On converter at 14.8v = 172.6w

There has to be some measurement error here. If you crank the numbers to get the element resistance, it's lower at 14.8 (1.27 ohms) than at 13.6 (1.37 ohms) That's opposite of reality. Also, for good data, the voltage needs to be measured at the element.
___________________________________________________________________
On paper, going from 12.8 to 13.6 volts should have increased power by about 21% to about 147 watts.

I don't see that happening. The element resistance at 12.8 volts is 1.35 ohms. 12.8 volts/1.35 ohms=9.48 amps*12.8 volts=121.3 watts. Using the same resistance at 13.6 volts: 13.6/1.35=10.1 amps*13.6 volts=137.4 watts, a 13.3% increase.

[BFL13]

I don't know why the watts are not as high as might be.

Starting the long test, the fridge got down to proper freezing on gas, (took three hours, but ambient is about 5C) and switched to 12v to maintain. Except the 12v did not come on -indicating but no watts pull. So I figured it must need to wait and then come on for a cycle whatever the temp range is for that. After an hour the freezer temp was back up quite a bit, so back on gas one more time and try again.

I see that everyone is trying to say the 12v is not enough watts to get the temp back down once it does cycle. That would mean it can't maintain, but just slow down the rise in temp, which is good for a few hours. Actually just leave the fridge doors closed and you can do a few hours ok.

Something is not right with my fridge I think, but whatever. It works on gas. I don't really need the 12v if this is all a bust.

[pianotuna]

On paper, going from 12.8 to 13.6 volts should have increased power by about 21% to about 147 watts.

Going from 12.8 to 14.8 volts should have done 72% more, or 208 watts.

Did you happen to check the voltage at the element end?

My fridge has #10 wire.

[BFL13]

12v element wattages:

On battery 12.8v/9.5a = 121.6w
On converter at 13.6v = 135.2w
On converter at 14.8v = 172.6w

Note the manual's example of 14v and 16a = 224w, so the element can do more than what I am seeing. I haven't looked at the voltage at the fridge to see if much voltage drop. Have to check that out.

So now trying the long test again to first get the fridge down to proper cold temps on gas, and then to maintain that with 12v at 13.6v on converter.

If doing this for a long period and you don't want to float the battery bank at 14.8v, then 13.6 it is. If you did need a higher 12v voltage, then you would need a second converter just on the fridge.

[BFL13]

To be clear, we operate the fridge on gas when camping. Only time it was ever on 120v was at home for the day before going camping.

I am just looking at replacing that 120v time with 12v time on converter--so it is actually still from 120v stick house power, and that is only to save a little propane that day, same as before.

OK I fixed the wiring issue and we are back in business. Freezer temp is 2 C no cool down, so the 12v element measurements will be at whatever its full power is and it will stay on, no cycling.

On battery it did 9.5 amps at 12.8 volts, sort of like yesterday. I can't try different voltages and see the amps on converter using the Trimetric. The Tri shows battery only and the converter will do it all. However, I have the RV shore power on the Kill-A-Watt, so I can see if there is any change in fridge watts when I crank up the converter's voltage (adjustable voltage PowerMax converter)

Results to follow once the converter finishes topping up the battery bank to 13.6, so any watts will be mostly fridge.

[MrWizard]

It's pretty simple
The 12v element does not produce enough power for normal regular operation

It's winter and you are not camping, so maybe it will keep temp..maybe
But not when camping in the summer

You can maybe remove the 12v element,replace it with a 120v element, then use 120v

Makes more sense to me than trying to use the 12v element on continuous duty

[BFL13]

Ok, first my test overnight failed because a wire came off a blade terminal somehow, so I have to start over on that.

On the wiring in general, I take the specs I posted earlier to mean from the fridge to battery.

The 20 amp fuse is on a wire between the 12v element and PC Board, maybe two feet long. The actual wires on the 12v element are #16 AWG. The actual wires on the 120v element look the same but are marked (can't read it) AWM. Both of those sets of wires are aluminum.

I do have the manual statement that the 12v is only for short duration and it is not enough to do a cool-down, only good for maintain.

I can't figure out if they want that short duration only because they are worried about battery going down, as they seem to say, or if there is another reason you can't leave it on longer maintaining after the cool down using gas.

My plan is for more like 16 hours at 12v maintaining when on shore power at home, so the converter is on and no worries about battery going flat.

So if there is another reason for not leaving it on a longer time besides battery worries, I would like to know, so I don't wreck anything.