Automatic cutoff relay for refrig. - Fiberglass RV


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Old 04-16-2020, 03:13 PM   #1
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Name: Jack
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Automatic cutoff relay for refrig.

Help,
I am trying to find a suitable relay to shut off my Dometic refrig running on 12 v.. when I stop for gas or take a break when traveling. I have a Scamp 13 2018 and I have tried searching the forums here without success. I know it has been discussed here, but I cannot find it.
Any help with part #, source for part or actual post here will be helpful, and no I do not want to run the refrig on propane when traveling.
Thanks,
Jack
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Old 04-16-2020, 05:01 PM   #2
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This relay operates on 12V, is rated for continuous duty, & has contacts capable of handling the 12V heater element current.

What are you planning to use as a sense source for the relay coil & where are you going to place the relay contacts?
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Old 04-16-2020, 06:00 PM   #3
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Good question Jon. I am just started to consider doing this project when its gets warmer. At present I am in the planning stage (since we are still home bound here in MA) and looking to see exactly what has to be done to accomplish the task. Was hoping that someone on the forum would share their experience with the subject.
Jack

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Old 04-16-2020, 06:37 PM   #4
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It seems like a voltage sensing relay on the power lead to the fridge would work. When you shut off the tow, the voltage drops and the power is disconnected to the fridge. Start the tow, and it re-connects. This might also turn on when you connect to shore power. Then have a simple bypass with a manual switch so you could run it on the battery while camped.

But this idea means it will run until you are parked next to the gas pump, and it will re-start before you leave the gas pump, when fumes might still be present. Is that a problem?
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Old 04-16-2020, 06:39 PM   #5
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Why? Are you worried about discharging your tow vehicle battery, the trailer battery, or both? How long are the stops going to be? Why disconnect the fridge at gas stops (under 10 minutes I assume). Its not a danger like an open flame while running it on propane.

Typically the recommendation is to install a device to disconnect the tow vehicle when you are stopped and the engine / alternator are off. This prevents the tow vehicle battery from being overly discharged by the running fridge. There are a few ways to do that.

If the fridge then continues to run on the trailer battery then the question is, how long are the stops. A fully charged 100 AH battery can power the fridge safely for up to five hours, so short stops are no problem. Any stop over 2 hours.. I would shut the fridge off myself.

If you really want to disconnect the fridge when the tow vehicle is off then a voltage sensitive relay triggered by the charge line will work. But I really dont see the point in using that for the fridge alone. Its a fine solution (but not the cheapest) for disconnecting the tug from trailer however.
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Old 04-16-2020, 07:31 PM   #6
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Hey Guys,
I've been cooped up in the house for a month and it looks as if it may be another 2-3 weeks before the Gov. says we can relax a bit.



Anyway I've been thinking of things that might need my attention and I remember a post about shutting off the 12v to the fridge when stopping for a time on the road. Was giving it some thought, and decided to research it here on this site. Like I said It had been discussed on this site before, and yes I was more concerned about the TV battery.



I normally disconnect my 7 pin plug from the camper when I know we will be stopping for an extended time. So if its not that big of a deal I will not worry about it.
Thanks
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Old 04-16-2020, 07:39 PM   #7
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Originally Posted by Lockman View Post
Hey Guys,
I've been cooped up in the house for a month and it looks as if it may be another 2-3 weeks before the Gov. says we can relax a bit.
Please dont base your decision on any politician. Listen to the doctors. I predict a resurgence when people think its OK to go out and socialize too much.

Quote:
Originally Posted by Lockman View Post
.... and yes I was more concerned about the TV battery....
The just use an isolator / relay / cutoff device to disconnect the charge line then the tug is not running. The one I posted is easy to install. Other options are cheaper but require more effort to install.

Or just pull the 7 pin anytime the stop is not a "short" one.
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Old 04-16-2020, 07:51 PM   #8
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[QUOTE=gordon2;774917]Please dont base your decision on any politician. Listen to the doctors. I predict a resurgence when people think its OK to go out and socialize too much.



I agree with you on that Gordon. It will be back.
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Old 04-16-2020, 08:07 PM   #9
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Quote:
Originally Posted by gordon2 View Post
Please dont base your decision on any politician. Listen to the doctors. I predict a resurgence when people think its OK to go out and socialize too much.



Thank you Gordon, I agree.
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Old 04-17-2020, 05:35 AM   #10
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Quote:
Originally Posted by Lockman View Post
Good question Jon. I am just started to consider doing this project when its gets warmer. At present I am in the planning stage (since we are still home bound here in MA) and looking to see exactly what has to be done to accomplish the task. Was hoping that someone on the forum would share their experience with the subject.
Jack

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I would think a little Arduino with a led readout screen could do this for you. Not sure but they are very versatile little computers. It could sense the voltage drop and turn a relay off.
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Old 04-17-2020, 08:09 AM   #11
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A relay in the TV charge line that is switched by a hot wire from the switched ignition will shut off the line from the battery to the 7 pin trailer plug.
That is what I did on my T&C that did not have a relay in the factory line.
Then when you switch off the ignition the trailer will be disconnected.
Easy-Peasy.
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Old 04-17-2020, 08:20 AM   #12
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Some thoughts on adding a relay (well, overthinking, but that is what happens when you sit in the house too long).

I don't see how pulling the 7 pin will shut down the refrigerator unless you don't have a trailer battery.

A voltage sensing relay may not work because in some cases the refrigerator current draw is high enough to pull the 12V tow vehicle charge line down to the trailer battery voltage.

Where do you put the relay? If you put it in the 12 V supply, you will need to add a switch (and remember to use it) to turn it on when you want to run the refrigerator on 120V or propane since most refrigerators (at least the newer ones) require 12V for the electronics. While the 1.6 watts the relay coil draws isn't much, it would be another draw for those dry camping.

If you put the relay in the 12V heater element lead, that will have no effect on running on 120V or propane, but with an electronic loaded refrigerator, might throw a fault code or even switch the refrigerator to propane.

If you use the tow vehicle charge line to provide the sense for a standard relay, you need to prevent it from also sensing the trailer battery. Again, a voltage sensing relay might do this, but might not. Adding a diode would work, although it would add a bit of voltage drop between the tow & trailer battery. It also presents a problem if you have a breakaway switch or power tongue jack (which a Scamp 13 probably doesn't have) since the trailer battery won't get past the diode to power them.

A completely fool proof & automatic system may not be as simple as it seems...
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Old 04-17-2020, 08:53 AM   #13
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The relay's only purpose it to keep from killing the TV battery so that you can crank up and drive off.
One could put a switch in the line from the battery in the trailer to the refrigerator power line and have the TV power input after that switch so that the refrigerator would draw power from the TV when the ignition was on and no power (DC) otherwise.
If you had a 3 way fridge you could feed the TV power to an inverter to give 120 VAC when on tow and help eliminate the voltage drop associated with the long lines involved.
Many refrigerators will select the 120 VAC in preference ti the 12 V (or LP sometimes).
You could also use that 120 VAC to power a battery charger to also eliminate the voltage drop that makes it difficult to charge the trailer battery.
In that case there would have to be some switching involved to go back to battery power.
Perhaps power the existing battery charging system ( multistage charging perhaps) form the 120 volt inverter, but the limit would be the approximately 10:1 current transformation involved. For example a 12 volt 20 amp power from the TV would only provide 2 amps at 120 volt AC minus the losses in the inverter.
Just some mental gymnastics brought on by self isolation.
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Old 04-17-2020, 09:43 AM   #14
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Maybe we need to clarify what jack's thinking is here.

He wants to shut off the fridge when fueling and when taking a break. He does not want to use the fridge on propane while "traveling".

So, shutting it off while fueling is not needed for protecting the battery, because it is a short term load of 15 minutes or so. But maybe he's worried about fire. Dunno.

Shutting it off while resting is not about fire hazard, but could be long enough to run the battery down.

Traveling, might mean driving, but it could also mean anytime he is not home. So. maybe he uses propane while camped, but not while driving. I like doing that too.

Running the fridge on 120 volts from the inverter would only work if the tow provided enough charging power through his system to run the fridge and the inverter losses of 10%.

I don't understand the advantage of running a battery charger on top of that load, or where the power will come from. 12 volts tow current to run an inverter to make 120 volts to run a battery charger to charge a 12 volt battery?

The way I increased the available power to the trailer while towing was to add a 50 amp Anderson plug at the rear bumper, that is plugged into a separate pigtail from the trailer. This comes straight from the tow battery, through a 60 amp breaker, which also is a switch. The pigtail feeds directly to the trailer battery. I can run the fridge on 12 volts and charge the batteries at the same time while driving. My truck voltage regulator is at least two stage and charges at 14+ volts first, then drops to 13 volts when the batteries are up. It may be that a voltage sensing relay, in the trailer, will not work. But shutting that line off would not keep the fridge from running down the trailer battery, or reduce any fire hazard.

Does the 12 volt fridge system have any fire hazard? It seems less dangerous than starting your engine to leave the gas station.

Jack. Come back and tell us more about what you are trying to achieve. Fire hazard, protect the tow battery, the trailer battery, are you willing to click a manual switch, or must it be automatic?

Jon makes a very good point about the voltage drop fooling the relay.

With my setup, I don't worry at all about leaving the Anderson plug, plugged in for short term stops, or during a nap on the road. But I unplug it for overnight stays and switch the fridge to propane. My seven pin 12 volt wire is not connected in the trailer, so that has no affect on any of this.
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Old 04-17-2020, 09:59 AM   #15
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The only point about the 120 volt inverter is that they are cheaper than a 12 volt boost inverter as there are more of them. As to the efficiency I am not as concerned IF the power is available and replaceable.
The voltage drop from the TV battery to the trailer is of, perhaps, less importance.
The relay take care of dropping the current when the ignition is off refueling or stopping to get lunch.
If you hooked up a battery maintainer (low draw on 120 volts) to keep the trailer battery topped off and ran the fridge off 120 volts (assuming enough power at 120 volts is available) the voltage drop from the TV battery is of less importance to battery charging. However the drop would be considerable with 20 amps being drawn by the inverter, perhaps.
Basically I have the relay that kills the TV connection and I also have 300 watts of solar power on line when stopped or towing, I also have two 104 AH 12 volt batteries.
I also have a 12/120 volt swing compressor refrigerator and no LP to worry about.
I think that the relay in the TV battery line will accomplish what the OP is looking for and depending on the wiring setup that may already be present.
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Old 04-17-2020, 10:24 AM   #16
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let's at how silly htis is

First off the reason most gas stations wath the fridge turned off is open flame during propane operation.
Second you most like ly have at least 50 amp hours of battery between the trailer and tow vehicle. 50 amp hours / 10 amps (fridge draw )= 5 hours.


Does it take you 5 hours tho fill the gas tank?
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Old 04-17-2020, 11:09 AM   #17
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Automatic relay for refrigerator

Another option you might consider is very simple to install and use. I installed a battery cutoff switch next to the battery on my 17' Casita. I did this so that when the camper is plugged in at home the charger won't overcharge the battery. I believe you may still need to unplug the seven-pin connector, but doing both should keep from running down your car and camper batteries.
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Old 04-17-2020, 11:20 AM   #18
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Just a recap on what I was thinking

Was only interested in adding a cut out relay to save the TV battery when taking a break on the road traveling. I have been disconnecting the 7 pin plug when I thought we might be stopped for a longer time in a rest area.



Since I only run the frig on 12v. when traveling, I was concerned about running the TV battery down and having to get me a jump start to leave. I do carry jumper cables.



But since it does not seem to be a problem stopping for short amount of time I'll continue with my present way of working around the situation. Now I have a better idea of how long it would take before I started to get into trouble.


Thanks for all the input.


Jack
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Old 04-17-2020, 12:46 PM   #19
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Power cut off.

To get a power cut off you need to go to the HAM (amateur) Radio community.

Go to West Mountain Radio;

West Mountain Radio - DC Power Models

and choose which one will work for you. The one I have is capable of 40 amps,
PG40S is the one I have.

You will also need a product called Powerwerx to connect to your system, this is also a Ham Radio item.

https://www.hamradio.com/detail.cfm?pid=H0-009889
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Old 04-17-2020, 01:25 PM   #20
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Quote:
Originally Posted by Lockman View Post
Was only interested in adding a cut out relay to save the TV battery when taking a break on the road traveling. I have been disconnecting the 7 pin plug when I thought we might be stopped for a longer time in a rest area.



Since I only run the frig on 12v. when traveling, I was concerned about running the TV battery down and having to get me a jump start to leave. I do carry jumper cables.



But since it does not seem to be a problem stopping for short amount of time I'll continue with my present way of working around the situation. Now I have a better idea of how long it would take before I started to get into trouble.


Thanks for all the input.


Jack
Since it doesn't sound like your tow vehicle includes a relay to shut off the charge line with the ignition, adding one would be practical. While the relay I posted would do this, there are ones that are designed specifically for the purpose. This is the one I added to my RAV4 when towing an Escape 17.

This relay will prevent depleting your tow vehicle, but does nothing to protect the trailer battery. While a short stop won't kill the battery, a long lunch could have you arriving at an overnight stop with a mostly dead trailer battery.

Using a relay to specifically shut off the refrigerator would protect both.
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