Using Dometic refrigerator on 12 volts

[Raspy]

Originally Posted by Raz

A wire is a resistor. By definition it limits current.


Wires do have resistance, and they do limit current, but that is not the function here. The function of the charging wire is to deliver 12 volts at a certain amperage. Not to be a resistance wire that drops 100 amps to 10 amps through resistance. We are not designing an electric heater here, just charging a battery, and there are better ways to limit current than routing a red hot wire from the battery to the trailer.




This may have been true 20 years ago but with all the additional electrical components on any new vehicle I'm not sure only 11amps is the term I'd use.

Not sure what you mean here. The 12 volt load on a Dometic resistance heater is about 150 watts. So, yes, it's about 11 amps. This is far below the typical alternator output (mine is somewhere round 130 amps) and even at idle will have far more than 11 amps extra after all vehicle loads are factored in.




Since the factory charge line is typically included in the tow package why didnt the manufacturer use 10 ga wire to begin with? I doubt it's cost. Also, if you connect directly to the battery on many vehicles, my Nissan included, the computer will not know about the additional load. The current sense is between the battery post and the frame. I wouldn't add anything without knowing what effect it will have on every thing else. I'm a retired E.E. I love to tinker. I never worked in the Auto field but am familiar with what is being done. At least enough to know to be cautious. I'm sorry to say the days of seat of the pants backyard engineering are gone. Like I said, I love to tinker.



The manufacturer did not include a 10 gauge wire, for their own reasons. You'll have to ask them that question. The computer does not need to know about the additional load that an added trailer load will incur. The charge controller, or regulator looks at battery voltage and tries to maintain it a 14.1 volts. Loads reduce the apparent voltage at the battery and the regulator responds by upping the current from the alternator It will do this up to the full output of the alternator. That is the beauty of a charging regulator, it does not need to know the load on the system, just the battery voltage. That is how regulators work

Modern car electrical systems are canbus. This means the computer manages all electrical outputs from the starter motor to all lights, etc. It also allows the computer to notice if a light is out or if an additional load is applied to a circuit, or the current level of battery voltage. This constant monitoring and complete system control allows the computer to shut off any loads to protect the battery charge and prevent a dead battery. An example of this is if you leave your lights on, the computer will shut them off even if the switch is still on, after a period of time, or when the voltage drops to a pre-determined level. This is also why you can't tap into your tail light circuit to add a load in the trailer.

Another examples of current limiting are the cigarette lighter, which supplies 12 volts, but only 15 amps, and the onboard inverter that puts out 120 volts, but only 100 watts.

You may think its "shade tree" and you may use that term in a derogatory way, if you wish, but there is nothing wrong with installing a larger circuit and circuit breaker directly from the battery to the trailer. The charge controller in the vehicle will still maintain the battery at 14.1 volts. A properly sized wire will allow for the resistance over the run and deliver the needed amperage. As the battery nears full charge the load is reduced, further reducing the line resistance, so the trailer battery can come up to 14.1 volts, or close enough that it doesn't matter, even if there is some line resistance. In my Ram, once the voltage can be held at 14.1volts with a very low amperage, (fully charged) the controller drops to a maintenance or float charge of just over 13 volts. BTW, this is exactly how winch wiring is installed on Jeeps, but they can draw 400 amps. The alternator will come up to full output and the battery will carry the balance of the load to it's full capacity. Often second batteries are installed to supplement this load. They are simply wired in parallel, and again, the charge controller just does it's duty, without a problem, by measuring the primary battery voltage, and in some cases, the amperage required to hold that voltage at 14.1.

Bottom line: The voltage regulator does not need to know the load downstream of the battery. It is not based on that. That idea went out in the '50s with Lucas lighting, the Prince of Darkness. The regulator only needs to know battery voltage and in some cases the amperage required to hold the battery at that voltage. Any load can be added up to the full output of the alternator, which varies by RPM and temperature, and there will be no affect on the vehicle.

[John in Santa Cruz]

If you DO run your own heavier trailer charge wire, I would add an isolation relay to it, triggered by the same circuit as the current charge relay in the tug, I'd probably use a 50A relay for safe margins here (if your trailer battery is seriously discharged, and you've run 10 gauge, the dead battery could easily pull 30A down that wire.

the relay's function is to disconnect the trailer when the tug engine is off.

[Raz]

The manufacturer did not include a 10 gauge wire, for their own reasons. You'll have to ask them that question.


okey dokey

[John in Santa Cruz]

IK just checked the wiring diagrams for a gen2 tacoma like mine. the trailer 'batt chg' pin is on a 30A fuse.. but the diagrams don't give the wire gauge. there are two connectors after the batt chg relay, one at the engine compartment, and the other near the trailer connector.

[Raspy]

Originally Posted by John in Santa Cruz

If you DO run your own heavier trailer charge wire, I would add an isolation relay to it, triggered by the same circuit as the current charge relay in the tug, I'd probably use a 50A relay for safe margins here (if your trailer battery is seriously discharged, and you've run 10 gauge, the dead battery could easily pull 30A down that wire.

the relay's function is to disconnect the trailer when the tug engine is off.

This is a very good plan. A common way to do it is with a Ford type, continuous duty relay, but I'm sure there are other good ones too.

[David Tilston]

Originally Posted by Raspy

This is a very good plan. A common way to do it is with a Ford type, continuous duty relay, but I'm sure there are other good ones too.

I use something like this:
https://www.princessauto.com/en/deta...or/A-p8511875e

A 140A, voltage sensing relay. At 13.3 volts, it closes, (turns on). At 12.8 volts it opens, (turns off). This prevents the trailer from draining the vehicle battery. Not cheep, but everything at Princess Auto eventually goes on sale.

[sokhapkin]

Originally Posted by Raz

Since the factory charge line is typically included in the tow package why didnt the manufacturer use 10 ga wire to begin with? I doubt it's cost.

Yes, it's the cost. Copper is expensive nowdays. Few cents saved on thousands of vehicles bring sizable profit to shareholders.

[Raspy]

Originally Posted by David Tilston

I use something like this:
https://www.princessauto.com/en/deta...or/A-p8511875e

A 140A, voltage sensing relay. At 13.3 volts, it closes, (turns on). At 12.8 volts it opens, (turns off). This prevents the trailer from draining the vehicle battery. Not cheep, but everything at Princess Auto eventually goes on sale.

That looks interesting. Invisible to the user, which makes it more foolproof and easy to use.

Does it measure the trailer battery voltage for it's reference? If it measured the TV battery it might have a continuous battery draw even when off.

Thanks for that link. Very useful.

[cmartin748]

Never know what the risk is for sure! I was coming south on I 75 in Lake City Florida and there was a contractors wheelbarrow in my lane in heavy traffic! Just missed it.
Always get fuel in a station with a canopy over the pumps. You never know when space junk is going to fall from the sky!

[John in Santa Cruz]

Originally Posted by cmartin748

Never know what the risk is for sure! I was coming south on I 75 in Lake City Florida and there was a contractors wheelbarrow in my lane in heavy traffic! Just missed it.
Always get fuel in a station with a canopy over the pumps. You never know when space junk is going to fall from the sky!

not sure that tin canopy would be much help... just sayin'

[cmartin748]

Maybe someone can give us the specs on what is actually needed to do the job.

[Cathi]

Originally Posted by Raz

I don't run my fridge on propane while driving down the road because it's a risk I prefer not to take. It's my understanding the flow restrictor on a twenty pound tank limits to 5 psi. The output of the regulator on my trailer I believe is 1/2 psi. If true, a ruptured line will continue to supply gas despite the safety device. But what concerns me more is the use of hydrogen gas as a component in the refrigerant. With no moving parts these fridges often fail through metal fatigue, most likely caused by vibration. The smell of ammonia is the tip off. But if the hydrogen encounters an open flame, an explosion occurs. Do a search for "RV refrigerator explosion", lots of stories, law suits etc. Interesting that Norcold is producing absorption refrigerators that use helium in place of the hydrogen.


When I encounter a guy smoking a cigarette while he's pumping gas, I drive away. He's been doing it that way for years. So what? Been driving for years with the fridge running on propane. I have the same reaction. I really don't care what others do, I just don't what them to take me with them. Raz

I looked at a number of RV and propane associations and they all said not to drive with the propane on. The FMCA (Family Motor Coach Association) looked at the safety devices and concluded that, while a small risk, and despite the cut-off, there would still be enough propane out there to cause a catastrophe, and they recommended driving with the propane off. I saw on a forum where certain propane associations gave the same recommendation, but do not know if it was this forum. Another organization noted that the number of ppm's of propane needed to cause a catastrophe was tiny and they also recommended not driving with the propane on.

While it may be very unlikely to have something happen, that is not the same as it cannot happen. Many prefer to take the risk. Many claim that there is no risk. That is not, however, what a number of organizations very familiar with this say. Now I am sure that many people who drive with their propane on will want to bash any organization or any persons who do not, and who used facts such as the above to make their decision. That is always the way it will be on many issues.

It is not impossible that I saw old information but I don't think so. If those organizations did not recommend not driving with propane on, we would not be doing it. Also saw the report of the average 5000 fires per year at gas stations in the U.S. You all can do what you want and we will do what we want. We use ice packs that we freeze overnight while stopped and switch them with others in a cooler in the car. May be interested in trying 12v with solar some time as some have had success and others not.

[David Tilston]

Originally Posted by Raspy

That looks interesting. Invisible to the user, which makes it more foolproof and easy to use.

Does it measure the trailer battery voltage for it's reference? If it measured the TV battery it might have a continuous battery draw even when off.

Thanks for that link. Very useful.

I found the manufactures web site for this product:
AOPEC Automotive Electronics - Products

They only give this spec for the 100A, (not 140A) version, but I suspect they would be the same:
Standby current:<3mA@12V;<6mA@24V
As for which battery they reference, it would have to be the tow vehicle battery. Otherwise, the unit would never see when the alternator is producing voltage. Though in the wiring diagram, and instructions, it says to wire the negative reference to the accessory battery, (not possible on a trailer). This implies that it is measuring the accessory battery. Since both batteries are shown with the negative terminals connected, it would make no difference.

On the Princess Auto web site they link the user manual:
https://images11.palcdn.com/hlr-syst...875_manual.pdf

[Raz]

Cathi, this is one of those hot button issues. It typically provokes arguement. "I've done it for years and never had a problem. " is a common response. Or "there's no law against it" is common too. Since this is a forum, all points of view are valid and should be tolerated. That said, one of my favorite sayings is " We all dig our our holes." But " let's take all the warning labels off and see how things sugar off" might be more appropriate. Stay safe, Raz

[Raspy]

Originally Posted by David Tilston

I found the manufactures web site for this product:
AOPEC Automotive Electronics - Products

They only give this spec for the 100A, (not 140A) version, but I suspect they would be the same:
Standby current:<3mA@12V;<6mA@24V
As for which battery they reference, it would have to be the tow vehicle battery. Otherwise, the unit would never see when the alternator is producing voltage. Though in the wiring diagram, and instructions, it says to wire the negative reference to the accessory battery, (not possible on a trailer). This implies that it is measuring the accessory battery. Since both batteries are shown with the negative terminals connected, it would make no difference.

On the Princess Auto web site they link the user manual:
https://images11.palcdn.com/hlr-syst...875_manual.pdf

David,
You're right. It has to measure the primary battery or it can't work, but then there is a continuous draw on that battery. I know it's small, but still. Might be better to energize it with the ignition key. Have you noticed your primary battery getting discharged?

[Raz]

Originally Posted by Raspy

David,
You're right. It has to measure the primary battery or it can't work, but then there is a continuous draw on that battery. I know it's small, but still. Might be better to energize it with the ignition key. Have you noticed your primary battery getting discharged?

There is a parasitic drain on your battery already. It maintains the radio settings, the engine settings, the alarm, etc. It's easy to measure. I doubt 3 mA more will matter. Raz

[Raspy]

Originally Posted by Raz

There is a parasitic drain on your battery already. It maintains the radio settings, the engine settings, the alarm, etc. It's easy to measure. I doubt 3 mA more will matter. Raz

Good point. I wonder how often a new car has to be started to maintain itself?

[Raz]

Originally Posted by Raspy

Good point. I wonder how often a new car has to be started to maintain itself?

Good question. I just bought a new pickup that sat on the lot for over a year. Battery is still good. They must have done something. I don't run my pickups in the winter to avoid road salt. I disconnect and put the charger on occasionally. I should measure the parasitic current next time.

[David Tilston]

Originally Posted by Raspy

David,
You're right. It has to measure the primary battery or it can't work, but then there is a continuous draw on that battery. I know it's small, but still. Might be better to energize it with the ignition key. Have you noticed your primary battery getting discharged?

Typically, I disconnect the battery, if I plan to store a vehicle for a long time.

[Raz]

The key off current on my simple Frontier settles to 140mA, which is higher than I expected. I'm sure that would increase if it had an alarm and other bells and whistles.