Powering a 12 volt fridge and battery

[ecasey]

I am looking at some of the options for powering my Dometic 12 volt DC/Propane fridge on the road. I have a 71 Boler which has no house battery, converter or 12 volt wiring inside the trailer. I currently have a four way connector for the lights on the trailer but I would like to upgrade to the standard seven way connector. I bought a Progressive Dynamics converter and will install a battery box with a couple of 6 volt batteries on the tongue. I plan to add a charge wire from my Jeep to the house batteries protected by the proper fuse and a continuous duty solenoid to isolate the vehicle's battery. I am unclear on the best way to keep the batteries charged and power the fridge at the same time. Here are my options:

1. I could use the charge wire from the vehicle to charge the batteries on the road and let the Fridge draw from the house batteries until I reached the campsite.

2. i could run separate wires from the tow vehicle to the fridge and the house batteries with no connection between them.

I don't like option 1 much. I plan to dry camp most of the time and the draw from the fridge will lessen the chance of fully charging the house batteries in transit. The seven way connectors I have looked at are generally limited to a AWG 10 wire for the power line which I think would be the minimum size required for charging the battery alone. The second problem is that whenever I stop along the way, the fridge will still be drawing 7 amps from the house battery unless it is turned off which would be a hassle. I am quite sure that somewhere along the way I will either forget to turn the fridge off or back on leading to a fridge full of bad food or a dead battery at the campground.

Option 2 seems like the best bet but I am not sure of the best way to go about it. Could I have two wires leading from the trailer side of the isolator/solenoid to the 7 pin connector. Would it be possible to wire the fridge to the auxiliary pin in the connector and from there directly to the fridge. This would protect both batteries from being run down. Is there a potential problem with running two wires from the solenoid. I would prefer to have both the fridge and the battery charge lines running through the 7 way connector rather than separate connectors but that wouldn't be a deal breaker. I am curious how others have solved this problem.

[Per Walthinsen]

ecasey:

This is an issue I've successfully dealt with, but it took me quite some time and effort to learn how these things interact. My problem was that the 12v element on my Dometic was 123 watts and the 120v element was 175watts. One was inadequate, the other works quite well. That is not really your issue if the 12v side of your Dometic works well. What is at the heart of this is that unless you have adequate wiring all the way from the alternator or battery in your tow vehicle to the converter and battery in your trailer the voltage drop will not allow the juice from the tow vehicle to force an adequate charge into the batteries, especially if you try to run the frig at the same time. A fraction of a volt as far as voltage drop is concerned can make a lot of difference.

I believe you have correctly identified this issue already, so here is what I did: installed 6 gauge charge wire and ground wire all the way between car battery and the converter/battery. There is a Pollak 9-pin connector which supposedly truckers use, but it uses the very same housing as the 7-pin. This connector will accept up to 6-gauge wires for charge and ground. The 6-gauge is a pain to work with, but it can be done.

My van alternator puts out 105 Amps max. and is able to run an inverter powering the 120v element in the frig, charge the batteries at a fairly rapid clip, and run a towel warmer for good measure (I use a Xantrex Link 10 battery meter which tells me everything I want to know about what is going on).

As I see it the wire gauge is the key. I suspect if you install 6-gauge or even 8-gauge your scheme will work.

[ecasey]

I haven't heard of the Pollak connector. I will have to look into that. I just checked the manual for the fridge and it has a 95 watt/12 volt element. I don't have AC but the AC version is the same.

[Bigfoot Mike]

I haven't heard of the Pollak connector. I will have to look into that. I just checked the manual for the fridge and it has a 95 watt/12 volt element. I don't have AC but the AC version is the same.

Here ya go...
http://www.marksrv.com/wiring.htm

[Brian B-P]

The MarksRV diagram shows only the common 7-pin connector, as far I as can see.

I suppose it wouldn't be the same as the Pollack, but one of the Dexter manuals shows a 9-pin variation of the Bargman RV-style 7-pin... it appears to have the same type of contacts as the 7-pin.

The aux connection in the 7-pin is available for any desired use, so it certainly could be connected in the proposed manner, as a dedicated refrigerator circuit. I have noticed that the Euro-style towing connections, one 7-pin for vehicle functions (running/brake/signal lights) and another 7-pin for coach functions (such as battery charging) have multiple powered circuits, presumably to divide up the load. This would disconnect the 'fridge whenever the trailer is unplugged, but to keep it from running down the trailer battery when the tug is stopped the two circuits (battery charge and refrigerator power) would each need their own relay/solenoid (or separate poles of a double-pole relay).

If I could fit all required function in the nearly standard 7-pin, I would rather do that than have a non-standard towing connection. In case an alternate tow vehicle connects something else to the aux pin, or another trailer pulled by the same tug expects something else on the aux pin, I would put disconnect switches in this circuit on both tug and trailer.

I would like to share how I solved this problem... but I have not solved it. My Boler is supplied with power via a solenoid, and I have run down the trailer battery by forgetting to switch from 12VDC to propane for refrigeration when stopped along the way.

One potential method to fix only the accidental running down problem: put a relay in the refrigerator circuit in the trailer powered by the tail light circuit - the 'fridge would only run when the tail lights are on, which would not be when camped or parked (but would be when driving, if you always drive with the lights on).

[ecasey]

I would also prefer to keep a standard trailer connection in case I may be called upon to pull another trailer or require the trailer to be towed by another vehicle. I like your idea of using a relay powered by the tail light circuit for the fridge. I will use this if I use a single line to the trailer battery. I am fairly new to pulling a trailer and I find it enought of a challenge to remember to turn my lights off when stopped and not locking my keys in the car in addition to all the other trailer care issues. I don't really need any more memory tests if it there is a simple solution. Also a good idea about the disconnect switches on the auxiliary circuit. I was concerned about that as a potential problem.

This is the crux of the issue for me:

The aux connection in the 7-pin is available for any desired use, so it certainly could be connected in the proposed manner .... This would disconnect the 'fridge whenever the trailer is unplugged, but to keep it from running down the trailer battery when the tug is stopped the two circuits (battery charge and refrigerator power) would each need their own relay/solenoid (or separate poles of a double-pole relay).

So connecting both wires to the trailer end of the solenoid with one circuit breaker/fuse is a no go? The solenoid I have (but have not installed) is not a double pole solenoid. I have a 95 amp alternator in my Jeep Cherokee. I am not concerned about running a single wire to the house battery since it is a common set-up but I am not sure what effect running two wires, with either two solenoids or one solenoid, would have on my primary charging system.

[Gerry]

I would also prefer to keep a standard trailer connection in case I may be called upon to pull another trailer or require the trailer to be towed by another vehicle.

Ecasey: the 7 pin connector I got for my tow vehicle had the other 4 pin connector right along side of the 7 pin so I can haul my utility trailer on the 4 way or use the 7 way when I pull my Boler.
This system of keeping the fridge cold while traveling works great for me but as far as recharging the trailer battery + keeping things cold it doesn't happen...not enough power comeing back on charge wire to make up for the draw the fridge is taking out, but every little bit helps.
Gerry the canoebuilder

[Pete Dumbleton]

Some comments:

Bargman and Pollack are essentially the same thing; with Pollack coming first, I believe. Some sites use Bargman/Pollack as one word.

When terminating heavier wire on the 7-way connector, I just snip a couple of strands of the wire off so it will fit in the connector.

One drawback to having heavy loads on the charging system is that the system is seeing a composite of the trailer load and the TV battery.

Here's some food for thought:

http://www.hellroaring.com/rv.htm

The first diagram seems to have some potential, although it requires an additional wire between the egg battery and the isolator, but maybe that's finally a use for the wire in the center of the 7-Way B/P!

Pete

[Al Nelson]

I Have been pulling trailers for 30 years and I have always simply run a 10 guage wire from the alternator to the trailer and never had any problem with running the fridge and charging the battery at the same time. You do of course, have to shut off the fridge if you stop for any length of time. I don't bother if just stopping for fuel but if stopping for more than10 minutes or so, we just switch the fridge off. It will keep cold for a couple of hours or so. You should switch to 110 or propane of course as soon as you stop at your destination. That's always the first thing we do when we pull in for the night. We always have a fully charged battery and a cold fridge. I guess one could install a relay in the line from the trailer battery to the fridge that would open when you stop the TV engine.
Al

[Frederick L. Simson]

The first diagram seems to have some potential, although it requires an additional wire between the egg battery and the isolator, but maybe that's finally a use for the wire in the center of the 7-Way B/P!

...For those who are not using that pin for Back-up Lights on the trailer.

[Gerry]

I Have been pulling trailers for 30 years and I have always simply run a 10 guage wire from the alternator to the trailer and never had any problem with running the fridge and charging the battery at the same time. You do of course, have to shut off the fridge if you stop for any length of time. I don't bother if just stopping for fuel but if stopping for more than10 minutes or so, we just switch the fridge off. It will keep cold for a couple of hours or so. You should switch to 110 or propane of course as soon as you stop at your destination. That's always the first thing we do when we pull in for the night. We always have a fully charged battery and a cold fridge. I guess one could install a relay in the line from the trailer battery to the fridge that would open when you stop the TV engine.
Al

I to do not have a relay in my chargeing system but do have a 30Amp in line fuse to protect the tow vehicle system from the trailer system.
I have a trailer battery so do not worry about draining TV battery down while fueling up but if we stop for extended time I just unplug trailer from TV and fridge keeps going on 12V from trailer battery alone.
Gerry the canoebuilder

[Pete Dumbleton]

I to do not have a relay in my chargeing system but do have a 30Amp in line fuse to protect the tow vehicle system from the trailer system.
I have a trailer battery so do not worry about draining TV battery down while fueling up but if we stop for extended time I just unplug trailer from TV and fridge keeps going on 12V from trailer battery alone.
Gerry the canoebuilder

I put an auto-reset circuit breaker in instead of a fuse (but something should be there, for sure!!). My reasoning is that if the fuse blows, I might not realize it until the egg battery is dead (which will happen quickly if the fridge is in 12VDC mode). I expect that if I had a hard short to ground I would either hear the ARCB clicking away or it would burn up (and they are almost as inexpensive as fuses).

[Brian B-P]

So connecting both wires to the trailer end of the solenoid with one circuit breaker/fuse is a no go? The solenoid I have (but have not installed) is not a double pole solenoid.

The commonly available solenoids for this purpose never are double-pole... but an adequate one (probably described as a "relay") should still be available... but not likely from an RV supply store. The only reason I'm suggesting two is that if both circuits are connected to the same solenoid output, they're really one circuit, not two, and the trailer battery will keep running the refrigerator even when the solenoid is off.

The relay to power the refrigerator controlled by the tail light circuit would eliminate this problem, and you're back to one ordinary single-pole solenoid working fine.

I have a 95 amp alternator in my Jeep Cherokee. I am not concerned about running a single wire to the house battery since it is a common set-up but I am not sure what effect running two wires, with either two solenoids or one solenoid, would have on my primary charging system.

I can't think of any reason why it would be a problem. There are lots of circuits in the vehicle. Connecting anything with it's own battery introduces the issues which Pete has mentioned, but other than that what's one more circuit?

[Brian B-P]

Just in case there's some confusion about what the centre "aux" pin is used for...
It's for whatever you want to do with it. So far, I've heard of three uses:

• back-up lights - factory setup on some trailers, especially larger ones
• stop (brake) light - when separate stop and turn signals are used (wire is provided from the factory in many Bolers for this purpose)
• additional power circuit - this discussion

[Byron Kinnaman]

Just in case there's some confusion about what the centre "aux" pin is used for...
It's for whatever you want to do with it. So far, I've heard of three uses:

• back-up lights - factory setup on some trailers, especially larger ones
• stop (brake) light - when separate stop and turn signals are used (wire is provided from the factory in many Bolers for this purpose)
• additional power circuit - this discussion

My Dakato tow package has the center pin wired to the back-up lights, as does my Scamp.

[Byron Kinnaman]

I was curious about this whole thing and did a bit of testing. Here's what I did.

1. I made sure the battery wasn't fully charged.
2. Pulled the fuse at the battery and connected an amp meter.
3. Turned on the fridge to battery and measured the current. It was approximately 9.5 amps.
4 With fridge on and meter still in place I connected the pig tail to my truck.
5. Started the truck and checked the meter. For a couple minutes the meter read -4.5 amps, then dropped to -2.5 amps.

Notice the sign associated with the current.

This indicates, at least in my case the TV will charge the battery with the fridge running on battery, maybe not real well, but maybe enough. Also the wiring in both the truck and the Scamp are factory. The truck is a 2005 Dakato 4.7l with tow package.

If you have doubts you might want to try the above test, even then I doubt the 12 heater in the fridge will keep things really cold. I plan on using ice while traveling or at least supplementing with ice.

[Greg A]

Byron,

If all the wiring is factory, do you know what gauge they ran for the charging line?

[Byron Kinnaman]

Byron,

If all the wiring is factory, do you know what gauge they ran for the charging line?

I have no idea. If I was to guess I'd say probably 12 AWG, although it could 10 AWG. The truck also have a heavy duty battery, so I would guess that it's also got a pretty strong alternator.

[Steve L.]

I did a similar experiment as Byron did several years ago. I make no representations as to absolute truths but will report what I did.

I'd installed a Link 10 battery monitor in the trailer. Without going into to much detail, one of its functions is measuring total amp-hours consumed and replaced. Amp hours drained show up as negative numbers and as you charge the battery, the negative numbers count backwards to zero. The theory being that x numbers of amp hours drained needs to be replaced by X numbers of amp hours of recharge (with charging inefficiencies in there somewhere).

Before going on a long trip, I topped up the charge on the battery and zeroed out the amp hour count just before pulling out of the driveway. The reefer was set to 12v operation.

I also travel with a GPS. I zeroed out the trip odometer functions. One of the functions on the odometer page of the GPS measures time stopped.

My tow vehicle (a '03 Explorer at the time) has the tow package and it includes a relay that disconnects the trailer charge circuit from the tow vehicle's battery when the ignition is off.

I did this experiment only on a travel day and repeated it twice. The short of it was that there was a net drain on the trailer battery that equaled the stopped travel time (basically rest stops and gas stops) times the amp draw of the reefer on 12v.

I concluded, that for my vehicle trailer tow package at least, the tow's alternator kept up with drain but put few if any amp hours or charge back into the battery.

Again, I make no claims that my results apply to anyone else.

[Pete Dumbleton]

If all the wiring is factory, do you know what gauge they ran for the charging line?

Not dead sure about this, but I believe Scamp generally uses 10G for the internal and umbilical cable power wire.

At a recent RV show, I noticed that the BulgeMobile trailers had 6G to the battery and the smaller trailers had 8G.