Powering a 12 volt fridge and battery - Page 2 - Fiberglass RV


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Old 10-22-2007, 11:53 AM   #15
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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.
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Old 10-22-2007, 12:27 PM   #16
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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.
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Old 10-22-2007, 01:35 PM   #17
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Byron,

If all the wiring is factory, do you know what gauge they ran for the charging line?
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Old 10-22-2007, 03:11 PM   #18
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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.
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Old 10-23-2007, 07:05 AM   #19
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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.
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Old 10-23-2007, 05:18 PM   #20
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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.
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Old 10-23-2007, 06:20 PM   #21
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One has to remember that the wire going from the alternator or battery of TV to the trailer battery is only one small part of the equation. The bigger issue is the alternator. Can it handle the larger current required to charge 2 batteries and run the fridge at the same time? The size of the wire only determines how much loss over the 20+ feet of wire.

I just ran some numbers on voltage drop. I used 20 as the approximate length of wire and 12 Amps of current.

12 AWG voltage drop = .3888 Volts
10 AWG voltage drop = .2448 Volts
8 AWG voltage drop = .1536 Volts

As you can see there's not a lot of difference. Going from 12 AWG to 8 AWG you save .2352 Volts. Not enough to make very much difference. However the higher the current the greater the drop. at 15 Amps you loose a bit less than .3 Volts.
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Old 10-23-2007, 07:03 PM   #22
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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.
Never thought of that. Good point. As you wrote, the relay in the tail light circuit (or even remembering to shut off the fridge when stopped) will solve the problem of the fridge draining the battery even with one circuit. I suppose this is still not ideal for battery charging though since the battery will have to keep up with the draw from the fridge. I plan to use two 6 volt GC batteries so there could a lot of charge to replace. Using two circuits would give the best chance for the battery to charge while keeping the fridge running. The continuous duty solenoids are not that expensive.

Based on the posts on this thread and previous threads, I will use an auto re-setting circuit breaker as Pete suggested.
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Old 10-24-2007, 06:57 AM   #23
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West Marine used to have a really good article about why automotive alternators aren't good for recharging deep cycle batteries. This being a topic of real interest for cruising sailboat owners. I can't find it quickly, but the gist of the article was something along the lines of the internal voltage automotive regulator is keyed more to voltage than returning amp hours to the battery. I suppose that while the engine is running it sees the reefer as a drain which it is willing to offset but not much more. There's no doubt that it will recharge the tow's battery over time. Relatively inefficiently since that's not really its purpose. West Marine's Alternator Discussion

We've talked before but I don't remember a consensus as to what happens when you present two different batteries at different states of charge to an automotive alternator. Heck, I work at an auto company. I should track down a fellow geek to quiz him or her. (I did that on the alleged reduction in drag from those fabric truck tailgate thingys, but that's another story.)

In the sailboat marine application the preferred set up seems to be to use a heavy duty continual use alternator with an external regulator with two or more circuits and sophisticated 3 phase charging programs.Balmar Voltage Regulator

As an aside, I stumbled across some web sites for people who modify their cars for those extreme thumpin' stereo systems. Not the merely annoying systems in the car next to you at the stop light, but serious almost competition cars. These systems use serious amounts of amps. The standard practice seems to be a second battery, usually deep cycle, and a second alternator just to recharge the deep cycle. (Search on "second alternator" and "sound system" for scads of urls.)

Har! We should submit the whole recharge your trailer battery with the tow vehicle thing to the Mythbusters gang!

(Edited to add some links found later.)
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Old 10-25-2007, 07:27 PM   #24
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The size of the wire only determines how much loss over the 20+ feet of wire.

I just ran some numbers on voltage drop. I used 20 as the approximate length of wire and 12 Amps of current.

12 AWG voltage drop = .3888 Volts
10 AWG voltage drop = .2448 Volts
8 AWG voltage drop = .1536 Volts

As you can see there's not a lot of difference. Going from 12 AWG to 8 AWG you save .2352 Volts. Not enough to make very much difference. However the higher the current the greater the drop. at 15 Amps you loose a bit less than .3 Volts.
There's more to it than that -- Checking the table below shows that even with 8 AWG, the trailer battery is going to get less than a 100% charge (somewhere in the high 80's, presuming the alternator/regulator would deliver the full 12.6VDC and then shut off, which may not even be true given the presence of the tug battery in the system) and with 12 AWG it will be far less than desirable (somewhere in the low 60's under the same conditions).

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I also like the larger gauge wiring between my egg's battery and fuse block because I want all my battery's energy to go to lighting or whatever, not to heating (albeit miniscule amounts) wires.
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Old 10-25-2007, 07:42 PM   #25
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There's more to it than that -- Checking the table below shows that even with 8 AWG, the trailer battery is going to get less than a 100% charge (somewhere in the high 80's, presuming the alternator/regulator would deliver the full 12.6VDC and then shut off, which may not even be true given the presence of the tug battery in the system) and with 12 AWG it will be far less than desirable (somewhere in the low 60's under the same conditions).

Attachment 10644


I also like the larger gauge wiring between my egg's battery and fuse block because I want all my battery's energy to go to lighting or whatever, not to heating (albeit miniscule amounts) wires.

You're right there is more to than that, or is there? Typical vehicle electrical system runs at 13.8 volts. Fully charged battery according to that chart is 12.6 volts. With a slightly larger voltage drop across the wire it'll take a few minutes longer to get the full charge.

Heat? Heat = watts. The difference in my examples in heat measured in watts is 2.9 Watts between 12 AWG and 8 AWG wire over 20 feet. Not much is it?
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Old 10-25-2007, 09:21 PM   #26
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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
Howdy, I use mine as an auxilliary ground.....use two grounds.....Benny
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Old 10-25-2007, 10:57 PM   #27
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Howdy, I use mine as an auxilliary ground.....use two grounds.....Benny
This is another one of those things won't hurt anything, but doesn't do much either. It's like putting a "Y" on the camp site water faucet running two hoses to the trailer and putting another "Y" at the trailer water connection. It doesn't hurt anything, but it don't do much.
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Old 10-27-2007, 05:49 PM   #28
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One has to remember that the wire going from the alternator or battery of TV to the trailer battery is only one small part of the equation. The bigger issue is the alternator. Can it handle the larger current required to charge 2 batteries and run the fridge at the same time?
I agree this is a concern. I know some smart solenoids designed for charging batteries will lock out the auxiliary battery circuit until the vehicle battery has recovered its full charge. Failing that, I suppose it wouldn't be a bad idea to run the vehicle a bit to bring the starting battery up to charge before plugging in the trailer. I doubt the draw on the refrigerator on its own would be a problem. The 8 amps constant draw of the fridge would be similar to a pair of off-road lights or a pretty standard aftermarket car stereo. Alternators deal with these kind of loads quite well as long as there are not too many of them.

The trailer battery is another issue. Unlike the refrigerator, which would be a constant draw, the draw of the trailer battery would vary depending on the state of charge. My understanding is that the way that a standard alternator works tends to protect the alternator from being damaged. The regulator is essentially a constant voltage charger that tapers off quickly as the battery voltage rises. That is good for protecting the vehicle battery from being overcharged but not so good for charging deeply discharged house batteries. The voltage at the house battery doesn't directly indicate the state of charge. The alternator tapers off the charge well before the battery reaches a full charge. Marine charging systems use external multi-stage regulators (similar to smart chargers) especially designed to charge deep cycle battery banks. I suspect those would kill a typical vehicle alternator which are supposedly designed to provide about 70% or less of their max output for a short period of time. Marine systems with smart regulation use high output alternators that can handle the full load for an extended period of time. I think I might be overthinking this though. People seem to be able to charge their batteries (or at least keep them topped up) without blowing up their alternators so I will just use the heaviest wire I can squeeze through the 7 way connector and run a 12 awg wire in the Aux for the fridge with a separate solenoid.
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