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Old 11-07-2010, 06:01 PM   #1
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2011 25' FB Flying Cloud
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Inverter to run refer while driving...

Ok, so we're not big fans of having the refer run on gas while driving down the road, so I'm looking into using an inverter to run the refer/freezer off the 12volt system.

So while installing an inverter to run the refer is easy enough, does anyone know what will happen to the charge level of the AS batteries? I don't really know what our TV(a Tundra) will do when it's own battery is charged after driving awhile.

I've looked into inverters and the like, but haven't really found any info on this specific situation, so any help would be appreciated. We've just picked up our new AS, after a brief stint in the Class B world. Our refer there ran on 12v, so we've kinda been used to that.

Thanks,
Jeff
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Old 11-07-2010, 06:15 PM   #2
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Originally Posted by lilynhoward View Post
Ok, so we're not big fans of having the refer run on gas while driving down the road, so I'm looking into using an inverter to run the refer/freezer off the 12volt system.

So while installing an inverter to run the refer is easy enough, does anyone know what will happen to the charge level of the AS batteries? I don't really know what our TV(a Tundra) will do when it's own battery is charged after driving awhile.

I've looked into inverters and the like, but haven't really found any info on this specific situation, so any help would be appreciated. We've just picked up our new AS, after a brief stint in the Class B world. Our refer there ran on 12v, so we've kinda been used to that.

Solar would help a bit, but you really need a much larger battery battery bank and a way to keep them charged to do what you want. Another avenue would be to replace your Dometic fridge with a marine unit that uses a Danfoss compressor and will run on either 120VAC or 12VDC, which ever is present. They are very efficient and draw only 3-4 amps DC while running. I'm waiting for my Norcold to die (should be any day now) and I will be replacing it with a marine unit.

Thanks,
Jeff
Jeff,

You will need a very large battery bank to run your refer on AC thru an inverter. The elements in a Dometic fridge your size draws about 120watts AC. That becomes 1200 watts DC which is 120amps that will be drawn every hour. Even if the element is active only 50% of the time, you will still require 60 amps/hour not even factoring in the inefficiencies of the inverter. Even the best run at 85%, so add another 15% to 60 and you get almost 70 amps DC/hour. Electric heating elements really burn up DC power in an RV.

Assuming that you have the stock Interstate group 24 batteries (160 amp/hours for the pair), you could run your refer thru the inverter for less than an hour before you reach the 50% discharge level on your batteries. Your TV will NOT recharge these batteries at that rate, and may give you another half to one hour of run time.

These refers are designed to run on LP WHILE TRAVELING. Many, many folks (including myself) run the refer on LP while on the road with NO consequences, other than cold beer when you stop for the night.

If you really don't want to use LP, you can cool it to temp on electric while connected and then pack some dry ice into the refer and freezer box and keep it closed until you reach your destination, when you can hook up to 120VAC. This really converts your refer to a big cooler.

Just remember that for every minute you keep that refer door open, an RV fridge requires ONE HOUR of run time to recover the lost chill in the box.
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Old 11-07-2010, 06:16 PM   #3
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The alternator in your TV should be able to provide sufficient current to operater the refrigertor through and maintain charge on both TV and Airstream batteries. I have done this on earlier Airstream with no problem. You will need to provide a transfer switch to transfer refrigerator from normal AC power feed to inverter and back again. Good Luck
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Old 11-07-2010, 06:19 PM   #4
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12v dc or 120v ac

Jeff,
Is your refer a 2-way (lpg-120v ac) or 3-way (lpg-120v ac-12v dc)? If you run it on either 12v dc or go through an inverter (NOT CONVERTER) it will take batteries down fairly quick. We run ours on lpg while driving and while dry camping and use 120v when we have hookups. We've done this for years without any problems even while fueling (biggest concern). Quite a few gas stations post requirements about turning all lpg pilot lights off prior to fueling. It's your call as to what your comfort level is. Gary
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Old 11-07-2010, 06:25 PM   #5
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Another option is the AC/DC marine fridge that uses a Danfoss compressor. These are very efficient units that draw only 3-4 amps DC when running. They can operate 30* off level and because they use a compressor, they really get your box down to 34* quickly and keep it there! Ahhhhh, COLD beer for a change and HARD ICE CREAM!!!!!

I will be replacing the aging Norcold in my MoHo with one of these units very soon. They have more interior space than a standard RV fridge, and will switch automatically to either AC or DC (which ever is present) with a priority on AC if it is present.
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Old 11-07-2010, 06:26 PM   #6
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Dometic quit making three-way refrigerators years ago because of (A) Improved design that would enable propane during running and (B) So many folks ran their batteries down by forgetting to switch them over when stopping at a hook-up campground. Our Dometic switches over automatically when it senses shore power.
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Old 11-07-2010, 06:31 PM   #7
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If your refrigerator operates at 120Watts at 120 volts AC It will also operate at approximately 120 Watts at 12 Volt DC. It will draw approximately one amp at 120 volts AC and 10 amps at 12 Vots DC.
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Old 11-07-2010, 06:58 PM   #8
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Thanks everyone, good info and suggestions. I know most people travel with the gas on, but some states don't allow this and if you're on a ferry, it must be shut off.

According to the label, the refer draws 2.7 amps, so 324 watts AC. I also found some info that says the Tundra provides 12 constantly at the plug. So I'm thinking this should work fine, as our only goal is to run on 12v when driving. It appears that some inverters have automatic transfer switches, so when we plug-in, that should solve the memory issue(switching to AC).

On another note, the refer has a function that when you shut off the truck, the gas and spark are shut off for 15min, such as at a gas station. Ours is not connected, has anyone connected this?

Thanks,
Jeff
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Old 11-07-2010, 09:22 PM   #9
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Jeff

I have tried operating my refrigerator while driving using a 400 watt invertor. It worked fine for about an hour, but then the battery was down too low in voltage to operate the refrigerator. My refrigerator draws 180 watts or 15 amps, so it is a significant load. I have a Tundra with a 150 amp alternator. However the charge line to the trailer, I believe, is limited to only 5 amps. So with 5 amps to the trailer and 15 amps to the frig, it is just a matter of time before your battery voltage is too low.

There has got to be a way to get around the 5 amp charge rate to the trailer.

It also helps to make sure your frig is completely cold before your trip and we carry a one gallon container of water frozen to keep the frig from running. Like Lew suggested, you could carry some dry ice too.

I jumped thru these hoops when my frig was not working on LP. It works on LP now, so I just run it on LP when we are driving. I don't have a safety problem with it.

Dan
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Old 11-07-2010, 09:59 PM   #10
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There's no doubt a way to do this, but I have to go with those who run the frig on LP while towing...it's simpler, it's faster...and your newer frig is designed to be operated that way. We have done it for years and have never had an issue or a worry...in this trailer or in a small motor home we had before the AS. If it makes you feel more comfortable, just go inside and turn off the frig when you are pumping gas, then turn it back on after you pull away from the pumps...
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Old 11-07-2010, 11:04 PM   #11
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Our 2001 Lance TC had a large 3-way refer in it, and we ran it on the 12 volt mode when on the road...

I wired a constant duty relay from the alternator output directly back to the 12 volt power terminal on the camper harness - I used 6 gauge wire in hopes of keeping voltage loss at a minimum...

We never had any discharged battery issues with this set up - just had to remember to turn to gas mode when stopped - it would auto change to AC when plugged into shore power, of course...

The 12 volt TC wiring was much shorter in length than wiring required to run back to a TT, so I'm not sure it would really be an acceptable option without lots of rewiring, etc...

Ray
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Old 11-08-2010, 08:47 AM   #12
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Jeff

I have tried operating my refrigerator while driving using a 400 watt invertor. It worked fine for about an hour, but then the battery was down too low in voltage to operate the refrigerator. My refrigerator draws 180 watts or 15 amps, so it is a significant load. I have a Tundra with a 150 amp alternator. However the charge line to the trailer, I believe, is limited to only 5 amps. So with 5 amps to the trailer and 15 amps to the frig, it is just a matter of time before your battery voltage is too low.

There has got to be a way to get around the 5 amp charge rate to the trailer.

It also helps to make sure your frig is completely cold before your trip and we carry a one gallon container of water frozen to keep the frig from running. Like Lew suggested, you could carry some dry ice too.

I jumped thru these hoops when my frig was not working on LP. It works on LP now, so I just run it on LP when we are driving. I don't have a safety problem with it.

Dan
Okay, this is what I was concerned about, not having the current to carry the full load of the refer. So it sounds like it's LP unless I rewire the truck!

So the question remains, what about the alternator sensing wire that needs to be connected to the refer? Does anyone have this or does Airstream just not connect it for some reason?

thanks for all the help,
Jeff
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Old 11-08-2010, 09:20 AM   #13
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Thanks everyone, good info and suggestions. I know most people travel with the gas on, but some states don't allow this
What states are these? I was not aware that there were any state-wide prohibitions.
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Old 11-08-2010, 09:28 AM   #14
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Jeff,

You will need a very large battery bank to run your refer on AC thru an inverter. The elements in a Dometic fridge your size draws about 120watts AC. That becomes 1200 watts DC which is 120amps that will be drawn every hour. Even if the element is active only 50% of the time, you will still require 60 amps/hour not even factoring in the inefficiencies of the inverter. Even the best run at 85%, so add another 15% to 60 and you get almost 70 amps DC/hour. Electric heating elements really burn up DC power in an RV.
As rptradewind noted the wattage would be approximately the same (324 watts) with either electrical source, approximately 3 amps at 120 volts or approximately 30 amps at 12 volts.

On a hot day it is not at all unusual for the element to run continuously for several hours particularly if the fridge has been opened frequently or warm food recently added as might be the case at the beginning of a trip.

Yes you can get 30 amps from the tow vehicle alternator back there but it requires tow vehicle modifications.

I would be more as concerned about fire safety with a large inverter load like that as with a gas fridge. Both are fairly safe if properly maintained, but pose risks.
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