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Fridge on inverter


jmowrey

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On a recent trip I set it up so my frig (propane and 120 only) is plugged in to my inverter while driving. Has anyone ever done this? It seemed to work fine for about a 2 hour drive, but I'm unclear about how may amps my frig draws. It's a Dometic 2400. From what I can decipher in the manual it seems it draws 10 amps. Could be wrong about this though. It's unclear. My concern is whether or not the alternator generates enough juice to maintain the cab battery while there is a 10 amp draw happening on the coach battery, since the alternator is also charging the coach battery too. I don't want any surprises. It's an 1100 watt inverter wired direct to the coach battery. The frig is plugged into a heavy duty extension cord which runs from one of two plugs provided by the inverter. Once we parked, I switched the frig to propane.

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i checked mine once with a watt meter at 175 watts so the inverter should run it but it will suck up 14-15 amps DC.

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I cannot think of any reason why you cannot do it while driving.  Just not very efficient, but who cares along at it works?  Most of these 4 cubic foot Dometics draw 8-10 amps constantly and never shut off.  At least according to Dometic.  I just read this recently from Dometic.  They state the electric mode - regardless if AC or DC is for keeping things cools once things are already cold from using propane.  Dometic says the electric-heaters are not good enough to use full-time for cooling.

Now - if you were driving a lot at night - with the headlights on, maybe AC on, and with a small alternator - you might start seeing the charge light come on when the engine was at idle speed. I had that problem with my 1973 Champion motorhome.

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6 minutes ago, linda s said:

I've never noticed any difference in cooling between AC and propane. 12 volt doesn't cool as well. I always precool before a trip by plugging it in. No need to waste the gas. Gets cold just as fast

Linda S

Mine seems to work okay when I plug it in the night before. I am just citing what Dometic says.  I know my propane burner in the frig is rated around 650 BTUs.  My electric chimney heaters - 12 volt and 110 volt only make 350-400 BTUs. 

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Yeah if I'm headed some where I use the grid the day before but switch to gas until I get home. Never blew up a gas station or had the flame blow out.

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1 hour ago, linda s said:

That would mean the 110 is as inefficient as the 12 volt. It's not.

Linda S

You lost me. I made a reference to MY refrigerator; not your's.  Some Dometics have same-wattage chimney electric heaters for 12 volts and 110 volts.  Some others have unequal wattage electric heaters for 12 volts and 110 volts.  Some Dometics have just one heater that has both coils and some Dometics have two separate coils.  If you feel your refrigerator heats better with 110 VAC then with 12 volts DC - then I suppose you have a model with two different wattage heaters. Or perhaps there is something wrong with it.   Watts are watts.  As far as I can tell, any differences in wattages has nothing to do with efficiency. Just a matter of one making more heat then another.  Being electric-resistance heaters - either one will be near 100% efficient in creating heat from electricity.  Now - if one is not making as good contact with the chimney as the other - that can change were that heat energy goes (i.e. "efficiency").

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2 hours ago, linda s said:

That would mean the 110 is as inefficient as the 12 volt. It's not.

Linda S

A few specs on two Dometic refrigerators I have here. One is the RM211 that came OEM in my 1978 Chinook.  It has a dual-element heater, works on 14 volts or 110 volts and puts out 95 watts of heat in either mode.

I also have a RM2604 that has two separate elements.  The DC element is rated 388 watts on 14 volts (214 watts on 12 volts).  The AC element is rated 252 watts @ 110 volts.  285 watts if grid power is 117 volts.

Not sure about the RM2401 I have. Never looked up the specs.

 

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The heaters for the 2401 are 160 watts for 120v and 175 watts for 12v

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9 minutes ago, WME said:

The heaters for the 2401 are 160 watts for 120v and 175 watts for 12v

If so, seems the DC heater would  be making around 238 watts when cruising down the road.  175 watts @ 12 volts means .82 ohms resistance.   Same .82 ohms at a running voltage of 13.5 volts comes to 238 watts unless my Ohms-Law math is wrong.

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To add to the discussion.  To calculate the approximate BTU from the resistance heater element, multiply the wattage by 3.12142  So the 175 watt heater at 12 volts produces 546 BTU.  The heating element is simply a strand of nickel chromium wire that runs an electrical current through it to produce heat.  Not exactly the most efficient way to produce heat but it is simple and cheap.

Another thing to note is that with heat, the wire element resistance increases throwing the calculation off a bit for the higher voltage vs temperature.  You'll get a bit more heat out of the element at the slightly higher running voltage but the calculation is not linear.

Edited by Back East Don
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The 120 and 12v are just generic terms to tell them apart. Who knows at what actual voltage was used to calculate the wattage.

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  • 4 weeks later...
On 9/28/2016 at 4:35 PM, Maineah said:

Yeah if I'm headed some where I use the grid the day before but switch to gas until I get home. Never blew up a gas station or had the flame blow out.

I always turn off any pilot lights or open flames before gassing up. I HIGHLY recommend you do the same for yours and others' safety!

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