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Old 07-06-2012, 10:31 PM   #1
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inverter to power fridge while driving?

Hi everyone, I just put a new fridge in the SowBug and a thought came to me. It's a Dometic with the 12v control panel, and 110v or propane driven. I realize most people run propane safely while driving, and I also know how to put a bag of ice in there before we leave the house. This fridge draws 110-120v at 5A and that comes out to 600watts, so I'm wondering why I couldn't run an inverter driven by the AS battery, charged by the truck? I do understand that when I drop the voltage to 12v and increase the wattage of the inverter then I could drop the amperage draw, and of course use large cables. I'm not an electrical engineer or even an electrician, so for any input I would be very grateful. Thank you , Doug.
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Old 07-06-2012, 10:44 PM   #2
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600 watts / 12 volts = 50 amps. That is a whole lot of 12 volt power. Even if you put the inverter in the engine compartment of the truck, it is a really big demand on the alternator. If you try to get 50 amps back to your trailer to run the inverter there, good luck. Takes a big cable and good ground system back to the truck, and very heavy duty electrical connectors.

Bottom line: anything can be done, given time, and money, but this is just not the best way to spend either.

Even the 12 volt option on some RV refrigerators takes in the range of 25 amps, and does not really cool all that well. It can work somewhat in a Motorhome, where you can run larger wires to the refrigerator easier than you can in a TT, but even then, it is not really a great idea and many refrigerators no longer are available with that option at all.
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Old 07-06-2012, 10:46 PM   #3
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The charge line from your TV will not keep up with the power required to run the refer. If the refer runs constantly it will drain the battery in no time at all.
Let's say the heating element is 240 watts; at 12 volts plus the loss thru the inverter it will draw about 22 amps. With a group 24 battery rated at 100 ampere hours. That's 4 hours max at best.
If your refer requires 600 watts, you are looking at a dead battery in less than 2 hours. Also as the battery drains, it will eventually drop the output voltage to where the inverter will shut down automatically. Which I would venture to say would happen in about an hour or maybe an hour and a half at best. With a single group 24 battery.
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Old 07-07-2012, 12:25 AM   #4
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Doug,

Because my trailer is deisel powered, I use an LG fridge, 110v-1.5 amps. I've used it for over 5 years now.

While enroute, or when I am overnighting at a shopping centre or some such, the fridge is powered by a 1500w inverter run off the two standard AGM batteries in the trailer.

The latest example was yesterday while the trailer was in a shop having some work done. The record setting high temperatures sent some shop workers home early but my fridge held its 38 degree temerature perfectly, for about 6 hours.

Sergei

Enroute with my iPad; please excuse the typos.
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Old 07-07-2012, 06:45 AM   #5
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Why are you worried about running LP while traveling? It is done all day every day by thousands of RV'ers with no problems. I am not trying to be a wise guy, I am just curious.
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Old 07-07-2012, 08:34 AM   #6
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Bruce,

This can be a hot potatoe subject, something like trying to discuss gun control or medical insurance in your country.

That thousands every day chose personal convenience - a cold beer maybe - or personal comfort - a pre- warmed trailer perhaps - over safety doesn't change the fact that it can be dangerous.

There are countless documented cases of somebody stepping inside, switching on the lights, and leveling a home or trailer.

Lots of pictures on the internet of RV hulks burning to a crisp. Documentation there too of an RV that exploded at a gas station, leveling the place and killing a Good Samaratin.

Some of the photos concern the recent NORCOLD recall.

Propane is resticted or outlawed in many jurisdictions. It is forbidden on many bridges and in many, many tunnels. That this is sometimes not endforced doesn't mean its safe.

Manufacturers themselves advise against using propane on the road.

Sergei

Enroute with my iPad; please excuse my typos
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Old 07-07-2012, 11:42 PM   #7
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Thank you all for the replies. Idroba, I was going to increase the size of the inverter to 2000w which i figure would drop the amperage draw to around 16A, unless I'm missing something. TV alternator is 125A and this is only while we are enroute to go camping. When we arrive it would be shore power or propane. The Airstream also has a battery in it which the inverter would be hooked up to. Truck would only charge the battery, while we are in transit. All this would be unhooked upon arrival at the campsite. SmokelessJoe, the reason for the idea is to keep our daughter's chemotherapy drugs cool. We have traveled all over the world with a cooler and a spare with ice to keep her meds cool and everyone including hotels, ship's, airlines, and trains have been very helpful and compassionate. I thought camping would be fun for us so here we are fixing up an old Airstream to have some fun. My concern about the propane while driving is if the flame goes out, meds don't stay cool. The racer in me says, what can I do to make this a little better? You say your trailer is diesel powered. Does that mean you have a diesel generator or is the tow vehicle a diesel? Last question is , what is an LG fridge? doug
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Old 07-07-2012, 11:59 PM   #8
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I'd put a Kill a Watt on the fridge and see what it actually draws. I strongly suspect it doesn't draw 600 watts, it probably draws significantly less.

Before planning any course of action, see what the refrigerator actually uses. Test it when first run (initial cooldown) on electric to ensure the heating element is powered up.
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Old 07-08-2012, 12:01 AM   #9
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A larger inverter will not decrease the load. If you need 600 watts from an inverter it will require the amperage I mentioned in the post above, inverter size must be at least as large as your load, but a larger inverter will not change the load you are supplying. I have no idea where you got you 16 amp figure. Watts = volts x amps and thus 16 amps x 12 volts = 192 watts, not 600 you say you could need.

BTW, propane refrigerators seldom go out, and the newer electronic control ones automatically light, or re light if they do. I am in the camp that says that propane refrigerators used while traveling is quite safe and have probably 200,000 personal miles with propane on. But nothing is 100% safe of course.

There are some small 12 volt compressor units that look like a big ice chest, Norcold has several of them. They are fairly expensive as in the $700 range but use only about 3 amps at 12 volts and are very very reliable. You might consider one of them to cary the medicine you need.
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Old 07-08-2012, 07:43 AM   #10
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The lengths people go to to avoid propane really amaze me.

Yes, your alt is 125 amps in your tow vehicle, but don't forget it's using much of that capacity already... running the vehicle and recharging your batteries for the next start.

Yes, there is some spare capacity (or you could put a higher amperage alt in), but as was mentioned it's all but useless for this application because the charge line to the trailer isn't heavy enough, and making it heavy enough to provide a much higher charging current is difficult.

Your fridge should have a circuit to detect if the flame went out and should re-start it. At least I think that's how ours works; I've never actually had it go out.
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Old 07-08-2012, 10:10 AM   #11
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You could always just get a separate refrigerator for the meds that works on 12vDC/120vAC but not on propane.

My Interstate has one, a Nova Kool R3100 (3.1 cubic feet) though they make both smaller and larger ones to suit your specific needs (In your case, I'd get one that has enough space for the meds with only a little bit left over; a full fridge cools more efficiently than an empty one). Their smallest is 1.9 cubic feet, their biggest is a 9.1-cubic-foot upright double-door unit.

The Nova Kool is not an ammonia-cycle unit like the propane-fired refrigerators. Ammonia-cycle refrigeration is oldie-but-goodie technology that goes all the way back to Dr. John Gorrie in Appalachicola, FL circa 1850, who built the first one to help cool yellow-fever patients. It works, but it's hardly efficient, which is why you've got such a large power draw in electric mode.

The Nova Kool maxes out at 30 watts for their refrigerator/freezer units. You can start the unit the day before a trip to cool it down, and run it while you're underway, just drawing directly off the house batteries in 12v mode without risking battery depetion.

I travel with insulin, which must be kept refrigerated but can't be allowed to freeze, so I can't rely on iceboxes or portable coolers; I need something with a thermostat. I'm very happy with my Nova Kool refrigerator.
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Old 07-08-2012, 10:14 AM   #12
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Thank you all again for the replies. Tpi-I will find a Kill a watt and use it as prescribed! Idroba- I was praying that a larger capacity inverter would lower the load, kind of like a bigger hole lowers the pressure. I guess not ,huh? This is why I'm asking this stuff, Thanks. and Skater- I'm not against propane while driving. Just thinking about an alternative way to power something.
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Old 07-08-2012, 10:25 AM   #13
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Here's a different rationale. Why use the fridge at all? There are countless ways a fridge can become unreliable and fail. Properly designed coolers for medicine can keep meds cool for very, very long periods. I am not pre-judging, but how big could such medicines be? In the past I have kept a dozen viles of 50ml medicine very cold for days, in a small cooler.

There's a lot of room in a trailer for a small cooler. It is cheap, extremely reliable, and requires no modifications to your TV or trailer. Just a thought. Good luck, and happy trails.
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Old 07-08-2012, 11:21 AM   #14
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Hi everyone, Protagonist- I will look into the Nova Kool products. That sounds sounds real good, thanks. redwoodguy- this is what we are used to doing and what we will do on our upcoming trip. It's a 14 hour drive from home, across the Nevada dessert so you can all see how this got started. We first work with what we have....don't want to be trying anything new on that kind of a trip and put the meds, TV and Airstream in jeopardy. We'll save that for the Salt.
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