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Old 04-19-2013, 08:33 AM   #11
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The 110 volt option will require more battery than you have.

SoMoney is correct that heating the ammonia with the 110 volt electric heater uses way more electricity than the 12 volt system because the gas provides the heat.

Suppose you had a 100 AH battery and only used it to run the inverter to power the fridge AND it drew 325 watts when cooling AND it cycled "ON" 50% of the time.

325 watts @ 120 volts inverted AC is 27 amps of DC power required by the battery. Using the chart below, your 100 amp hour battery is reduced in capacity and can only deliver 55 Amps per Hour (NOT 100) at a 27 Amp load.

While there will be "some" recovery of capacity when the battery's load drops to whatever the "OFF" load is, it won't be the full rated capacity.

If the Fridge was "ON" 100% of the time, the battery would last 55 AH divided by 27 amps (or 2 hours)

Now, the Tow vehicle will be pumping amps back into the battery at the same time, but if you read the article on why alternators stink as deep discharge battery chargers you will see why it won't add significantly to the life of the battery.

Also the battery will be at reduced load when the heater is "OFF" so perhaps the life of the battery will be double the calculated "ON" life.

Remember this is for a 100 AH battery. If you have a 75AH battery, your life will be less than 75% of 100 AH to start (as you would need to shift the curve down).
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Old 04-19-2013, 12:09 PM   #12
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For 8 hours @325 Watts you'll need approximately 400Ah of battery capacity.
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Old 04-19-2013, 08:24 PM   #13
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And so, the advantage of multiple batteries (in addition to additional capacity) is the load is spread (evenly if the batteries are identical) so each individual battery "sees" only 1/2 of the load (for 2 batteries).

If you have 2 100 AH batteries and a 27 AH load, the load on each battery is 1/2 of 27 amps or 13.5 amps. Using the chart, 13.5 amps will reduce each battery to 70 AH capacity, so the total available AH in your bank would be 140 AH. 140 AH divided by 27 amps (100 percent "ON") is about 5 hours and 10 minutes till dead. Again, less ON time will increase battery longevity to perhaps double that.

That might be enough to accomplish what you are trying to do.
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Old 04-20-2013, 11:29 AM   #14
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When I had a popup, it had a small refrigerator that was 3-way. We had no problem with it staying cool while we drove just by drawing power off the truck alternator (12v). That one probably drew less power than what we're taking about here, but I just wonder if we're not neglecting how much of this energy draw is going to come for the tow vehicle. Herk mentioned that an alternator wasn't good at charging a deep-cycle battery, but it's not charging we're talking about here. 15 amps coming from the tow will be a significant reduction in battery requirement.

As long as the OP is willing to add batteries if needed, he should probably try it with what he's got. It may work just fine.
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Old 04-20-2013, 02:15 PM   #15
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In this scenario he's forbidden from using the 12VDC/Propane option while driving in his country (no idea how they would find out though)...

Its possible for a 12VDC / 70Amp alternator to supply the necessary 12VDC/35AMPS necessary to invert for 110VAC/325Watt usage but I bet running a 110V extension cord from your truck to your trailer is even more illegal
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Old 04-20-2013, 03:03 PM   #16
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Quote:
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In this scenario he's forbidden from using the 12VDC/Propane option while driving in his country (no idea how they would find out though)...

Its possible for a 12VDC / 70Amp alternator to supply the necessary 12VDC/35AMPS necessary to invert for 110VAC/325Watt usage but I bet running a 110V extension cord from your truck to your trailer is even more illegal
He's not forbidden from using 12v. He requested that fridge but didn't get it.

I guess none of us had thought about just putting the inverter in the tow vehicle. I don't see why it wouldn't be allowed to run a (another) wire from tow vehicle to RV. You'd just have to be careful not to discharge your tow vehicle battery when you stopped the truck for a break, etc.
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