Beginners conner

I have 300 watt panels, 20 amp mppt controller, 400 amp hour amg batteries, in a travel trailer, i want to run the fig on electric. Its one that automatically detects power then switches to propane. Trying to find a inverter with a adjustable low level cutoff so i wont ruin my batteries. Any ideas here? Thanks

I can't think of an inverter with user adjustable cut off until you get into the higher price area but pretty much all charge controllers have adjustable cut offs for their switched output. Even controllers that don't actually have a switched output, such as several of Victron's models. can control a relay via a connector. You could use the charge controller's switched output to control a heavy relay, or a contactor set, and have that switch power to the inverter.

My honest opinion is that this is a bad idea.

Typically when Travel Trailer fridges are running on electric, they are in the 500-600 watt range at about a 70% duty cycle. With your 300 watt panels, you are fighting an uphill battle trying to keep up with the demand. Your batteries will be pretty much constantly draining and over time you will kill some nice batteries in a hurry because that chemistry likes to stay fully charged.

You are better off to run on Propane as typically these fridges cool better on propane and the cost per BTU is WAY less with the propane than this solar system.

Thanks for all the help guys

HighTechLab said:

Typically when Travel Trailer fridges are running on electric, they are in the 500-600 watt range at about a 70% duty cycle.

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That's bonkers level of power. Is it using peltier devices or the like? Even an aqueous ammonia system wouldn't use that much power.

gnubie said:

That's bonkers level of power. Is it using peltier devices or the like? Even an aqueous ammonia system wouldn't use that much power.

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A while back I did a test will a Kill A Watt meter, but memory may not be serving correctly.

A quick google search for "rv propane fridge btu rating" says if it is 12 cubic feet fridge, then it can consume approximately 1.5 pounds of propane on a daily basis. With this, the energy of about 1,400 British Thermal Units (BTU) is produced every hour.

1.5 lb/day * 21,600 btu/lb = 32,400 BTU/day
32,400 BTU/day / 24 hours = 1350 BTU/hr
100 BTU/hr = 29.3 Watts
13.5 * 29.3 = 395 WattHr

So the math says 395 watts on electric, assuming the electric heating element is the same as the propane flame.

Our 8 cf dometic electric/propane fridge uses about 30 amps DC when the electric heating element is on through our inverter as measured by a battery monitor which would be including any inverter and wire losses. Your 300 watts of panels in good sun will be producing closer to 15 amps. Not enough to run the fridge even during the day without a discharge to the battery.

Ammonia absorption fridges are not very efficient compared to a compressor fridge. They have the advantage of using LP gas which can be convenient for RV travels and plugging in where power supply isn't an issue but not great for inverting battery power.

We do run our fridge through the inverter most of the time during the day when it's sunny but with almost 1100 watts solar which is enough to power the fridge and keep the battery charged.

There are inverters with low voltage cut off. Magnum Energy makes a nice product. Ours is set at 12v as a safety measure for our LFP batteries but the only times it has tripped is during testing. We try not to run the battery down that far as a course of normal practice.