Norcold 3-Way Fridge Running off DC vs AC w/inverter

AnOldUR · July 25

ReenieG said:

Your 3-way might be drawing 1.5 amps at 120V from the inverter, but to produce the 120V from 12V the inverter will consume about 15 amps from your battery when the fridge is cycling on. That doesn't include the inefficiencies from the conversion or that the inverter will be a steady draw on the battery whether the fridge is cycling on or not.

It will be interesting to see the results from your experiment over a 24 hour period.

MuttonChops · July 25

AnOldUR said:

ReenieG said:

Your 3-way might be drawing 1.5 amps at 120V from the inverter, but to produce the 120V from 12V the inverter will consume about 15 amps from your battery when the fridge is cycling on. That doesn't include the inefficiencies from the conversion or that the inverter will be a steady draw on the battery whether the fridge is cycling on or not.

@AnOldUR I believe the above chart did capture the efficiency of the Inverter and Duty Cycle of the Load for a typical (1st approximation) wet-cell battery. Of course the chart only has Fridge load and does use a very high quality inverter.

What is more worrisome is @pthomas745 observation that he did try using an inverter and thought the Fridge was On most of the time . . . so there was little benefit from having a thermostat control.

AnOldUR · July 25

MuttonChops said:
What is more worrisome is @pthomas745 observation that he did try using an inverter and thought the Fridge was On most of the time . . . so there was little benefit from having a thermostat control.

I was questioning that also. It would be interesting to hear from @pthomas745 what he meant by, " I think it ran almost constantly". Is there a condition that could cause the element to not provide enough heat? Maybe low voltage from his inverter. A larger, better quality inverter might make a difference in @ReenieG's case.

pthomas745 · July 25

I worked on this a day ago, and...got sidetracked with "stuff". I am still not sure if the 3 way fridge..running on electric...has any sort of duty cycle. Here is a photo of the setup: a 50 amp hour lithium battery, and a 600 watt inverter. This is one of the Will Prowse "Milk Crate" alternatives to the spendy "solar generators". It was a nice project for me, just my skill level. Needs a bit more work on the "wire dressing". I use it for my portable 12V fridge when I'm on the road. Hard to see ( I will edit this photo later)....edit, picture fixed so you can actually see the inverter!). The fridge is plugged in to the inverter over on the right side of the photo. My Aili battery monitor is showing the amp draw at 13.13, which seems to match the 11-13 amps the fridges are always mentioned as using, with probably about 1 amp for the efficiency of the inverter.

Image: https://us.v-cdn.net/5021717/uploads/editor/8h/2u38if164cah.jpg

I'll run this for a couple of hours tonight again, but I really think I should start earlier in the day, with a partially cooled down fridge. With the test from a day ago, that 13 amp draw was constant, but the fridge was cooling from around 68 inside the fridge, and I only kept going until I ran out of time, and the fridge was down to around 53 degrees. So...the fridge does work this way, and with a larger battery and a decent solar input, it could be doable.

Does a three way fridge, which needs to heat ammonia to produce the cooling effect, even have a "duty cycle"?

System · July 26

This discussion was created from comments split from: 2021 Tongue Box Dimensions (320).

AnOldUR · July 26

@pthomas745
So, your inverter is drawing 13.13 amps running the fridge at 120V. Have you checked the amperage that just your fridge draws when plugged into a 120V shore outlet? In previous posts 1.5 amps has been quoted. I'm curious what your actual number is.

Also, how many amps does yours draw when running directly on 12V? I've seen 11 amps, but again, a direct comparison would help to eliminate other variables.

ReenieG · July 27

So to throw a couple of wrenches in this discussion, about amperage and draining batteries and how long they will last, my solar panels were charging my batteries while I was running my fridge on 12v while driving. Of course, since I was driving, my car was also charging the battery through the inverter. At times, the solar was charging the battery more than discharging. I'd have to shut off the converter breaker next time to see how much charge I'd get just from solar alone.

So, with all things (close to)even on amp draw on ac or dc, and as it definitely appears the fridge cools much better on ac, the ac still might be a better option with an inverter.

pthomas745 · July 27

Sorry this is delayed for a day.

I precooled the fridge using shore power for a couple of hours. At 8:30, With the fridge temp near 40F, I plugged it into the inverter attached to the 50 amp hour battery. (The fridge "thermostat" setting was about the six o'clock position). The little battery monitor was showing about the same amp hour usage as the photo a couple of days ago: about 13 amps.

At 9:45.....the battery monitor showed I had used 15 amp hours. (About right for an hour and a quarter of constant 13 amp draw.) The fridge internal temp was down to 29 degrees. I turned the "thermostat" knob slowly "counterclockwise" until I heard the "click" that told me the "thermostat" had cycled "off", and the battery monitor showed a .33 amp draw...probably just the draw from the inverter itself.

At 10:30 am, I went back out...and the monitor showed the amp draw back to the 13 amp area. The fridge temp (near the 3 o'clock position) was near 36 degrees.

I grabbed a book and sat in the camper for a bit, and sure enough, about 40 minutes later...the fridge thermostat "cycled" and the amp drain dropped back to the .33 amp number again. The internal fridge temp was 37 degrees.

I did not have enough time to sit and "babysit" the fridge any longer, and ran an errand and was back by 12:45. At that point, the inverter and battery setup had shut down, probably due to low voltage from the battery.

The fridge was still holding a decent temperature, still near 44 degrees. I do not know how long it had been off.

So, I roughly got near 4 hours of power from the 50 amp hour battery, which was a bit expected. I had a cold fridge. And: the fridge does "cycle" in this combination of battery and inverter. The other interesting feature was much less heat from the fridge.

Unless I sit with the fridge for a while, I did not discover how long the "cycle time" is between the power starting and stopping. It would seem that the time the fridge is "off" is fairly short.

There are a lot of variables here: the way the fridge thermostat works with the positions of the sensor on the vanes could be a factor. I used an empty fridge, which probably did not hold the temp well. (The high temp here yesterday was only in the mid 70's.)

When I think about this, I wonder if there is a way to put a "timer" on the power going into the fridge, and cycle the fridge on and off from the battery/inverter? If you could knock down the power needs of the fridge by half, and still hold a safe temperature, that would/could make this method work. And, a decent solar input could recharge the battery if the amp draw was reduced to "only" 6 amps.

Of course, a 50 amp hour battery is not the best option!

Horigan · July 28

If your safe temperature is higher than what the thermostat was set to, just adjust it till it controls to your safe temp. Then you have your on/off timer switch. .

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Norcold 3-Way Fridge Running off DC vs AC w/inverter

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