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

ReenieG · July 24

This discussion began on another comment thread. Maybe one of the moderators will move the posts pertaining to this discussion. TIA

Here's where the discussion began, bottom of page 1and continues on page 2. 2021 Tongue Box Dimensions 320

I have questions regarding the efficacy of running the 3-way using an inverter to use the 120v option (like shore power) vs the 12v battery option, which we already know will suck up the battery bank. While I completely understand if the 2 settings work similarly, the math won't work on either setting, but that's not necessarily the case. The two settings might work differently and so using the 120v setting might actually work. Once I have my new battery and inverter setup I will be able to test it to see if there's any better function on the shore power 120v setting.

MOD NOTE: Relevant comments from the Tongue Box discussion have been transferred here.

ReenieG · July 23

@Basil48192 the cutting board... great idea along with the other DIYs. Thank you for the ideas.

As for the 3-way fridge I have, it uses much less amperage on shore power than the 12v setting. Of course propane is best but I want to try it with an inverter when hookups aren't available.

AnOldUR · July 23

@ReenieG
On shore power (AC mode), a 3-way refrigerator draws approximately 2.7 amps. That 2.7 amps will work out to almost 30 amps when using an inverter to convert 12V to 120V. It will eat through your battery pretty fast.

edit:
I pulled the 2.7 amps from a quick search. Looks like it could possibly be more like 1.5 amps. Still, at 15 amps or more, it's a heavy load.

ReenieG · July 23

@AnOldUR with 2-100ah lithium batteries and 2-105w rooftop solar panels, I think it would be pretty benign esp since that would be the max power used; wouldn't you agree? OTOH, do you know what the amperage is on the battery mode? I know it's way more but I can't seem to find the number anywhere. I believe it's like 3-4x the amount. Watching the reading on my bluetooth app when I'm towing in battery mode, I think the amp reading is something like 8-9amps while on battery mode and that's definitely not doable.

AnOldUR · July 23

ReenieG said:

... with 2-100ah lithium batteries and 2-105w rooftop solar panels, I think it would be pretty benign esp since that would be the max power used; wouldn't you agree?

No I wouldn't agree. Even being generous and saying that the fridge only cycled on for half the time during a normal day of use, the fridge and other common draws would be approaching 400Ah of use in 24 hours. Recovering that with your solar setup would be impossible.

By contrast, our 12V compressor fridge consumes 25Ah to 35Ah in a typical 24 hour period. Even with our 310W of rooftop solar and a 400W suitcase, I would never consider trying to power a 3-way absorption refrigerator on 120V electric with an inverter.

ReenieG · July 23

@AnOldUR I'm not sure about your math here, lol. :If we are talking about using the 3-way on 120v, (like shore power with an inverter) that would only be approx 2.7amps (your number) x 24hrs = 64.8amps. Like I said in my previous post, the battery option would never work at 8-9amps. On a good sunny day and/or driving, I can just about fully replenish my batteries up to 100% especially if they're not completely drained, which I don't see how they would be at 65amp draw in a 24hr period.

AnOldUR · July 23

ReenieG said:

@AnOldUR I'm not sure about your math here, lol.

2.7 amps at 120V is not the same as using a 12V source with an inverter to produce 120V. It's the wattage that you have to take into account. You'll draw at least 27 amps per hour while the compressor is running. That doesn't include the inefficiencies of the inverter.

Watts = Amps x Volts

2.7 amps x 120V = 324W

324W / 12V = 27 amps

edit to add:
If Watt's Law didn't apply, we'd all be running our air conditioners while dry camping.

ReenieG · July 23

@AnOldUR If I'm using an inverter, I'm running it plugged into my 30amp shore power outlet so I'm not running it at 12v. I'm running it at 120v. (324watts / 120w = 2.7a) They'll be some drop. My inverter says it has a 94% rating so we can add a little extra amperage but not a 12v rating. I'll leave it at that and I'll post my results after a few test runs. This is not hooked up yet.

AnOldUR · July 23

ReenieG said:

... I want to try it with an inverter when hookups aren't available.

I may see the problem. When you say "inverter", are you referring to an actual inverter, a generator or a power station? There are gas/propane generators that use inverter technology and there are power stations that have inverters as part of the package. Neither of them should be referred to as an inverter. An inverter is a single component that is often used as a standalone source of AC power using a separate battery bank as its DC source.

The problem may simply be not using the correct terminology. An inverter is simply an electronic device that converts direct current (DC) to alternating current (AC). It's not the same as a inverter generator (or any other generator) or a power station. Many owners use an inverter to power 120V devices and appliances using only their onboard 12V battery power. When someone says "inverter", that is what they should be referring to.

After reading past posts, it sounds like you are talking about a power station, not an inverter?

ReenieG · July 23

@AnOldUR said, "I may see the problem"

So what's the problem? Am I missing something in my calculation?

Here's the calculation I'm using. The pure sine wave inverter is 120v output, not 12v. I never mentioned anything anywhere about an "inverter generator" when discussing "inverter."

Amp-hours (Ah) = Watt-hours (Wh) / Voltage (V)

"Inverters are not 100% efficient. They lose some energy during the conversion from DC to AC. You might want to add a percentage (e.g., 10-20%) to your calculations to account for this loss."

So, like I said, some efficiency loss would be taken into account.

THIS VIDEO is a good calculation of run times.

In addition, I found this online regarding the amperage of the Norcold N180 3-way fridge:

The Norcold N180 series is a line of RV refrigerators that can operate on AC power, DC power, and LP gas

. The current draw of your Norcold N180 refrigerator depends on which power source it is using and whether it is in chilling or maintaining mode.

120V AC Power: The Norcold N180 typically draws around 3 amps when running on 120V AC.
12V DC Power:
- When initially chilling down, the N180 might draw 8-9 amps on 12V DC.
- Once the refrigerator reaches the desired temperature and is maintaining it, the amp draw will likely decrease to about 2.5 amps.

AnOldUR · July 23

ReenieG said:

... As for the 3-way fridge I have, it uses much less amperage on shore power than the 12v setting. Of course propane is best but I want to try it with an inverter when hookups aren't available.

I got into this discussion thinking it was about using an inverter to change 12VDC battery power to 120VAC and run a 3-way fridge using it's 120V AC feature. I apologize if I misunderstood what you were writing about. Running a 3-way on 120V using an inverter to supply power from a 12V source will be a big draw on your battery.

ReenieG · July 24

@AnOldUR FWIW, This is what Google says about 12v vs 120v re rv fridge efficiency. I didn't ask about a 3-way, just the electrical options.

Basil48192 · July 24

I think the inverter discussion is confusing things.  I am just a layman when it comes to electrical, but I'm pretty sure that the power consumption of a device (watts) is the same, regardless of the voltage.

My 2-way Isotherm refrigerator uses about 42 watts when running on the mid-range setting (#4). This equates to 3.5 amps when running off of a 12 volt battery
(42 watts / 12 volts = 3.5 amps). So, for every hour it is running, it is consuming 3.5 'amp/hours' of battery capacity.

If I was running on 120 volt shore power, the amperage would be much less
(42 watts / 120 volts = 0.35 amps). But I'm still continuously consuming 42 watts of energy. It's just at a lower current...so my wires and fusing could be smaller.

If I'm pulling the power from my battery and then running it through an inverter, I'm still pulling the same amount of capacity out of my battery (plus an additional 10+% to cover the inefficiency of the inverter). The amperage between the inverter and the refrigerator is low, but the amperage between the battery and the inverter is still around 3.5 (or 3.85).

So, if the primary power source is a battery, using an inverter will drain that battery about 10% quicker than if operating directly on 12 VDC.

Let me know if this makes sense.

ReenieG · July 24

@Basil48192 yes it makes sense, however when I use my app (like a shunt) the dc option on the fridge sucks up power & appears not to be "as efficient" as the ac option. That's what Google says too in my last post.

So one thing to be considered is if in fact the ac / dc settings work similarly or do they draw different wattages.

Obviously, right now I can't compare the ac /dc options without an inverter to see if the draw is similar or not on the 2 settings.

AnOldUR · July 24

Basil48192 said:

Let me know if this makes sense.

That makes a lot of sense. Keep in mind that your 2-way has a 12V compressor, so even when run on 120V there is a conversion to 12V taking place. However the energy consumption (wattage) of the compressor used is the same.

Grumpy_G · July 24

Same difference. The heating elements are very similar in wattage rating, if anything using the inverter is less efficient because inverters are not 100% efficient.

What problem are you trying to solve ?

ReenieG · July 24

@Grumpy_G it's sort of a lengthy discussion but there's a link in the original post above that will lead you to the questions. I suppose to sum it up, I'm wondering if the ac and dc settings on the fridge draw the same amount of wattage or if they are different since they actually do cool differently and there may be some differences internally on those two settings which in fact would mean a different amount of total wattage power being drawn from each.

AnOldUR · July 24

When running on electric, a 3-way uses different heating elements for 12V compared to 120V. Different voltage systems require elements with different electrical resistances to produce the same amount of heat. The 120V element is more efficient, but that advantage is lost to the inefficiencies inherent to an inverter and the power that the inverter draws even when the refrigerator is not cycling on. Running a 3-way on 12VDC battery power, whether directly or using an inverter, is going to be a heavy draw. Probably more than most systems can handle.

Compared to a 2-way, which uses a 12V compressor, so does not have the duel heating elements, there would be no advantage to using it's 120V connection and an inverter. In fact it would consume more power from the battery setup.

ReenieG · July 24

@AnOldUR and this was exactly my thinking on the differences in the cooling but the power consumption is still up for debate. Now, once I have this setup completed, I will be able to test and see if it's worth running it on 120v with an inverter or not. Only an actual field test can truly answer that. We can continue to discuss and debate but it doesn't truly answer the test, unless you can find me a good video where this model fridge was actually tested this way. I haven't been able to find one, yet.

Grumpy_G · July 24

My digital pocket machine tells me there is no significant difference in wattage. Are you wondering about efficiency or overall consumption as these are different things ?

AnOldUR · July 24

Hey @Grumpy_G, do you have the amperage reversed or am I missing something? Shouldn't the amps go down as the voltage goes up for any given wattage?

edit to say my mistake @Grumpy_G
"Increasing the voltage applied to a heating element will increase the current draw, assuming the resistance of the heating element remains constant."

Forgot about Ohm's Law ...

pthomas745 · July 24

I actually tried this earlier this year. 2017 320, 600 watt inverter, connected to a 50 amp hour lithium battery. Took the fridge plug out from under the sink, and plugged the fridge into the inverter.

This did work, and the fridge did get cold, but...I think it ran almost constantly.

I might just drag this back together and get some readings later today.

This did come up several times before, which made me to this test in the first place.

This thread from 2017 started by @Dalehelman

https://tab-rv.vanillacommunity.com/discussion/6053/operation-of-norcold-3-way-fridge-via-inverter-during-travel

Another one from early this year.

https://tab-rv.vanillacommunity.com/discussion/19125/refrigerator-powered-by-inverter-while-driving

Horigan · July 24

I’ll be interested in the test results. At 110V you may find some small efficiency increase on an absorption refrigerator over 12V, but I suspect the inverter inefficiencies will overwhelm any 110V efficiency. Compressor refrigerators (2-way) is the way to go if your seeking significant improvements.

ckjs · July 24

AnOldUR said:

Hey @Grumpy_G, do you have the amperage reversed or am I missing something? Shouldn't the amps go down as the voltage goes up for any given wattage? …

A simple resistor element has a nearly linear voltage-wattage heating curve. I.e., increase the voltage by 120/110 —> 9% and the power also goes up by that much.

The heating circuit would require extra electronics in order to keep the wattage the same. It’s not likely that the manufacturer would make the unit more expensive for this. As long as it uses a thermostat to turn off the heater when it has warmed up enough, it is likely that the energy used is about the same.

Grumpy_G · July 24

AnOldUR said:

Hey @Grumpy_G, do you have the amperage reversed or am I missing something? Shouldn't the amps go down as the voltage goes up for any given wattage?

I noticed that too, but this is copied straight from the Norcold service manual. That doesn't mean it is correct. The manual also states

which pans out for 11.7A x 12V = 140.4W and 1.3A x 110V = 143W but not for the other combinations. Oh well.

Edit: The numbers make sense when you consider that the resistance of the heating element doesn't change:

110V / 1.3A = 84.6 ohms

120v / 1.4A = 85.7 ohms

12V / 11.7A = 1.02 ohms

14V / 13.6A = 1.02 ohms

ReenieG · July 24

Horigan said:

I’ll be interested in the test results. At 110V you may find some small efficiency increase on an absorption refrigerator over 12V, but I suspect the inverter inefficiencies with overwhelm any 110V efficiency. Compressor refrigerators (2-way) is the way to go if your seeking significant improvements.

@Horigan I'm not interested in switching out the 3-way atm since I like the propane option, but... I'm interested in how the 12v compares to the 120v with the new inverter I'll be installing. Since I know the 12v is horrid on energy consumption, well, the 120v might be as well, but wo a full blown test... it's all a guess, imo.

AnOldUR · July 24

ckjs said
... As long as it uses a thermostat to turn off the heater when it has warmed up enough, it is likely that the energy used is about the same.

I was under the impression that the 12V element was a lower wattage, but stayed on constantly at whatever level you set it at and that the 120V element was thermostatically controlled. Could be wrong. Lots of conflicting information on the internet.

Image: https://us.v-cdn.net/5021717/uploads/editor/tf/6rn8204igf47.jpg

ReenieG · July 24

@pthomas745 thanks for providing those two discussions. I saved this pic posted by @MuttonChops on the 2nd discussion you shared.

So, with 2 lithium batteries and 2 105watt solar panels and driving/charging the batteries without any shore power, it still might work ok on 120v. This will be an interesting test and I appreciate everyone's input!

Image: https://us.v-cdn.net/5021717/uploads/editor/tp/47h3g3ymuo9j.jpg

AnOldUR · July 24

@Grumpy_G and @ckjs
Been out of school for too long. Was thinking wattage was the constant with a heating element, but you reminded me that it's resistance.

Basil48192 · July 24

@AnOldUR
Thanks!

Cherokee · July 25

ReenieG said:

This discussion began on another comment thread. Maybe one of the moderators will move the posts pertaining to this discussion. TIA
Here's where the discussion began, bottom of page 1and continues on page 2. 2021 Tongue Box Dimensions 320
I have questions regarding the efficacy of running the 3-way using an inverter to use the 120v option (like shore power) vs the 12v battery option, which we already know will suck up the battery bank. While I completely understand if the 2 settings work similarly, the math won't work on either setting, but that's not necessarily the case. The two settings might work differently and so using the 120v setting might actually work. Once I have my new battery and inverter setup I will be able to test it to see if there's any better function on the shore power 120v setting.

To run a 3-way on electricity, heat must be produced. NorCold uses a resistance heater for this purpose. Resistance heaters work better on 120v then 12v. That's why we pre-cool our 3-way on shore power before a trip. We've used one for 6 years and recommenced placing a thermometer inside a 3-way to enable more accurate adjustments. Math isn't needed, but cooling a 3-way before traveling on the 12v setting and frequent adjustment will keep you food cold,

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