Did a test w/12v fridge

[jsk125]

I have a new 2022 Rockwood 2511s with 190w panel on the roof, 30am Go Power controller, and two interstate deep cycle batteries (factory issue) hooked in parallel.

I turned on the fridge on a Friday at 3pm to test over a weekend. Partially cloudy Saturday.

Wow that fridge gets cold in a hurry. Uses a good bit of juice at first

Saturday morning I went over early, and batteries were at 85%. Went back over late afternoon at batteries were better, about 12.3v. So it recovered well. Fridge running fine. Set boost on just to try it. I turned the fridge setting from 3 to 2. Sunday is was still quite cold.

Went over Sunday at about noon, boost still on. Go Power message board indicates boost will stay on until it hits 14.1 (or 14.7, can't remember).

Anyway, Sunday, with fridge running, batteries were at near 100%. I checked via the controller, the camper sensor, and then with my 12v tester.

Long and short - batteries held up well for a weekend. I had nothing else running though. I imagine if I had the furnace on, or fans, my results would different. If I boondock (don't do it often) I'll bring along my 100w renogy suitcase panel, giving me 290w, and use several USB chargeable lights I bought, and limit power usage.

So for 3-4 nights bookdocking, likely it will work fine. Longer than that I don't know.

Just an interesting test. I have a boondocking weekend in October, will be another test. I don't have to tools to measure amps, so I'm sort of eyeballing it. But the batteries held up fairly well.

[SeaDog]

You may find when actually camping the frig will use more power due to opening and closing during the day. Plus with bright sun you may charge back much faster. Good to test before actually depending on a system camping.

[jsk125]

Originally Posted by SeaDog

You may find when actually camping the frig will use more power due to opening and closing during the day. Plus with bright sun you may charge back much faster. Good to test before actually depending on a system camping.

Thanks .. yes I agree, a cloudier day, using fans, etc. will impact the batteries. We don't boondock often. For now, if I used my USB charged lights, maybe turn down fridge at night, etc. we'll be ok for 2-3 day trip. Much more to learn!

[Chris61]

Great write up, it would be interesting to see actual usage with something like a Victron battery monitor.

[Hclarkx]

Originally Posted by SeaDog

You may find when actually camping the frig will use more power due to opening and closing during the day. Plus with bright sun you may charge back much faster. Good to test before actually depending on a system camping.

X2 on this!!! A half dozen plus door openings during meal prep and the fridge will run continuously for an hour or more instead of the usual duty cycle (typically 50% in a 70F ambient).

A lot depends on how well the fridge is insulated by cupboards or similar and whether it's on the sunny side of the RV. And of course how warm it is outside and inside and whether it's cooler at night. Under worst case conditions, the fridge compressor may run continuously through a hot afternoon (temps in 90's and/or sun on the wall behind the fridge). That's when you want the solar to cover 100% of the fridge current draw into the afternoon (when the sun is waning) in order to save the battery for overnight needs.

[Rhumblefish]

Originally Posted by jsk125

Long and short - batteries held up well for a weekend.

Great test! Nothing like some real-world experience and observation to reinforce understanding!

Originally Posted by SeaDog

You may find when actually camping the frig will use more power due to opening and closing during the day.

Keep in mind that a full fridge with pre-cooled items will act as a heat bank (er, cold bank), to preserve fridge temp despite openings and closings.

I'm a big fan of expanding the solar-and-battery capacity for even more boondocking capability.

Good luck with your rigs!

[Hclarkx]

Originally Posted by Rhumblefish

Keep in mind that a full fridge with pre-cooled items will act as a heat bank (er, cold bank), to preserve fridge temp despite openings and closings.

I would have said the same thing, but it doesn't work in practice. The thermal energy provided by the contents and walls to re-cool the air which has been largely replaced by room temperature air during a door-opening has to itself be restored. The only thing the contents and walls do is reduce the amount of air in the fridge which limits the amount of room temperature air that can be cycled in during a door opening. And, surely the contents inhibits air circulation out the bottom and in the top of the fridge when the door is open. But, I'm a bit of a nut on things like this (not enough to do in the RV) so have monitored the door opening problem. I didn't think it was nearly the factor that I read about in the forums, but it surely is. After seeing the fridge run 100% for an hour instead of the usual 50%, I started to calculate the energy required to cool 3.5 cu ft of 80F air down to 36F (assumes half of the 7 cu ft of air gets replaced) and relate that to the compressor running time. I didn't finish.

[Rolland]

Nice report and thanks for including the solar panel size and info on the controller.

Our Spirit 1943RB came with a 100w panel on the roof, a 10 amp PMW controller, then I had it set up with 2 6v GC2 batteries for 220ah capacity. It did well with our 12v fridge on a 4 day dry camp in full sun after about 10am. Never needed to break out the generator but had we been staying longer would have on the 5th day. It showed me what I had assumed before the, I need a bit more wattage on the roof and have planned to change to a Victron MPPT controller (installed a Victron 712 monitor last month.

Your post helps with some of my decision s winter.

[Rhumblefish]

Originally Posted by Hclarkx

I would have said the same thing, but it doesn't work in practice.

I think you've just violated the laws of physics, Hclarkx (not to mention the scientific process).

[Hclarkx]

Originally Posted by Rhumblefish

I think you've just violated the laws of physics, Hclarkx (not to mention the scientific process).

I beg to differ. If you cool down a chunk of air by moving thermal energy from it into the contents of the fridge, that air will cool down and the contents will warm up. If you compare the thermal mass of the air and the thermal mass of the contents, and know the quantity of air and contents, you can calculate the rise in temperature of the contents. For now I'll assume that a 40 degree F drop in air temperature through multiple door openings requires a half degree F rise in contents temperature (and the evap plate temperature). So if you open the door X times to prepare a meal, that's roughly a half degree rise in the temperature of the contents. I expect this is an overstatement, but the science is correct.

I think you are suggesting it's not necessary to re-cool the contents after it has absorbed thermal energy from the warmer air. Good luck with that. Because 12V fridges don't have a lot of excess cooling power, it can take hours to fully re-cool the contents.

BTW, I looked into this when a friend sent me a recording of the temperature in his fridge showing this process. I did not spend the extra bucks on a recording monitor as he did, but did get the linked monitors below. They show clearly that when we sit down to dinner, the fridge is warmer than it was before the meal prep. And it takes a couple of hours to fully recover though much of the recovery occurs in the first hour (an exponential increase in the contents temperature).

I've put the monitors in free air and put them inside a bag of something (both in the fridge) to see how much the contents temperature is affected by the warmer air that is circulated in. The effects are obvious. I keep the fridge at 34F (long story) and the air is almost always slightly over 40F by the time we sit down to dinner and the surface of the contents isn't far behind. About half of that is recovered after an hour.

BTW, the time constant on the sensors is about three minutes (five time constants to register 99% of a temperature change). And the readout is in 0.3 degree increments.

Contributing to the problem, of course, is that the evaporator plate has to pull thermal energy from the contents through the air in the fridge.

I found it interesting to monitor the change in contents temperature following a change in thermostat setting. With the temperature monitor located within the contents but not too deeply, and no air hitting it, the 0.3 to 0.6F rise that I see takes about 8 to fully.

BTW, you can think of the contents of the fridge as a battery. You discharge it to cool the air, but you then have to re-charge it.

So, all proper physics. Maybe not the greatest scientific method but the mechanics is clear enough. BTW, I got an A in my Thermal Dynamics course at Cal Poly in 1964. I don't think the physics has changed since then.

Cheers.

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[Rhumblefish]

No, I'm not stuck in the trap of free energy (thank you, laws of thermal dynamics)... You definitely have to "recharge the battery" of the thermal mass (in addition to that of the air).

It's the practical application of said energy: as you indicated, the thermal inertia of the contents (eg, water, tonight's casserole, beer) is so much higher than that of the air, you don't get the wild temperature swings in the thermal mass that you do in the air. And there's the practical approach that the available air volume is smaller thanks to said mass: Open and close the door: the volume of the mass itself prevents at least part of the air from circulating (thereby reducing the amount of thermal change introduced into the fridge air).

You DO have to pay to recharge the battery (not even counting real world inefficiencies and conservation of energy) ... and the greater the mass inside the fridge the greater the cost is (the harder it is to cram cold into the mass, even if - and in particular - the mass is large). It's the asymptotic curve at which the air changes temperature vs the mass. You open the door, but the thermal mass keeps the temperatures closer to the set point for the fridge. Consider, when you open your freezer, the ice doesn't instantly melt (even if 70° air is coming in).

So - YES - the fridge has to work to replace that energy lost (by both the air, AND the mass) but the temperature of the entirety of the mass doesn't change quickly, even if the surface temp has to give up some energy to cool the incoming air (thanks Stefan-Boltzmann).

Cheers, indeed. I raise a glass of 34° water in your direction!

[Hclarkx]

Originally Posted by Rhumblefish

. . . . . .

Seems we agree on the physics, but I reacted to your statement:

"I think you've just violated the laws of physics, Hclarkx (not to mention the scientific process)."

What laws of physics did the words in my first post violate?

[Rhumblefish]

Originally Posted by Hclarkx

What laws of physics did the words in my first post violate?

Perhaps I shouldn't have been cheeky; please forgive my attempt at humor. You aren't violating the laws of physics. You said "it doesn't work in practice." and "The only thing the contents and walls do is reduce the amount of air in the fridge which limits the amount of room temperature air that can be cycled in during a door opening."

This is technically accurate but practically incorrect.

A full fridge with pre-cooled items DOES act as a heat bank to preserve fridge temp despite openings and closings. Said mass doesn't preserve those temps AT fridge temp (which is why the fridge has to run to "recharge the battery" i.e. bring the internal temp back down to the setpoint) , but the pre-cooled thermal mass of a loaded fridge will help to cool any additional added mass (no matter air or whatnot) simply by the grace of having a larger temperature gradient between precooled mass and noncooled mass.

ie The existing cold mass is operating to cool the new mass just as the fridge is operating to cool the /entire/ mass (the existing cold mass that is now a fraction warmer, and the new, uncooled mass of the air). Granted, the fridge has to cool the air first, which becomes the medium to cool the mass (a very inefficient medium, mind you, but one that circulates reasonably well). The practical function being is that the existing mass temp doesn't rise significantly, and the new mass temp falls to closer to the setpoint temperature of the fridge thanks to the aforementioned gradient from surface temperature of the mass to the 'new' air.

Yes, this means the fridge has to run longer to put that fraction of a degree back into the existing cold mass (thanks to thermal density and thermal inertia).

Witness:

1.
34°*fridge ambient temp
34° precooled mass
74° coach air

2.
open fridge door
enter 74°*air, exchange half of cooled fridge air with coach, mix 50/50. <- worth measuring, btw, I suspect more than 50% of air is exchanging. Might be able to be seen with a circulating fan in the fridge and observation/graphic of temps over time
fridge air now 54°
close door

3.
Surface of 34° thermal mass now warmer (thanks to radiation) say 35°(Note: I'd want to measure this, too), as it cools the now-54° air.
Fridge kicks on to cool the now-54° air AND replace the same cold in the thermal mass. Thanks, thermodynamics and conservation of energy.

4a.
Observation shows the fridge has to run to get the existing thermal mass and the 'new' air back down to the 34°*setpoint.

4b.
Practical observation shows that the thermal mass (with its significantly higher energy density than air) is bringing the 'new' air down towards setpoint (limited by surface radiation coefficient) while the fridge itself is working to bring both the 'new' air and existing precooled-but-now-losing-some-cool thermal mass to the fridge setpoint. Fridge gets to 35°, and that last degree is a booger, b/c it requires the fridge to cram that lost degree against thermal mass/inertia into the precooled mass.

5.
Your fridge gets to 35° in fairly short order, but has to continue to work to reach setpoint, 34° to "recharge the battery."

The existing precooled thermal mass IS reducing the 'new' air temp, so long as there exists any temperature gradient between the surface of the cooled thermal mass and the 'new' air. You can even increase the 'cooling' effect of the thermal mass with additional surface area to provide additional temperature gradient via access to the thermal mass.

Additionally, depending on surface radiation, the new air will begin to approach setpoint "faster" than the thermal mass can be recharged (granted, they will arrive at the setpoint at the same time), but - again - asymptotic rate of temp change means this is negligible. Your ambient fridge air will be significant higher than the setpoint for the same amount of time, but the surface of the thermal mass will be only a fraction of that.

In practice:
Your air immediately becomes 54°
The surface of your cold beer warms to (again, guestimating) 35°, while the appreciable mass of the entire beer stays at 34° and the temperature gradient between the beer and the air brings the ambient air temp towards 34°.
Yes, your fridge now has to cool the new air and the beer both to 34° (theoretically both requiring the same amount of energy, not figuring for packaging or still air, placement in the fridge, air dispersal effectiveness, etc), but the temp gradient btw the surface of the beer and the 'new' air cools the 'new' air below its newly mixed 54°, so long as the surface of the beer will support such radiation.

From a practical standpoint, the fridge reaches 35° quickly, but takes a long time to reach 34°. Yes, the compressor runs a long time (using energy from other sources, notwithstanding inefficiencies/losses), but the fridge contents are practically cold. (note: This is the problem with fixed setpoints. Fuzzy logic built into a thermostat could help address this issue, and allow an acceptable temperature range within the fridge, perhaps to allow for energy use at different times/rates to increase efficiencies and reduce wasteful consumption).

Summary:
Your existing pre-cooled thermal mass is still colder than the air coming in, and will still work to reduce the temperature of any new mass (no matter what it is), so long as a temperature gradient exists between the existing mass (beer) and the new mass (air) - and there is nothing interfering with that gradient (like putting the beer in a cooler and then putting the cooler in the fridge). Yes, the fridge has to 'replace the cold' in both the air and the beer, but the beer is bringing the air temp down... it can't bring it to setpoint (but, thanks to its significantly higher energy density, can bring it close).

Or do you see it differently?

[SeaDog]

Wow talk about eye strain. The new camper should not have to think about the mass of this or that just that the more you open and the longer you leave the door open the more power your frig will consume (either electricity or propane) to keep things cool (remove heat) JMHO

[ScottBrownstein]

It is a lot easier to think of this as only in how energy is added to the box and its contents, allowing them to increase in temperature. That is the heat leak through the walls and fittings which are proportional to the temperature difference between the interior and the outside and the cooled air (much lower thermal mass) that is replaced with room air when the door is opened. If the box were totally full with no air space, opening the door would do very little to add thermal energy to the system.

In short, nothing spontaneously cools only energy is transferred from hotter items to cooler items. The compressor needs to remove that heat to maintain internal temperature.

[Half Ton Heavy]

Bottom line, plan on at least 80 ah a day, maybe 100ah for hot ones and higher use. Better have 300 ah of lithium on board to get through a couple of crappy days unless you want to be a slave to your generator.

[Rhumblefish]

Originally Posted by SeaDog

Wow talk about eye strain. The new camper should not have to think about the mass of this or that just that the more you open and the longer you leave the door open the more power your frig will consume (either electricity or propane) to keep things cool (remove heat) JMHO

Agree 100%, Seadog. Hclarkx and I were just p*ssing up a rope (in true internet forum fashion).

OP - and your initial reply - are still both golden! Great to test, and open door has a lot of impact.

<- *almost* fridge temperature beers

[jsk125]

Thanks all .. I will be actually boondocking in about three weeks, so I'll track again, but this time under actual circumstances (and with the 100w Renogy suitecase panel for extra juice).

This weekend is our first trip out - 3 days in Cape May - shore power thought haha!

[bbells]

The fuller the fridge is the less power it will use.

[Hclarkx]

Originally Posted by ScottBrownstein

If the box were totally full with no air space, opening the door would do very little to add thermal energy to the system.

True! But, totally full, there would be much more thermal energy coming in through the walls ....... because the air between the contents and walls slows thermal energy flow from outside to the contents. I.e., air is an insulator; not perfect but better than the contents.