DC compressor fridge power usage question

Hey guys - Looking for some help on a fridge topic. First post and have been reading as much as possible on this forum as well as others on the topic... has been very helpful.

Quick summary of my question and shared experiences from the public: Will I get at least 4days of fridge usage before depleting battery capacity with the following conditions:
-Lance 815 (small camper)
-100W solar (non-mppt charge controller)
- ~200AH (2x 6V)LA battery
- Pacfic NW weather: typically overcast with camping under tree cover so temps at a high of 80F with lows around 40f... ballpark.
-Conservative usage, minimize opening door, preconditioned contents, etc.

Fridge options:
-roughly 3cubic ft capcity (fire damaged fridge was a Norcold N300)
- 2-way (12v/120v) with Danfoss compressor
- Looking at Nova Kool R4500, Dometic CR1100

Long dissertation:
I have a small camper (Lance 815) which came with a 3-way Norcold fridge. Beyond the typical frustrations (level/temp stability), the propane lines were poorly routed and chafed a hole. This caused a small fire when I started the fridge the other day. Fortunately, I was able to identify and put it out before the entire rig went down with only permanent damage to the fridge and the vent (minor damage to the TPO roof). I always had concerns on fire safety of the 3-way setup, but one could sympathize that I will not go back to a 3-way fridge.

I've been looking at alternative options and the Danfoss 12V compressor fridge alternative is really appealing. I think it improves on my issues with the absorption fridge, but I may have power limitations and wanted to get feedback from the general public that has similar exposure to this setup.

I've read anywhere from 1-5A consumption on 12V (12-60 AH) and obviously conditions can take it to either of the extremes. I've also read duty cycles average 30-50%. Taking 30% as a more realistic number and the 60AH max, that would put usage at about 12AH to 18AH per day from my batter bank. Let's assume 20AH for safety factor, mathematically I would get 5days of fridge capacity without factoring in solar and no other power usages. Let's take this theory of getting 5days (or 20AH) from the compressor fridge and put it aside for a moment.

Before looking at my solar capabilities, let's assume that my other power usage is about 10AH per day for non-fridge usage. Now with the 100W solar that I currently have, this would replenish, or at least minimize the draw on the battery banks. I do have the option to upgrade to an additional 100W (or 200W) solar bank with a possibility to upgrade with an MPPT controller. If it legitimizes the compressor fridge I would take this route as well. I'm not interested in a generator.

So total daily consumption would be 30AH which would get me a hair over 3days of usage. With 100W solar panel, this could get me anywhere from zero to 64AH (8hrs @ 8A). Does this math add up? Would you recommend a 2nd 100W panel with MPPT controller?

Keep in mind, this is a small camper (Lance 815) so having a 200AH bank is pushing the capacity of the camper along with putting 200W solar on the top. I will upgrade to a larger camper, but being the first one, I would like to get complete use out of it (learning) before I upgrade.

The target is to dry camp 4 days without moving. Do you guys think this is doable? Anyone with similar experiences?

Thanks for any useful comments and suggestions.

JimK-NY
Explorer II
May 20, 2015
I have a NovaKool, I believe it is 4.3 cf. The unit works great at moderate temperatures with compressor use of 30-50%. When the ambient temps increase into the 80 degree range the unit runs almost constantly. I was really surprised at the difference especially because mine is surrounded by additional insulation. Be sure you have plenty of ventilation around the unit. I hooked up a couple of computer fans to keep air moving past the compressor fins.

I believe the unit pulls about 3.5 amps so on a hot day you will pull 60 A/H or perhaps even more. On cooler days you will be as low as 20 AH. 40 AH is probably average. Be sure you use heavy gauge wiring. My unit was at the opposite side from the batteries and was installed by the Northstar with 10 gauge wiring. I had a drop of about 0.5 volts at the refrigerator. I rewired with 6 gauge. Some people (even manufacturers) seem to think that you only need light wiring for 12v use. The opposite is true. To prevent voltage drop due to resistance, you need very heavy gauge wiring for 12v applications.
kohldad
Explorer III
May 20, 2015
ydoggy - How about an update? I would also like to know where you stuck the 3 batteries.

Wife and I are trying to decide on keeping our 815 or moving up to a 915. Either way, she wants a DC compressor fridge so I can quit worrying about the camper being level when I park.
Horizon170
Explorer
Jun 06, 2014
We have an OLD Dometic RB2195 model refer and the front page of the owners manual stated that "Operation is by 12V D.C, Compressor."
It is a 12 Volt compressor according to the schematic.
It shows that it can use 120 volts but only if the 120 volts runs into an AC to DC Converter and then to charge the 12 volt battery that the feeds the 12 volt compressor.
westend
Explorer
Jun 06, 2014
I have a 1.7 cu. ft. dorm fridge that draws 80 W when operating. This summer, I'll put a long term usage test on it with the KAW and see what happens..
wa8yxm
Explorer III
Jun 06, 2014
Denny & Jami wrote:
One thing to remember when looking for a fridge with a 12v compressor is the compressor is still 120v AC it just has a dedicated inverter built into the box mounted on the side of it. You can do the same thing with a standard compressor but mount a dedicated inverter close to your battery to eliminate running heavy 12v wiring to the fridge.

Denny

Uh, NO, that is not correct.. The compressor is not 120 volt.. Not sure what the voltage is (48 perhaps) but it is not 120 volts.

In either this or another thread a refrigeration engineer who is very vry familure with the Danfoss and it's competitors posted the facts, Alas, I am not him and did not properly remember the voltage but it was either 24 or 48 as best I can recall.

I have not seen a residential fridge that draws less than 100 watts running, that is, when translated to 12 volt with the aid of an inverter, roughly 10 amps.. The Danfoss/sling/(one more name I do not recall) compressor units usually draw less than FIVE amps. So you can NOT simulate one with an inverter and a residential fridge, Simply not possible. Residential fridges burn at least 2x the power (closer to 4x in most cases).
wa8yxm
Explorer III
Jun 06, 2014
Denny & Jami wrote:
One thing to remember when looking for a fridge with a 12v compressor is the compressor is still 120v AC it just has a dedicated inverter built into the box mounted on the side of it. You can do the same thing with a standard compressor but mount a dedicated inverter close to your battery to eliminate running heavy 12v wiring to the fridge.

Denny

Uh, NO, that is not correct.. The compressor is not 120 volt.. Not sure what the voltage is (48 perhaps) but it is not 120 volts.

In either this or another thread a refrigeration engineer who is very vry familure with the Danfoss and it's competitors posted the facts, Alas, I am not him and did not properly remember the voltage but it was either 24 or 48 as best I can recall.

I have not seen a residential fridge that draws less than 100 watts running, that is, when translated to 12 volt with the aid of an inverter, roughly 10 amps.. The Danfoss/sling/(one more name I do not recall) compressor units usually draw less than FIVE amps. So you can NOT simulate one with an inverter and a residential fridge, Simply not possible. Residential fridges burn at least 2x the power (closer to 4x in most cases).
westend
Explorer
Jun 06, 2014
Looks like you have it well sorted.
Yes, if you can improve the fridge with insulation or lower the temp settings by using internal fans, as mentioned, you're going to be able to leave the generator at home.

I'd grab one or two of those BD35 freebies, for a backup and maybe experiment with 12V air conditioning. How nice would that be?
ydoggy
Explorer
Jun 06, 2014
Hey guys... I appreciate all of the posts. They have been extremely helpful. Just an update. I purchased an Isotherm fridge with the BD35. I'm still working on finalizing the insulation and managed to pack in 300+AH of battery and installed an extra 200W solar. A bit overkill given my small camper, but 300W and 300+AH of battery seems doable based on the data shared.

I got an opportunity for an overnight shakedown test on a campground. Unfortunately I forgot to reset my battery monitor settings so I don't have a good basis on data. However, I learned a couple of things: 1) You really don't get much charging amps under tree cover even at peak solar. 2) The fridge rocks and even gives me quite a bit of extra space over the earlier version 3) When under fully exposed peak solar, I get the full 15A from my PWM 15A controller and it really charges the batteries back up quickly. With many of my camping spots being under cover, I'll just make an extra effort to head out to a clearing during peak solar to recharge. 4)I need to do some baseline testing in addition to completing insulation. I may even camp out in my driveway to get a more realistic scenario.

I did run the setup in my shop the other night and started tracking the data. It's not heated, but I'll bet it fluctuates between 50-70deg F throughout a 24hr cycle. Since running inside there is no auxilliary power from solar so truly running off battery. I was able to run 3full days and averaged the first two days at 1.7A with the three day total at 2.2A. This tells me ambient swings play a large role and additional insulation should improve those numbers. I had AH to spare and was roughly at 14% remaining (that is 57% total capacity to prevent my flooded batteries from damage). Of course I only opened the fridge once per day to grab a coke and I only had an inverter going to run a timer clock as well as the camper sensors. So it's not necessarily realistic from a perspective of actual operation.

As a summary, I am able to get 3days off of just battery and my even improve that with better insulation. It would also be possible to sustain given that I'm conscious of getting line of sight to the sun for my panels.

The unfortunate things is that after I dropped $1200 on the fridge, the test engineers that I work with all have similar models of BD35 compressor fridges at their desks. Working at a heavy truck OEM, when they scrap test trucks, they take out useful items and I could have obtained one for free. Oh well.

Thanks again for all of your input.
bdosborn
Explorer
Apr 21, 2014
Here's what my fridge uses for energy:

I use around 10 amp-hrs a night around here (where it get's pretty cool at night) for the fridge. Our 80 watt portable panel will keep up (just barely) if we don't watch a movie at night. 200 watts should keep up just fine on sunny days. I had 225 watts on the roof but I had issues when it rained for a week so I bumped our total wattage up to 400. I haven't had to run the generator since.

Tundra TJ22 Tests

Bruce
Almot
Explorer III
Apr 21, 2014
Ydoggy - it IS possible, but not at all times.

Yes, a lot depends on where you are parked (in the open or not), and if there is any sun at least part of the day. 100W panel won't cut it if you're under tree canopy and/or if there is overcast all 4 days.

If you want to know how much a flat 100W solar panel will collect in PNW, here is a PVWatts calculator. I found it to be quite accurate. It doesn't accept watts, only kilowatts. So click on, say, Seattle, enter DC rating=100, Array tilt=0, and it gives you 13065 WH for June (ignore that is says KWH, it just doesn't accept watts). Divide this in 30 days and in 12V, and you get 36 AH a day.

36 AH is your average day, could be better and could be worse. When there is an overcast for 4 days in row (it can be 2 weeks in row in this area, as you know), you'll get a lot less than 36 AH, - maybe 6 or 7 AH. Then what are you going to do - run home with tail between legs? Or try charging it through the weak alternator of your truck, burning gallons of gas and inhaling fumes for hours? Get more solar wattage, you won't regret.