Dometic 3 way on DC

[muttmaster]

Knowing my propane was approaching low level and decided to test using the Dometic 2.7 cf 3 way on DC. As FWC does not recommend it unless you are driving and charging battery via alternator. Actually, they don’t recommend that either due to charging wiring issues and high amp usage of these fridge. I just added 120 watts semi flex to the factory installed 160 for this test.
At 9:45 am, ZAmp controller shows 13.6 v at float. 100% charged battery when I switched to DC mode. In 30 minutes, I got 12.7 v and 11.0 A charge current and 11.2 ah. Still 100%.
At one hour on DC at 10:45, ZAmp shows the battery out of 100%, but does not show %. At 12.6 v, 11.6 A, 16.4 ah charged capacity.

So my question is how long can the 280 watts keep my two 75 ah AGM charged enough to run the Fridge in DC mode? Seems worthwhile test before I run out of propane. Also, why FWC didn’t install heavier gauge wires so I can charge batteries via alternator or solar more efficiently.

 

[billharr]

At it best DC on a 3way is not good. If you are worried about running out of propane buy a steak saver. I carry one and have never used, but I know it is there and will keep me working.

 

[muttmaster]

Got one of these “steak saver”. I have tested it and works fine on FWC propane system. Not sure it will have much gas for hot water heater or furnace but fine for fridge and stove, good for several hours on one lb canisters.

Yes, I know it is not ideal to use DC for the 3 way as it draws 12-18 amps according to Dometic, FWC says 10 amp, but have anyone did real time test to see how much drain in the batteries? With enough solar wattage, and full summer sun, if might just be fine to be in DC during the day once the fridge had already been cooled in propane mode.

I only tested it for a hour in a not so hot day, and it seemed to worked fine and my batteries didn’t drain much that I could tell anyway and it recovered rapidly after switching back to propane mode. I was just wondering if anyone or the factory have given it a through test. FWC Manual just recommend don’t use DC without giving clear reason beside the high amp draw. I was worrying about other things such as charge wiring getting too hot, or potentially have other issues. FWC manual just says don’t use in DC unless you are driving, but nothing on solar charging.

It bothers me a bit that FWC install something and recommend that you don’t use it in certain mode. Couldn’t they find or suggest solutions to overcome shortcomings of a product they choose to use in their camper? Aren’t FWC liable if something like wires getting fried when they installed and knowing it is under gauged wiring and “recommend” that you don’t use it.

I like my 3 way. I want a compressor fridge, but I like using it in propane mode too. Maybe in the near future, there will be a 3 way fridge that is affordable and I can use it without concern.

 

[cwdtmmrs]

It isn't FWC or ATC or whoever. It is the nature of the beast. 12v is only to maintain the temp while driving for folks that won't use propane when on the road. You can get around the 12v issue by putting a big bank of batteries in with lots of solar and run an inverter. Or, install a compressor fridge. I don't get it though. It is pretty hard to run out of propane uder normal usage.

 

[muttmaster]

What is normal usage? My propane tanks can last me over two weeks with hot showers. My post was about seeking test done by 3 ways owners. Don’t need opinions without actual tested facts. Pretty silly to suggest getting a big bank of battery and inverter. Be real.

 

[klahanie]

... So my question is how long can the 280 watts keep my two 75 watts AGM charged enough to run the Fridge in DC mode? Seems worthwhile test before I run out of propane.
...

... Be real.

Seriously, are your batteries really 75 watts each ?

Seem like a hard marker. Have you considered testing yourself ? I'm thinking you'd find out in 24-36 hrs.

 

[muttmaster]

I meant 75 ah agm. Was hoping someone have tested it and can give me some facts. Maybe I will have to do the test myself instead of getting quirky suggestions.

 

[pvstoy]

Interesting idea Having solar put out enough amps to keep it running while the Sun is up. You have to do a test for more than an hour or 2 to see how it does long term I'd be intrested in Seeing that result. The 3 way refrigerator has 3 different heating elements. Each time you switch to one it takes longer for the element to heat up and work efficiently. So running on the DC after a while maybe it will get more efficient and run less and use less amps. Only your testing will know for sure.



If so then during the sun of the day you can run in off the solar panel then when the solar panel shuts down turn off the refrigerator overnight but things will get a little warm overnight if it's hot. That is when the steak saver kicks in. It might use more propane at first to get the element up to temperature.

 

[ardvark]

The difficult with questions like this is calculating charge rates assumes constants between systems.

The DC element in the three-way is what Dometic calls a "sustainer". It is meant to maintain box temperature only as long as the door is not open. It is not meant to be capable of dropping box temperature (per Dometic factory training for techs). The amp draw to the DC element in our current fridge is 13.9 amps and since it is either off or on and mostly on, the draw is pretty much constant. The element in my last three-way which was older (o4 versus 19) was 17.5 amps so you get some idea of whether your solar panels can keep up.

As an aside, in some cases owners assume higher charge rates from their converters and alternators than is often the case so anything having to do with battery charging also has to be determined with a meter to get a true measure on a case by case basis. All pop-ups suffer in cooling efficiency because they rely on two side vents rather than a side and a roof top. Since convection is crucial to efficient function in absorption refrigerators cool down and recovery times are much longer than with compressor models.

 

[muttmaster]

Thanks earthpig for the thoughtful input. I am guessing that I can probably run in DC for 4-5 hours with my 150 ah batteries, that is not being solar charged. Assuming 15 amp constant use in my 2016 3 way, but I read somewhere these fridge is on 1/3 duty cycle? But with solar charging and with all the variable factored in, I shouldn’t damage my batteries, I hope. I know more ah capacity in my system can allow me to run longer in DC if I had to use it when the propane runs out.

From What little I researched with my limited knowledge electronically inclined mind, I can safely run 15ampx4hrs=60 ah without going over 50% depletion of my batteries. I just don’t know how much my solar panels is replenishing the juice while I am in DC during the day. I am think that I might test it in the Morning with a fully charged batteries, run it for 4-5 hours and see how much the batteries is depleted and then switch back to gas and allow the solar to charge up batteries again. Maybe a longer test next day to see how long I can push it. Really hoping someone has done this already.

 

[ckent323]

Muttmaster,

If you want to operate this way going forward I think it wise to be able to monitor the charge on your battery bank. Therefore, I recommend you get a good quality battery monitor. You can use the battery monitor to easily run the tests you describe as well as have a way to always know what the battery bank charge state is.

I have and like the Victron battery monitor. The BVM-712 is the latest model and it comes with built in BlueTooth so you can download the free Victron app and monitor your battery bank from up to about 30 feet away. The Victron App keeps a history of the battery charge/discharge so you can go back and review that from time to time or use it to run tests/know when to start rationing power, etc.

Best price I have found for the BVM-712 is about $206. There is a cheaper version without Bluetooth which sells for about $180. You can add a Bluetooth dongle to it later, if desired, for around $50 (total cost is clearly cheaper to buy it built–in)

PKYS has it at a good price or you can find it on Amazon and elsewhere for about the same price or more.

https://shop.pkys.com/Victron-Energy-BMV-712-Smart-Precision-Battery-Monitor.html

 

[muttmaster]

Kent, yes I know I need better battery charger and monitor. I installed Trimetric system and heavier gauge wire on my last Grandby 2007 model with the help from Vic’s advise and others and just haven’t upgraded yet with the 2016 model. I didn’t bother to understand 1/10 of what thx Trimetric was capable of. The ZAmp ZS-30A controller will suffice for now. That is also why I am lazy and hoping someone has done test with good monitors, and has knowledge they can share.

 

[Vic Harder]

Thanks earthpig for the thoughtful input. I am guessing that I can probably run in DC for 4-5 hours with my 150 ah batteries, that is not being solar charged. Assuming 15 amp constant use in my 2016 3 way, but I read somewhere these fridge is on 1/3 duty cycle? But with solar charging and with all the variable factored in, I shouldn’t damage my batteries, I hope. I know more ah capacity in my system can allow me to run longer in DC if I had to use it when the propane runs out.

From What little I researched with my limited knowledge electronically inclined mind, I can safely run 15ampx4hrs=60 ah without going over 50% depletion of my batteries. I just don’t know how much my solar panels is replenishing the juice while I am in DC during the day. I am think that I might test it in the Morning with a fully charged batteries, run it for 4-5 hours and see how much the batteries is depleted and then switch back to gas and allow the solar to charge up batteries again. Maybe a longer test next day to see how long I can push it. Really hoping someone has done this already.

You are right-on with the 60AH total draw, and calling it quits there at around 40-50% SOC.

And the good news is that your 280 watts of solar might just be enough to run the fridge during the day!

Keeping up with your fridge's 15A draw is our target for this. Assuming 80% efficiency (wait, is your ZAMP controller MPPT?)... you get about 220W from those panels under idea conditions. Assuming your batteries are already at 100%, the controller will try to keep them at float ... usually around 13.6V. 220/13.6 = 16A.

 

[muttmaster]

Ah yes, I didn’t think about factoring 80% efficiency from the panels. I probably can factor in the fridge not to draw 15 amps constantly as the temperature cools down and just kicks on when inside warms up. 1/2 duty cycle maybe at around 80F ambient in the shade? Will give it a try. My ZAmp 30 amp is PMW.

Another reason I wanted to find out how long I can run the fridge in DC is basically there aren’t much information in the Dometic or FWC manuals. Few months ago, when one of my 10 lb tank ran empty while the fridge was in Auto mode, it switched to DC mode, automatically, which the manuals don’t tell you that. It suppose to switch to AC when available. So my fridge was running for maybe few hours in the morning before I noticed the lights, good thing that I had the second tank to switch to and if was relatively cool in the AM. The battery didn’t drain down much, but it could of drained down to 9.6 v before it shuts down. Maybe causing long term damage to the batteries.

These days, I only set the fridge on Gas mode to prevent the fridge switching to DC and drain my batteries when I am away. Probably not a big deal, but I think FWC should advise/warn 3 way owners about this.

 

[muttmaster]

Well, I worry too much...had been running in DC for three hours and battery never went below 12.5 v. and I have only the 160 watts roof panel hooked up.

Called ZAmp and learned that the controller doesn’t really tell you the SOC except in estimated percentage and they also told me to get a Victron 700 of 712. Called Dometic too and was informed that at “holding mode” in DC, it may draw just few amps depending on the variables. My fridge is rated st 14.6 amp, but that does not mean if draws that much constantly.

So, learned a few things, especially how much I didn’t know about my fridge and controller. I also learned that I can get a Victron 700 for less than $130 with free shipping. I think I better do longer and more through test after Victron is installed. No more worries.Thanks to all for helpful inputs and sagely advises.