RV Fridge - Diversion circuit switching from propane to electric

[Barry Timm]

I'm trying to maximize my surplus solar power, using the new DSSR20 with diversion and DEXT16.

I've already decided to utilize 2 of the DSSR20s to power my RV hot water tank heating on diversion.

The second thing I want to do is to use the diversion switching functions to switch the RV dual fuel fridge (propane and 120v electric) from propane mode to "Auto" mode  whenever the SBMS0 is in diversion mode. The "Auto" switch on the front panel of the RV fridge allows the fridge to sense if there is 120v power and if so, to switch it from propane mode to electric mode (which would work perfectly while the SBMS0 is in diversion mode). In this way, I can use my excess solar power during diversion period, to save propane.

So I believe I need to simply connect the fridge's "Auto" switch in series, to a SBMS0 diversion signal circuit or a Charge circuit signal (depending on the State fo the Auto switch). I need the Auto Switch to be ENABLED during the SBMS0 Diversion period, and then to switch the Auto mode DISABLED when the SBMS) is charging.

So, I think I first need to find out if the switch is Open for Auto (electric) mode, or Closed for Auto mode. 

If it requires to be OPEN for Auto mode, then I guess I can connect it to an optoisolator switch on as SBMS CHARGE port (Type 1) as Charging will be OPEN during diversion phase.

If the fridge switch is CLOSED for Auto (electric) mode, then I suppose I can have it in series with the SBMS diversion signal from the DEXT16.

Any of this make sense?

[Dacian Todea]

Barry,

Why will you use a dangerous and inefficient Propane fridge to begin with ? Especially as you have such a large solar system.

I recently looked at DC fridge/freezers and they use about 60W when running and around 0.6kWh per day https://uniqueappliances.com/product/unique-13-0-cu-ft-solar-powered-dc-fridge/

They have different sizes from 6 to 14 cubic ft  (170 to 385 liters) all using the same compressor that can work from 12V or 24V battery.

There are likely other brands making this and even if you get an 120Vac fridge with a small dedicated inverter it will still use about the same amount of energy and very insignificant.

While searching on some video reviews on those Unique branded fridges I got to this video and around this point  https://youtu.be/v-j2HvlQLlk?t=521  they mention the incident they had to propane fridge and the reason they where so emotional to have an electric powered fridge.

[Barry Timm]

Simple answer....we already have the fridge and have been using it for 1 year, full-time, and we have the excess solar power.

The longer answer is that I am looking at EnergyStar 120v fridges, but don't want one with an open drain tank like that Unique, because it can spill water everywhere if we move while the drain water has not yet evaporated.

And...it's not easy to find a fridge that has locking or physical door catches, to avoid having to add some ugly latch on the two doors while we move. Nearly everything made in a vertical apartment fridge format has insufficient magnetic "latches" that open during moves. 

I could, of course, pay double for a cheaply made RV compressor DC fridge, but that seems such a waste of money. Still looking for alternatives.

In the mean time....the diversion switching helps us reduce propane usage and propane time......

[Dave McCampbell]

Dacian.  Very interesting unit.  Having worked with refrigeration on our boats for over 20 years, I have a hard time believing that a refrig/freezer such as that only needs about 25 ahrs a day to keep up with the significant heat load it would be subject to.   Things like run time per hour and for what ambient temps need a closer look.  The box looks like it has less than 2" of insulation for the refrig and 3" for the freezer.  Unless they use some sort of super insulation or vacuum panels that is half what is required to get to max efficiency of R20 for the refrig and R30 for the freezer using what most commercial refrig/freezer units use.  That unit will be air cooled which is the least efficient heat exchange method for a refrig system.  It uses the standard Danfoss/Secop sealed compressors (we have two aboard now), so nothing new there, and will be most efficient if run at lowest speed, so probably one of the larger models.  Finally it is 13cuft and side loading with what will be a large daily heat load, increased every time it is opened.  We have an 8 cuft side loading refrig only, with BD-50 Danfoss compressor, keel water cooling, full R24 insulation, and it uses just under 30 ahrs a day in 80-85F ambient tropical temps.  Hard to believe a 13cuft refrig/freezer could do with only 25 ahrs a day if in similar ambient temps.  Before purchase of something like this I would ask a lot more questions or you might be disappointed in its efficiency claims.

Barry's concerns about box drainage and door latching make sense for a mobile refrig unit.  You might look at some made for the marine market.  Those that can use a remote heat exchange unit set in your fresh water tank and with proper insulation will be much more efficient in warmer climates..

[J.W van de Poll]

Hi All

I have a Weaco fridge with a Danfoss compressor in my Van and it draws 4.5 Amp when running.
Haven't got the doc's handy to tell you the size in litres.

Didn't check 24 Hr usage, but know that on cold nights (that means 10 degree C here in Australia) it only started twice.

Even on warm nights (23 to 25) it uses about 25% capacity of the 200AH available.

Once the sun comes up, the battery is back at 100% within 2 or 3 hours.

I have modified this fridge (and why wouldn't you) to take the thermostatic control away from the fridge compartment .

It is being controlled with a separate controller and probe in the freezer to run between -16 to -19 degree C.

There are two vents behind the fridge in the Van wall and I have fitted a 12v fan that I switch on on very hot days (35 and above)

Whilst traveling, we snap elastic straps across the two doors, as the snap catches failed on the first trip.

Regards Jan van de Poll

[Rob]

Following, as we have a similar fridge which works great, but would be nice to have this feature to save some manual button pushing.

[Dacian Todea]

Dave,

The spec is for +25C (77F) ambient temperature and it is true assuming you do not insert some other warm products just keeping what is already there cold.

The thermal insulation is fairly thick and not bad compressor is around 60W so it assumes that it will work around 40% of the time the other 60% will be OFF so fairly typical of most fridges in +25C ambient.

If you need a super efficient fridge is all about thermal insulation so here is a chest freezer that can be used as a fridge and uses less than 200Wh/day https://cdn.shopify.com/s/files/1/0247/1546/0701/files/Model_ST-8CF.pdf?690

This has 4.5" of polyurethane insulation and likely uses the same compressor. 

[Barry Timm]

Getting back to my original question -  How to activate a switch on the RV fridge during diversion. (No actual high current diversion needed), would the following circuit work"

1. Assuming the Fridge "Auto" switch must be CLOSED during SBMS0 diversion and OPEN during charging.

Use an unused available "RCHeat+" terminal on the DEXT16 to signal an optoisolator input circuit to have the optoisolator output circuit in series with the Auto switch.This would allow me to override the DEXT16 control signal during diversion, with the Auto switch, if necessary.

2. Assuming the Fridge "Auto" switch must be OPEN during SBMS0 diversion and CLOSED during charging.

Use an available existing optoislator input circuit already used for charging, to signal another optoisolator input circuit to have that optoisolator output circuit in series with the Auto switch. This would allow me to override the DEXT16 control signal during diversion, with the Auto switch, if necessary.

Is there an easier way, without yet knowing the current and assumed 12v at the fridge "Auto" switch?

[Dacian Todea]

Barry,

I know nothing about your fridge. Is that propane and 120Vac or propane and 12Vdc ?   Also what will you try to achieve exactly ? Just run the fridge form the battery when solar charging is stopped ?

Is unlikely you will be able to use the excess PV solar energy directly as your fridge electric heating element will not handle the PV directly. 

[Barry Timm]

Sorry, OK. The RV fridge has a 12v control board which manages the switching between 120v ac and propane mode. There is a 12v switch on the front of the fridge called "Auto". Basically, if the "Auto" switch is enabled, then the fridge runs on 120v ac and if the Auto switch is disabled, it runs on propane.

All I am trying to do is to have the SBMS0 automatically ENABLE that 12v Auto switch circuit when the SBMS0 is in DIVERSION mode (so that the fridge switches to 120v AC when the SBMS0 is in diversion mode), and DISABLE that switch circuit when the SBMS0 is in CHARGE mode (so that the fridge switches to propane when the batteries are charging).

I am not yet clear whether the Auto switch must be OPEN or CLOSED for the 120v ac mode, so need to consider both types of control circuits until I can check the switch working in more detail.

[Dacian Todea]

Yes but what will be the advantage of this ? As you will not be using excess PV energy just energy from the battery. Also there may be many charge discharge cycles in a day so fridge will need to switch from electric to propane and back many times a day.

If you had an extra unused EXT IOx then you can have that set as type 4 and set a limit of say 80% SOC that way the fridge will start working on electric as long as the battery is above 80% so maybe for the entire day time thus fridge will run on electric during the day and then later before sunset it will switch on propane.

To test that Auto switch just take a multi meter set on  10A range see if there is any measurable current and if not or super small switch to the mA range to check more precisely. If is just a digital signal then it will be fairly low current but if that powers some electromagnet directly then current will be higher than what an EXT IOx can handle and maybe also higher than the 9V  160mA the RCHeat output from DEXT16.