Solar and 12V Refrigerator

 

Partly Cloudy - Rain Expected

We are preparing for a trip and I'm checking the systems of the Roadtrek.  One of the things I test is the 3-way refrigerator (AC-DC-Propane) model RM2554. For the test the refrigerator and freezer are empty. I then place a sensor in the freezer.  

Why empty? I wanted to see the performance of the refrigerator. Had I put frozen stuff in the freezer, the temperature of the freezer would have decreased or increased because of the temperature of the contents. 

I plugged the 210P into shore power and set the refrigerator to AC with a cooling setting of "5". I began this test at 4:30pm.  The freezer gradually reached a low of -15F with the outside temperature at 8am of 56F.

I then switched to DC to see how well the refrigerator can function on that power source. This would be the method used when traveling in the vehicle. According to Dometic "The DC mode is a holding mode not a full cooling mode. DC should be used once the unit is cooled down on gas or AC and driving (constant supply of DC) down the road." 

Refrigerator selected for 12V DC, maximum cooling

I'm curious about the apparent difference when on DC cooling as opposed to AC cooling.  According to Dometic the energy sources, both AC and DC are identical. The DC cartridge heater  supposedly provides the same energy for cooling as the 120VAC cartridge heater provides:

• 12 VDC cartridge: 175W (30A Fuse)
  

• 120VAC cartridge: 175W

12VDC Issue

With the Roadrek running on coach battery power and the refrigerator "OFF" the power meter indicated only 0.11A of DC battery power being consumed. At the time the roof vent fan and interior lights were off.

Battery System Status while on Solar and Overcast

I allowed the refrigerator to chill overnight on 120VAC power.  The freezer temperature gradually reduced to minus 15F.  At 8am I disconnected the Roadtrek from shore power. 

I then switched the refrigerator to 12VDC.  The power meter indicated a problem.  The RT was consuming only 0.41A.  That implies the cartridge heater of the Dometic was not getting 12VDC power. By noon the freezer had warmed to about 33F.

12VDC Battery running the refrigerator

Troubleshooting the Problem

The 12VDC power to the cartridge heater is controlled by a relay. I readily determined that the 12VDC cartridge heater was not hot. Using a DC Voltmeter I checked the 30A fuse.  It was okay. I then determined that the relay coil was getting power. I surmised that the relay had failed.

Existing relay, unscrewed from aluminum mounting

I went online and began looking for a replacement relay. The relay is a standard automotive DC relay, similar to a horn relay.  I didn't have much luck with online Dometic suppliers, who seemed to want to sell me a new circuit board and relay.  I did more research and found a generic supplier. I purchased two relays for about $4 each; I'll keep one as a spare.

I installed the relay and put the refrigerator on DC mode.  I checked the power at the heater and it was getting 12VDC. Success!

Rerunning the Test by chilling on AC Mode and then switching to 12VDC Mode

I followed the Dometic manual and chilled the refrigerator on AC mode. Normally I'd then fill the refrigerator with cold food and then switch to DC while travelling. For this test the refrigerator was empty.

A little morning haze turned to bright sun

On this day we had bright sun. The Roadtrek was parked mostly in the shade. At 4pm with the ambient temperature at 87F and the freezer interior about 80F I  plugged the Roadtrek into 120VAC shore power. I allowed the refrigerator to run overnight on AC and a cooling setting of "5".   It took 2 hours for the freezer to cool to 32F. At 6:00pm the freezer had cooled to 32F. At 7:00pm it had cooled to 20F and at 8:00pm it had cooled to 11F.  At that time the ambient temperature was about 76F.

Nighttime ambient dropped to a low of about 67F at 6am.  

The freezer temperature continued to decrease throughout the night. By 8am the freezer was about minus 10F.  I then selected DC mode on the refrigerator with maximum cooling setting of "5".  I let the refrigerator run on 12VDC from 8am to until 4pm.  At 4pm the freezer temperature was about minus 2F.  I assume the gradual rise in temperature was because of the increasing exterior temperature.  The interior of the Roadtrek was 94F at 4pm. It had been in mostly shade during the day. 

Refrigerator on AC and later on DC power

The Refrigerator was consuming about 3A DC according to the power meter. I have an in-line Ammeter I could use to confirm this.  However, if this is the DC consumption running on my solar system should not be an issue.

The temperature in the vicinity of the 12V cartridge heater was 145F:

General Refrigerator Information

According to the service manual, the refrigerator consumes:

• 12V Controls: 3 A.(That's the fuse size, too!).
  

• 12V Heater: 15A (175W).
  

Based on my power meter it seems the DC mode requires less Amperes than the manual indicates. That is a possibility. It would explain why Dometic describes the DC mode as a "holding mode".  If the DC cartridge heater was the same watts as the AC cartridge, I would expect the refrigerator to cool identically on DC and AC mode.

According to Dometic the refrigerator will function down to 9.6 VDC.  However, I am curious to see if decreasing battery voltage at night when there is no solar will impact the DC cartridge heater and if there is a lower voltage limit. It is also possible that the DC relay won't function properly at such a low voltage.  Relays of this type have a minimum "pick-up" voltage and also have a "drop-out" voltage. If the DC holding the relay "in" falls below the "drop-out" voltage the relay will de-energize. If this occurs the relay will disconnect the cartridge and the heater will no longer have 12VDC power.

Here's the circuit board in my refrigerator.  The photo indicates the part number. Yours may be different. Both the relay and the 30A DC fuse are mounted externally to the circuit board.

 Original material http://roadtrek210.blogspot.com/