For our RV conversion we purchased a Norcold NB 751 3 cubic feet, 12 volt electric refrigerator. The big attraction of this fridge is that it uses the Danfoss BD35 compressor and is said to be very efficient on electricity use. This is important to us in that the fridge is by far the largest electrical load in the RV. This test is about checking on what the fridge energy consumption actually is under typical conditions.
In order to get an idea how much electricity the fridge uses, I bought an inexpensive watt-meter on ebay and set up a test to measure the electricity use for a full day.
All the details are provided below, but the bottom line is that for this 80F day, the fridge used 42 amp-hrs (568 watt-hrs) for the full day.
When the fridge is on, it draws about 3.6 amps, or about 50 watts with our typical 13.6 volt battery voltage. Based on the 568 watt-hrs per day, the average power use is about 24 watts, so the fridge ran about half the time during this test.
So, I guess for these conditions, you could assume that the fridge will need about 42 amp-hrs for a typical day of use. You might want to allow somewhat more if you operate more in hot weather.
For our house battery, we have two 6 volt golf cart batteries hooked in series to make an approximately 220 amp-hr 12 volt battery. If we allow the battery to discharge to 20%, then it has a useful capacity of 177 amp-hrs, and the 42 amp-hrs that the fridge pulls would be 25% of this useful capacity. Since our other loads are light, we can likely go 2 days without any charge to the battery. Since the solar charges some even on cloudy days, we can likely go longer than 2 days without running the van engine to charge the battery.
When the fridge cycles on, the initial amperage is about 4.5 amps. Over about 2 to 3 minutes it settles down to about 3.6 amps and stays near this value as long as it stays on.
I put a couple half gallon bottles of juice in the fridge just to provide some realistic thermal mass. These were initially at room temperature, and it took quite a while for the fridge to cool them down — I also think that they delayed the time for the fridge to reach the the optimal 40F ish inside temperature.
The DC watt-meter is shown in the picture. Its a little hard to see, but if you click the picture to go to full size, you can see that it shows amperage upper left, battery voltage upper right, and wattage lower right. The lower left area rotates through: accumulated amp-hrs, accumulated watt-hours, peak watts, peak voltage, and peak amps. There are a bunch of them on ebay, but here is one example… about $20. To hook it up, you have to insert it in the wires that serve the fridge.
This may be just me, but I’d probably not operate with this watt-meter permanently installed. I think its fine for tests like this, but it does not impress me as being a high quality instrument for long term safe use. It did decide about 3/4’s of the way through the test to reset the accumulated amp-hrs to back to zero and start over for no apparent reason. It says that its rated for 130 amps, but the wires coming into it are only 12 gauge — I don’t think I would want to leave it alone at these high currents.
The plot above shows the energy use in watt-hours over the two day test. The 2nd day is probably more representative because in the first day I was adjusting the fridge thermostat and the fridge was having to cool down the two half gallons of juice which started at about 65F and were eventually cooled by the fridge down to about 35F.
For the last 24 hours the fridge used about 568 watt-hrs, or about 42 amp-hrs. This is an average power use of about 24 watts. Since it uses about 50 watts when its actually running, this means it runs about half the time under these conditions.
Our house battery has a useful capacity of about 175 amp-hrs, so the fridge load uses about 25% of this useful capacity over a one day period. For the overnight period, it should only bring the battery down about 10%.
The plot above shows the temperatures inside the fridge (Tfridge in orange), temperature inside the van (Tvan in blue), and the outside ambient temperature (Tamb in grey). The freezer temperature varied from as low as -10F up to about 15F — this is with nothing in the freezer.
At the start of the test, the fridge was loaded with two half gallon bottles of juice at room temperature. For the first few hours, the fridge is cooling that mass down and the temperature inside the fridge is running warmer than the recommended 40F. The fridge thermostat was also set to a higher (colder) setting during this time (not sure if that really makes any difference). Eventually, the fridge catches up with cooling the juice down and maintains a temperature in the 30F to 40F area for the rest of the test — this is with the fridge thermostat set at just below “3” on the “1” to “5” thermostat scale.
June 18, 2015