Geeky data alert! Since I work for a company that does automotive testing, I was able to instrument my solar panel install and take data
.
Data was collected for the Solar Panel Voltage, Aux Battery Voltage (camping battery), Amps used to charge the battery, bus interior temperature, and refrigerator temperature. I was keeping the fridge around 37 Fahrenheit, and it was getting around 100 degrees inside the bus on the day I am showing data from.
The setup includes the Engle MT45 refrigerator/freezer, a Kyocera 140 watt solar panel, and a SunSaver 20L charge controller. The SunSaver regulates the voltage, stopping at about 13.3 V so as not to cook the battery. When the panel voltage output drops below the battery voltage, it cuts the panel out pretty quickly. The data below is from a nice sunny day, but the panel had NO PROBLEM keeping up with the Engle even with the 2 consecutive rainy days we had in the middle of last week. I have had the panel running the fridge continuously for over a week now, and have not noticed the battery voltage drop below 12v during this time.
Ok, now for more graphs than anyone ever wants to look at! First is the data for a full day. Each vertical line represents an hour. The black line in the center is the battery voltage, and the pink line is the panel output.
Next, is a look at the morning until the battery comes to full charge. You can see at the beginning that the chart, the panel has cut out (0 volts). It picks up quickly, and while the battery is coming up to charge the battery and panel voltages are locked together. Once the battery hits its "float charge", the panel voltage climbs. You will notice the "charge amps", the current actually going to the battery, drops to zero when the panel voltage climbs like this. The dips in the panel voltage are from when the refrigerator cycles on.
Here is a close look at the current draw, and a calculation of the power use (watts) for a short period of time during the day. The Engle is very efficient. The internal temperature of the bus was over 100 degrees, and the fridge temp was about 37 degrees. Under these conditions the fridge was cycling "on" almost exactly 1/3 of the time. During the cool night, it would drop down to well under 20% of the time.
Just to finish the geeky part... assuming a 1/3 "duty cycle" (the amount of time "on"), and an average of 3A draw during that cycle (rough averages from the graph above) one would expect it to consume a whopping 1 amp/hour of current each hour. For a 10 hour night (late in the camping season) we are talking 10 amp/hours or less per night. Again, the Engle is very efficient, and the solar panel can MORE than keep up with that load.
If you made it this far, I'm stunned. But it made me smile!
DJ Roberts