Welcome to my J.A.M. (Just Ask Mike) Session, a weekly column where I answer your basic electrical questions. If you’re a newbie who’s never plugged in a shore power cord (or ask – what’s a shore power cord?), or wonder why your daughter’s hair dryer keeps tripping the circuit breaker, this column is for you. Send your questions to Mike Sokol at mike (at) noshockzone.org with the subject line – JAM. This is Part 1 of the test results to determine how long an RV fridge will run from a single 100 amp-hr lithium battery.
I’ve now tabulated some basic kWh usage numbers for my Dometic DMC4101 12-volt DC compressor refrigerator. This is a 10-cubic-foot RV fridge with a separate freezer and refrigerator section. It’s designed to be mounted in an RV in place of a standard 3-way fridge.
I’ll not go into all the various shelf and lighting features here. I’m mainly focused on just how long it will run on a single 100 amp-hr lithium battery before the SOC (State of Charge) hits rock bottom (0%). Remember, with lead-acid and AGM batteries you should only allow them to discharge down to 50% SOC for best life. But lithium batteries are rated to discharge down to 0% without damage.
Plan A: Failed
So I first thought that my trusty HP 6286A power supply with an inline kWh meter from Amazon was going to get me started. It all seemed to be working at first, but then I noticed that the battery usage meter showed 1.2 kWh of energy use in maybe 4 hours of run time.
Something was clearly wrong, so I reset everything and tried again the next day. This time I carefully monitored amp usage with a DC clamp meter on the power line. While the 5.59 Amp reading was pretty close, there was no way this fridge was using 1.2kWh of energy in 4 hours with the compressor drawing an average of 6 amperes or so. That 70 watts of power times 4 hours of time should have been around 280 watt-hrs, not 1,200 watt-hrs. That’s nearly a 250% error compared to my paper and pen measurements, so something was clearly wrong with this meter.
Plan B: Limited success
I repeated this experiment with a battery since I know that’s what the ultimate test is anyway. I happen to have a very nice Briter Products 100 amp-hr lithium battery with a built-in SOC monitor, and a Progressive Dynamics charger connected. So I was able to fully recharge the battery in a few hours and turn off the charger. I then monitored how long the battery took to discharge down to a few percent while powering the Dometic refrigerator. That’s a real-world test…
Baby, it’s cold outside…
I had plenty of frozen things in the freezer and water/beer/ribs in the fridge section, since that will affect how these refrigerators respond to temperature swings. Plus, I like beer and ribs….
It’s getting hot in here…
Air temp in my garage would drop down to the high 70s at night, and get as high as 98 degrees during the day. So I figured this was a pretty good test of these 12-volt DC compressor refrigerators in the wild. Yes, even Maryland is feeling the heat wave right now. While I know other states have topped 110 degrees in recent weeks, for Maryland to hit 100 degrees is almost unheard of this time of year.
What about the defrost cycle?
Hey, I think that having a defrost cycle with a 110-watt heater in an RV fridge that you’re boondocking with is a bad idea for energy usage. It’s a big waste of energy while you’re on battery power and can certainly wait until you get home.
However, there didn’t appear to be any way to shut off the defrost cycle while in Performance mode, but maybe I’m missing something. But, I did find a schematic inside of the door which shows there’s a fuse you should be able to pull to get rid of the defrost heater. I’ll look for that later.
Just the facts, please…
Here’s my quick table of how long this refrigerator will run from a single 100 amp-hr lithium battery in 95 degree ambient temp during the day. I need a better DC data logger if I want to correlate the air temp with energy usage per hour. But this is pretty good for a start.
Mike’s SWAG (Scientific Wild A$$ Guess)
I’ll need to do a few more test runs to be sure. But it looks like this fridge in Performance Mode would run around 20 hours on 100 amp-hrs of battery, 22 hours in Silent Mode, and 25.5 hours in Eco Mode. The run times could be longer once the defrost mode is disabled, but I don’t know that yet. But this is a good start on expected boondocking times.
I’ll follow up in a few weeks with more data once I get back from the FROG Rally in Goshen in a few days. Yes, I’m sure there will be a few FROG tales to tell from Goshen….
OK, everyone. Remember that electricity is a useful and powerful force, so we all need to pay attention to safety precautions while using it.
Let’s play safe out there….
Mike Sokol is an electrical and professional sound expert with 50+ years in the industry. His excellent book RV Electrical Safety is available at Amazon.com. For more info on Mike’s qualifications as an electrical expert, click here.
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