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 week I discuss running a 2-way fridge on battery power alone.
How long would my 100 amp-hr battery last doing it? Can my truck alternator power this through the 7-pin trailer connector? —Toni
Great questions… and it shows that you’re thinking this through before plunging in and trying it. So let’s run a few numbers on what the fridge requires and see how you can power it.
A tale of electrons and propane…
The problem with a 2-way refrigerator is that it needs a source of heat to make things cold. When in propane mode, this is using a flame to boil ammonia. But in 120-volt mode, it’s using a heating element very similar to an electric space heater.
In your case, it needs 400 watts (edited) of electric power at 120 volts to heat the element that keeps your beer cold in your fridge. Yes, it does seem like a long way around to keep things cold, doesn’t it?
How much energy is that?
In this case it works out to 400 watts times 1 hour equals 400 watt-hrs per hr (400 watts x 1 hr = 400 watt-hrs).
How much energy does my battery have?
A 100 amp-hr battery at 12 volts works out to 100 amps x 12 volts = 1,200 watt-hrs of energy. Of course if you have two batteries you can double this, or four batteries would quadruple the total power available.
In your case, this is an AGM battery rated for a minimum SOC (State of Charge) of 50%. So there’s 600 watt-hrs of available energy that can be used to power your refrigerator. And, of course you’ll need an inverter to step-up the 12-volts DC to 120-volts AC. So throw away maybe another 10% in conversion losses.
A simple calculation…
If you have 600 watt-hrs of available energy from your battery, and your fridge uses 400 watts to heat the coil, that’s only 90 minutes of running time at 100% duty cycle before you’re out of battery power. Even if your fridge only runs 50% of the time (a 50% duty cycle) that’s still just 3 hours of available battery power. So, no good if you need it to run for at least 6 hours.
What about 7-pin trailer plug power for my 2-way fridge?
If you want to know how much energy your 7-pin plug can supply between the tow vehicle alternator and the trailer, it generally maxes out at around 5 amps or so, which would be 60 watts (12 volts x 5 amps = 60 watts), so that’s not going to extend your cold beer and ice cream time by much.
Remember, its total current is limited by the size of the conductor feeding that plug from your alternator, as well as the current carrying ability of those contacts, which I believe are rated at 10 amps max.
You could get a DC-DC charger
So you could get a 50-amp DC to DC charger and run a husky (4 or 6 gauge) cable between your tow vehicle alternator and your RV battery, since that would supply maybe 600 watts of power (enough for your refrigerator, but not an air conditioner). However, that seems like a lot of work just to run a refrigerator on DC power alone.
Danfoss compressor to the rescue
I think you should consider replacing your 2-way refrigerator with a 12-volt DC compressor refrigerator that only uses around 80 watts of power while running. In fact, I’ve done tests where I was able to run a 10 cu. ft. refrigerator for 36 hours on a single charge of a 100 amp-hr lithium battery. I’m guessing a refrigerator upgrade may be in your future.
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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