By Mike Sokol
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.
I think you’ve written about this before, but why won’t my truck’s alternator keep the house batteries charged in my 5th wheel trailer while I’m towing? I mean, it charges the battery a little, but not enough to keep my residential refrigerator running while I’m towing for 8 hours or more. When I get to the campsite my single RV battery is practically dead. Is something wrong? What gives? —Rich
Here are the basics. That connector and wiring harness connecting your pickup truck alternator to the trailer’s battery system is probably only rated for 10 amps of current, or so. And while that seems like a lot of current, it really isn’t.
For comparison let’s convert it into watts by doing the really simple calculation of Volts x Amps = Watts. So even if you were transferring 10 amps of current to your RV from the truck, that’s only 10 amps x 12 volts = 120 watts.
And that’s best case conditions since most of the time when I measure this charging connection it’s perhaps half of that, just due to the length of wiring and difference in battery chemistry between your truck (which probably has a flooded cell lead/acid battery) and your trailer’s house batteries (which might have AGM or even Lithium-Ion batteries). So you’re lucky if that connection can provide maybe 5 amps of 12 volt power to your trailer, which is 60 watts.
Now let’s see just how much power your residential refrigerator needs to operate. If you look on the nameplate or even measure its current draw with a Kill-O-Watt meter you might find that it only needs 4 amperes of current to operate. Sounds good so far, doesn’t it? But au contraire – that 4 amps is at 120 volts, so to determine the wattage we again do the calculation and see that 4 amps x 120 volts = 480 watts. Wow! That’s a lot more wattage than the 60 watts my trailer plug might be able to transfer from the truck’s alternator.
And even if the refrigerator compressor only cycles 50% of the time (50% duty cycle), you’ll still need at least 240 watts long term to keep it running. That extra 180 watts of power needs to come from somewhere. And that somewhere is the charge stored in the battery itself. If you have a 120 amp/hour battery you don’t want to discharge it more than 50%, so it really has 60 amp/hours of energy. And we can multiply that 60 amp/hours x 12 volts = 720 watt/hours, which means it can provide about 720 watts of power for an hour. So if it needs to make up 180 watts of power for even 8 hours, that’s more than 1,400 watt/hours, which is twice your battery capacity of 720 watt/hours.
This is the first lesson of running a residential refrigerator in an RV. Unless you have at least two house batteries, you’ll never be able to run your residential refrigerator a full 8 hours without shore power or a generator.
But doesn’t your truck’s alternator make way more than an extra 6 to 10 amps of current? Well, yes, it does. But to get it to your RV’s house batteries (and your residential refrigerator) you’ll need something called a DC to DC charger. Here’s a basic 50-amp version from REDARC. Of course you’ll need heavier gauge wiring and a big connector between your truck and RV’s batteries, but 50 amps of current at 12 volts is a full 600 watts of power. So your refrigerator at 50% duty cycle is using less than half of the charging current your truck’s alternator is capable of sending to the RV. The rest of the current can charge your batteries and such.
Showing you how to install a DC to DC charger is way beyond the scope of this JAM Session, so I’ll cover it in full later in one of my RVelectricity newsletters. But at least you know what’s needed to keep your residential refrigerator happy while towing down the road. In the meantime, see a link to a REDARC DC/DC charger on eTrailer HERE.
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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