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’m in Yuma and was checking my 5th wheel RV battery today. I turned the battery selector switch from “on” to “off” and removed the cables from the battery to better see if I needed to add any water. The levels were fine. I checked the battery voltage, which was right at 13 volts. I switched the battery selector to “on” and got a reading of 13 volts coming in from the converter. With the battery selector in the “off” position I was still getting a reading of 1.4 volts at the disconnected battery cable ends. Do I have an electrical leak, or is this normal? —George
A very interesting question. So let’s break this down a bit for our readers and go from there. If I’m reading this correctly, you were measuring 1.4 across the cables normally feeding 12-volt power to your RV from the battery, even though the converter was disconnected from them, correct? If that’s so, then it’s as if there was something in your RV’s electrical wiring that was supplying 1.4 volts DC to an unenergized electrical system.
Well, it appears that you’ve encountered something quite normal in modern RV electrical systems. One of the things that electronics don’t like is noisy electrical power. And so a variety of 12-volt powered gadgets in your RV include noise reduction capacitors across their 12-volt power supply/inverter inputs which are used to step that 12 volts up to some other voltage needed by your television set or whatever. And each of those capacitors acts just like a tiny battery that can hold a charge for a while.
I’m betting that when you connected the converter to your battery and turned on your battery selector, you simply charged up all those little capacitors (batteries). And after you powered down and disconnected your battery cables, the 1.4 volts you were reading was simply the residual capacitor voltage gradually draining down to zero, as you can see on this little chart. If so, then after a few more minutes you would have measured zero volts and your curiosity wouldn’t have been piqued.
So, you likely don’t have any electrical leaks at all – you simply measured something perfectly normal as it was discharging down to zero volts.
Hope this helps. Boy, there sure is a lot know about electricity, isn’t there?
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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