Electrical Question

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New member
Oct 8, 2014
I have a 2017 28RKS Catalina tow trailer.  I'll detail the symptoms.

I returned home from a trip and plugged my trailer into home outdoor plug (120V). I removed the batteries to charge them. I don't like leaving the batteries on my trailer because of the extreme heat in the summer. Typically my home outlet will operate the AC because of my 10 gauge extension cord. I returned to the trailer and my slide out did not work. Also the interior lights, fridge and AC didn't work either. Interestingly, the microwave worked! I decided to reconnect the trailer battery and everything started working again, even the AC. Why is a battery connection needed to operate the DC lights and slideout when the trailer is connected to my home power? Also why does the AC and fridge not work unless there is a battery connection? Is it an inverter issue? I checked all my fuses and they all look good.

Thanks for your help; I'm puzzled.


Lou Schneider

Site Team
Mar 14, 2005
Welcome to The RV Forum!

If your air conditioner uses a wall thermostat, it needs 12 volts to operate the thermostat and interface relays.  Same for most of the other appliances in the RV including the refrigerator.  The lights and slideouts also run on 12 volts.

You might have blown the output fuse(s) on the converter if you tried running the slide in or out without the batteries connected.  It's a heavy draw and usually the batteries provide reserve power to avoid overloading the converter.

If you did blow the fuse, the trailer is running on the batteries.  If you have a voltmeter, measure the voltage at the battery terminals with the trailer plugged into electricity and again when it's not.  Plugged in you should see 13.5 volts or better, if the battery voltage is at 12.6 volts or lower the converter isn't working.


Well-known member
May 7, 2015
Western Kentucky
Welcome to the Forum!

Lou is exactly correct. 

Everything that is NOT working requires 12VDC.  The slides, especially, often want a battery and shore (converter) power.  The easy test, as Lou said, is measure voltage at the battery.  Battery output (fully charged) should be about 12.6 volts.  IF the converter is working, you should measure about 13.5 or higher volts at the battery.  If you only get 12.6V, the converter is either not working or os not getting power to the battery.

I would suggest starting at the converter, checking for blown fuses and output voltage.

Nomenclature note:  A CONverter takes 120VAC input and converts it to 12VDC.  An INVerter takes 12VDC input and converts it to 120VC.

Gary RV_Wizard

Site Team
Feb 2, 2005
West Palm Beach, FL
Just to be clear, when your RV is plugged into 120v shore power it still uses 12v for most functions. Shore power allows the built-in converter/charger to produce 12v power so that it doesn't need to come direct from a battery, but it still uses 12vdc.  It's a mistake to think that "everything runs on shore power".

The thing we can't tell you from afar is just how the batteries are wired into your system, i.e. whether the converter 12v is still functional with the batteries removed.  However, most such wiring systems allow the battery to be removed and leaving the converter as the sole 12v power source.  The converter has fuses to protect against shorted or miswired batteries, and those could have been  blown during removal.  It's also possible that more than one wire is connected to the battery positive post and the additional wires may power somethig critical.
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