12V to 120 V Inverter not working properly

1995 Ford F350 Coachman motorhome

I am trying to rig up a 12v dc to 120v ac converter to be able to use all AC outlets for computers without having to run the generator.

I used the DC12v from the main panel input for the motorhome. + from inverter went to the red side of the 12 v fuse box. - from inverter went to the main ground that all other wires were grounded to. I used 0 gauge wire. It gives 14.5 volts with the motorhome engine running.

I then plugged in the shore power line for the motorhome into the inverter and instantly it gave an alarm. I did this several times and it gave an alarm every time. All appliances and AC was off. Then I turned off all AC breakers, and it still gave an alarm.

I tried this with two different inverters and both gave alarms instantly.

Any ideas?

Thanks, Ken

You likely need a direct connection from the battery to the inverter. This shoould be as short a distance as possible. I suspect the wiring from the battery to the 12v panel is so light that you are getting an undervoltage alarm.

You do need to put the inverter close to the batteries and connect it to them with thick wires to avoid voltage loss on the 12 volt side.  With 1/10th the voltage, you'll have 10 times the current and 10 times as much voltage loss compared to 120 volts.

Beyond that, there are two other possibilities for the alarm.  First, you may have left the 12 volt converter on when you plugged the motorhome's cord into the inverter.  This would make the converter draw up to it's full rated capacity as it tries to replace the power the inverter is drawing from the batteries.  In other words, you're trying to make a perpetual motion loop from the batteries, through the inverter, into the converter and back into the batteries.

The solution is to unplug the converter if it's plugged into a standard outlet or turn off it's circuit breaker to break the loop.

But you said you still got an alarm with everything turned off.  This may point to a second problem.  Most inverters produce balanced power, where each power pin is +/- 60 volts from ground.

RVs are supposed to keep ground and neutral separate through their electrical panels.  If yours is improperly wired with neutral and ground connected together, this would place a direct short on one side of the inverter's output since the negative battery terminal is also connected to the battery ground.

With all AC power off, measure the resistance between the ground pin and each power pin on the RV's shore power plug.  There should be no continuity there.  If you see low resistance then neutral and ground are improperly tied together.  Most likely there's a bonding screw in the breaker box connecting the neutral and ground busses together.

Thanks for the tips...I am getting another inverter and will start investigating the cause of the issue in a couple of weeks. Thanks for the ideas.

Ken

OK, here is the solution. The ground wire from the inverter 115V output had to be disconnected. So I made a short extension cord that I left the ground wire off. It runs from my inverter to the normal shore power 115V plug. Once I did that, it all works perfectly with no more alarms from the inverter.

I checked all the plugs with a simple three light checker and it showed they are all wired up correctly (no crossed white and black wires.)

For the life of me I dont know why I had to leave the ground wire off, but it works.

Whats really wierd is that my three light checker still says it has a proper ground, even when plugged into the inverter with the ground wire off.

Ken

Sounds like you have neutral and ground bonded together somewhere.  This is normal practice for the main panel in a house, but the RV breaker panel is a subpanel where neutral and ground must be kept separate.

You can use an ohmmeter to measure the resistance between the neutral (white wire/large slot) and ground at any of your outlets.  If there's continuity between them neutral and ground are connected somewhere.

Look for a screw bonding the neutral and ground buss bars together in the RV panel.  Sometimes it's put in by mistake, so if it's there, remove it.