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Old 05-02-2007, 11:26 PM   #1
1968 28' Ambassador
1971 29' Ambassador
Moorhead , Minnesota
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Shore power and battery power problems

I seem to have a problem when I have my battery and shore power both connected. My lights are either dim or will go out all together, though the power light on the control panel will show as being on. I disconnected my battery and got good lights with shore power. Similarly, I can use just batter power and get good power. But when I have both systems hooked up I seem to have problems. Any ideas?

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Old 05-02-2007, 11:39 PM   #2
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1992 34' Limited
Falls Church , Virginia
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Totally guessing here... but possible ground issues?

My .02 cents are likely WAY over valued here, shoulda stayed on the sidelines and let the FAR more experienced chime in. Hope you get 'er sorted...!


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Old 05-03-2007, 06:07 AM   #3
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What is the condition of your battery?
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Old 05-03-2007, 07:40 AM   #4
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Test the battery; it is a very likely culprit.
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Old 05-03-2007, 07:53 AM   #5
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Sound like the convertor is connected to the batteries backwards. The lights are 12 volts DC. and polarity sensitive if connected to 2 or more power sources. If they work off the convertor when the batteries are disconnected and off the batteries when convertor is disconnected, but not when both are connected, that says they are bucking against each other.

Using the trailer body as Ground check the polarity at the output of your convertor. With shore power on. Disconnect the battery cable, should be a red wire, at the convertor and check the polarity on that cable and on the terminal you disconnected it from. They want to be the same polarity. If not that is your problem and trace the wires back to the batteries and correct as needed.
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Old 05-03-2007, 08:23 AM   #6
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Surprise , Arizona
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I’d start by making a more careful study of the electrical system in three separate situations: batteries only, batteries + shore power, shore power only. And I’d not only look at the lights, I’d check the fans and other appliances. And I’d definitely use a multimeter to measure the voltage at the 120VAC and 12VDC outlets. The voltage at the 12VDC outlets should tell you a lot, I’m guessing.

If everything “downstream” from the converter runs okay when either batteries only or shore power only is connected, then I’d guess those branches of your wiring are okay. And my attention would then turn to the converter itself…where shore power and battery power meet.

When both shore power and the batteries are connected, I think you said the lights are dim (or don’t work at all?). That seems to show that the lights aren’t receiving their full 12VDC under those conditions, and I think you’ll be able to measure that voltage loss at the 12VDC outlets. That would be a nice confirmation of the idea that the converter’s having trouble producing 12VDC when the batteries are connected.

But why? If it can produce 12VDC when the batteries are not connected, why can’t it produce 12VDC when the batteries are connected? My hunch is that the answer is that the “battery charger” component of your converter is at fault. Without knowing much about your trailer’s wiring, it seems like that’s the only part of the system that becomes active when BOTH shore power and the batteries are connected. In other words, that’s when your converter uses some of the 120VAC shore power to recharge the batteries. So if that portion of the circuit has a problem, then you would probably see other problems, too…like dim lights and low voltages at the 12VDC outlets.

There are plenty of people on the forum who know more about these things than I do, and some of them should jump in soon. In the meantime, I hope these ideas are helpful.

Oh! And don’t forget that electricity bites, sometimes very hard. So please be careful.

Good luck!


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