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Old 10-03-2016, 13:36   #1
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Magic DC

My batteries were disconnected, both positive and negative cables removed, all winter. Yesterday I went to the boat to start spring prep and I reconnected the batteries in order to charge them. As normal I connected all the positive cables first. When I connected the + terminal that my Battery Monitor is connected to the monitor came on. Note that at that time not a single battery had a negative cable attached, the was no way for a circuit path from and to the battery to occur.

So - how/why did the Battery Monitor come on?

One part of me says it doesn't matter (it's done that for 4 years since it was installed). But another part of me says I should figure it out in case it is indicating some other problem.
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Old 10-03-2016, 13:51   #2
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Re: Magic DC

That can't happen.

I'd be figuring out how/why that's happening for sure. But I'm a engnerd so It'd bug me. It'd bug me bad. I'd sit in the boat and hook the + up and remove it several thousand times with ammeters, voltmeters, maybe even a digital sampling oscilloscope. Maybe some sort of conductive path to - on the outside of the batts?

OTOH, if it's done it for 4 years and everything is fine and it doesn't bug you...
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Old 10-03-2016, 14:06   #3
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Re: Magic DC

Quote:
Originally Posted by jeepbluetj View Post
That can't happen.


OTOH, if it's done it for 4 years and everything is fine and it doesn't bug you...
It does

and

It does
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Old 10-03-2016, 14:31   #4
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Re: Magic DC

What battery monitor?


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Old 10-03-2016, 14:37   #5
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Re: Magic DC

Victron BMS600
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Old 10-03-2016, 15:44   #6
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Re: Magic DC

Shore power connected?



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Old 10-03-2016, 16:14   #7
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Re: Magic DC

I have an old landrover and the amp gauge is not connected at all when I crank her over the gauge shows a current draw now work that out
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Old 10-03-2016, 16:19   #8
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Re: Magic DC

Quote:
Originally Posted by Sailmonkey View Post
Shore power connected?



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My reasoning here is the possibility that there is a connection from the DC ground to the shore ground, and that enough of a difference of potential exists to power the monitor.


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Old 10-03-2016, 17:56   #9
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Re: Magic DC

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My reasoning here is the possibility that there is a connection from the DC ground to the shore ground, and that enough of a difference of potential exists to power the monitor.


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It was, but the breaker was open/off. I can not see how power could go from the + battery terminal to the AC ground system. That's not a complete circuit.
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Old 10-03-2016, 19:06   #10
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Re: Magic DC

Quote:
Originally Posted by sailorboy1 View Post
It was, but the breaker was open/off. I can not see how power could go from the + battery terminal to the AC ground system. That's not a complete circuit.
AC ground is not switched, and shouldn't be!
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Old 10-03-2016, 19:08   #11
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Re: Magic DC

Quote:
Originally Posted by sailorboy1 View Post
It was, but the breaker was open/off. I can not see how power could go from the + battery terminal to the AC ground system. That's not a complete circuit.
Like was stated above, the ac ground is not switched. and if the battery negative bus is tied to the ac ground bus.....it's possible.
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Old 10-03-2016, 19:14   #12
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Re: Magic DC

ABYC requires the DC negative to be bonded to the AC ground,

I have studied electricity for years ( maybe one day I'll understand a little bit of it) and noticed recently that all of what we think of as known science in electrical matters is still noted as "theory" in serious scientific journals.
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Old 10-03-2016, 21:10   #13
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Re: Magic DC

When you connect the + terminal of the battery to the system you are connecting a + potential even if the - is not connected. If the system goes to a place of lower potential voltage than the + battery terminal then there will be a voltage difference between those two points. Doesn't have to be the - battery terminal.

In the normal setup, battery + and - connected then you have electrons moving from the negative side of the battery through a circuit to the positive terminal of the battery. As long as there's a place for the electrons to go (or in your example, a place that has extra electrons that can go to the + battery terminal) you can have a complete circuit. The ground to the AC or bonding to the ocean would be a place electrons could originate.
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Old 10-03-2016, 22:41   #14
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Re: Magic DC

Quote:
Originally Posted by skipmac View Post
When you connect the + terminal of the battery to the system you are connecting a + potential even if the - is not connected. If the system goes to a place of lower potential voltage than the + battery terminal then there will be a voltage difference between those two points. Doesn't have to be the - battery terminal.
No.

If the battery negative terminal is unconnected then there is no positive potential anywhere. A battery with a floating (-) terminal has absolutely no effect on the rest of the circuit.

How could it???

But something is happening. I suspect that jeepbluetj is right when he suggested that there is a conductive path on the battery case that connects the (-) terminal to the house ground somehow. A battery monitor draws very little current, so there might be enough leakage for the thing to work.

The only other explanation is that there is a hidden connection to (-) that you're not seeing.
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Old 11-03-2016, 00:13   #15
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Re: Magic DC

Haven't looked but guessing my shore power charger might light the meter without batteries connected.

Memory cell in something might be enough just to light the meter's led display briefly if you switched the main isolator on.
Capacitive charge in SSB tuner?
Solar charge controller, wind generator?
Galvanic isolator?

I wouldn't leave it unresolved.
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