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Old 08-02-2009, 08:59 PM   #1
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General Electric

Pardon the long newbie question. I'm trying to learn 12 volt systems.

When I first hooked up to shore power after having the SMB for a couple of months, the converter (that was delivering 14.+ volts when plugged in) developed a massive short immediately when unplugged and was pulling the lifeline battery down past 11.6 volts and dropping like a rock.

I took everything off the battery as quickly as possible, reconnected the alternator wire and the a ground and drove off to get a charger.

In my haste to get everything off the battery, I didn't label or note what was what. I thought it would be obvious or color coded. It's not.

Here's my question:

I've got 120 volt to a circuit box and then on to power a GFIC and other 120 volt stuff-check.
(I didn't touch the 120 volt system other than to unplug the old converter).
I've got a main 12 volt (big 6 gauge) power cable from the alternator-check.
I've got a positive cable going to a bus bar on the 12 volt panel-check.
I've got a couple of ground wires grounded to the frame near the battery-check.
I've got a couple of dedicated ground wires out of the wall(smaller gauge)-Fine I guess.
I've got a mysterious wire that I think was hooked up to the positive terminal. Much smaller than the 6 gauge main, and even smaller than the 8 gauge from the converter to the battery. it goes off into the wall.

The only 12 volt candidates are the starcool, the refrigerator, the co2 detector, and the furnace (I think). Is there another fuse back there? Is there some reason why there would be another wire going straight to the positive terminal of the battery instead of through the 12 volt fuse box?

I'm new at 12 volt. Does this make sense? I should be able to check the ground wires against the frame and determine continuity. If I check this wire and it is "open" does that mean it is a positive lead to some load (probably with a fuse) and that it can safely be hooked put to the positive terminal?

I've got my new Xantrex converter and I want to hook this up with as little drama and smoke as possible.

Many thanks,

Tom

P.S.-Hey! post number 100.
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Old 08-03-2009, 07:52 AM   #2
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Re: General Electric

First dropping from 14v to 11.6 when unplugging shore power (or shutting down the engine/alternator) is normal. Dropping past 11.6 as you describe sounds like there is a high draw item (my invertor running a toaster pulls it down quickly to around 10v) but I'm not sure you have to disconnect everything- the batteries can take quite a hit... just do it repeatedly and you'll kill them, but they shouldn't get ruined immediately (so says the guy who's got one house batt hanging disconnected because it's fried). Speaking from that experience I've brought both sets back from the dead more than once... now I've got one 4D out of two that is probably 70% and two new starter batteries (and the dead 4D which is ballast).

If the starcool or something wasn't on then you might have a bad battery- masked by charging from driving or being on shore power- although that you should have seen other evidence of previously. I doubt you could pull that much out through a short without starting a fire, or at least setting off fireworks.

Anyway, as far as your small wire, I would deduce the same thing- it's a positive wire. However you can leave it off and see what is no longer working. It might sense something (voltage?) as opposed to powering something. The proper way to wire a CB is directly to the battery, but I can't think of any other small gauge wires going to any of my batteries.

Incidentally are we talking about house or starter batteries here? Do you have an isolator or separator between? Are you working at the batt terminals or the isolator/separator?
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Old 08-03-2009, 12:41 PM   #3
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Re: General Electric

Thanks.

All this is at the house battery. I have the original separator that looks ancient. Not sure it is even hooked up correctly. I do know that the starter battery is working and the house battery is receiving 14+ from the alternator.

Next step is to wire in the new converter.

I didn't know the voltage dropped like that right after being taken off the charger. I figured it would drop from 14 something to full charge 12 something. Are you saying it bounces?

Oh well. The new Xantrex won't hurt.

I guess I can hook up the known wires and see if anything catches fire or blows a fuse. Then I can see what doesn't work.

Next, I'll turn everything on and see if the spare wire has current. If not, I'll air out the van and touch the mystery wire to the positive terminal and see if something turns on. Minimal risk, right?
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Old 08-03-2009, 01:35 PM   #4
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Re: General Electric

The voltage shouldn't drop like that unless there's a heavy load on the battery. With no load it'll stay quite high initially, and voltage at the terminals will drop to 12.8 or so after a few hours if it's in good shape (or you can put a draw on the battery for a short period instead of the wait - this is to remove the 'surface charge' by (if I recall correctly) allowing sulfuric acid to disperse from the area of the plates.).

The short in your charger would explain this drop.

Re: your extra wire - could it be a battery temp sensor? I have two of these that go to the battery terminal - one from the solar controller, and one from my battery charger.

Cheers,
Mike.

PS - edited to note that you should have a fuse in between the battery positive terminal and your charger/converter, for this very reason.
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Old 08-03-2009, 07:35 PM   #5
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Re: General Electric

The Xantrex has two 30 amp fuses built in. Do you think I need another on the line? If so, what size? I'm not sure how the two built in fuses are wired.

Not a temp sensor. No solar yet...

Thanks

Tom
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Old 08-03-2009, 10:24 PM   #6
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Re: General Electric

I'm not sure how your new charger is configured. Sounds like the old charger didn't have a fuse.

Fuses on the positive cable are usually located pretty close to the battery terminal - I think within a foot or so is recommended. But Xantrex knows a lot more about chargers and batteries than I do!! so I'd just follow the instructions carefully.

(I just took a quick look at the instruction manual for the XADC converter online - it mentions the need for an additional fuse near the terminal. Fuse size would depend on your charger's amperage rating - I'm using a 50A fuse for my 40A charger.)

Cheers,
Mike.
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Old 08-04-2009, 05:38 AM   #7
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Re: General Electric

Doh!

I thought they sent me two "owner's guides" so I could keep one in the van and one at home. There is an installation guide and it does suggest a fused disconnect. I think a 50 amp fuse on a 40 amp charger sounds reasonable.

thanks


Tom
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