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Old 07-06-2011, 06:35   #1
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Voltage Drops Under Load

Hi all. I'm having issues with my electrical system on a 1979 Cal 34. The other day I noticed that with shore power off and engine off, if I turn my DC electronics on they will all die after about 40 seconds. The voltmeter will drop and bounce between 0 and 2 ish volts. If I turn some devices off at the switch panel, volts may rise but it's still low and bouncing. With shore power on this does not happen. I thought my batteries were toast so I got 2 new identical batts (2 westmarine 6 volts combined to 12 volts) and fully charged them. Same problem. I know the batts are hooked up right and work because they show 6 volts each and 12 total.

The starter battery and starter works fine (I have a combiner that isolates house from starter batts). So I think the problem might be a short circuit drawing excessive load somewhere in the switch panel. Does this make sense or does anything else pop out to anyone as a likely place to look? I haven't change any wiring or devices recently. The only thing I've done is haul out and bottom paint. Also, my engine died under power the same day I noticed thi but I changed the fuel filters and it works fine now so I dint think they're related

Thanks so much for any ideas.
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Old 07-06-2011, 06:53   #2
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Re: Help: voltage drops under load

Clean all your connections. You have got resistance somewhere that is causing a voltage drop. Start at the batteries themselves. Sometimes you cannot see it you just need to wirebrush unitil you find that bad place.
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Old 07-06-2011, 07:37   #3
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Re: Help: voltage drops under load

What does your voltage read at the panel or switches? It does not matter what the voltage is at the batteries at this point. It sounds like a dead short, you should smell burning, or a loose or corroded connection. You are going to have to check all connections. Many times a wire will chafe through from movement or vibration. Start at the batteries and move towards the panel. Be sure and not overlook any wiring. Once at the panel, move from the panel to each load. Don't overlook the grounds, since a loose of bad ground will also cause this problem. Chuck
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Old 07-06-2011, 11:04   #4
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Re: Help: voltage drops under load

I vote for a poor connection causing voltage drop.

Check both sides of the circuitry. A bad ground connection is just as likely as a bad connection on the + side.

If it was a short, your boat would probably have already burned to the waterline.
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Old 07-06-2011, 11:12   #5
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Re: Help: voltage drops under load

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Originally Posted by senormechanico View Post
I vote for a poor connection causing voltage drop.

Check both sides of the circuitry. A bad ground connection is just as likely as a bad connection on the + side.

If it was a short, your boat would probably have already burned to the waterline.
Agree. 98.5% it is a corroded connection. Since the starter is working well you can eliminate the battery terminals as the problem area. If the problem is all 12V circuits then I would start at the main + power feed from the battery or bus bar to the switch panel AND/OR the main ground line from to the negative bus or engine block, however your system is set up.

Just look at the back of the switch panel and see where the main + and - lines go to the panel, check the connections there and follow those wires back to where they originate and check the connections on that end. If that doesn't solve it, keep working your way back. Don't forget that the bad connection could be inside a switch.
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Old 07-06-2011, 11:22   #6
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Re: Help: voltage drops under load

First , I'd look at the ground wire to the main buss or panel...that's likely the one common element to the auxiliary items you mention.
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Old 07-06-2011, 18:43   #7
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Re: Help: voltage drops under load

Spent a couple hours today digging around. I polished/greased the battery terminals and heavy gauge leads, and the heavy gauge ground to the engine block. I also bypassed the DC switch...same problem. The same problem occurs with everything jurry-rigged to the starter battery as well as the house batteries...leading me to believe it's hidden somewhere else. And since the bilge pump is wired straight to the battery (and it still dies), it leads me to believe it's something non-related to the breaker panel as well...maybe a short between the bilge circuit and the breaker panel/other circuits? If I recall the light for the bilge pump panel is wired into the breaker panel busses...could I be "inheriting" a short on the other circuits to the bilge circuit via this connection?
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Old 07-06-2011, 18:53   #8
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Re: Help: voltage drops under load

What are the actual voltages you are talking about. Exactly?
in your initial post you said the drop was "The voltmeter will drop and bounce between 0 and 2 ish volts"
So are you saying that you have 12.5 volts and then when you turn something on it drops to 11.5 volts?

If so you have a serious problem.

If you mean 0.2 volts then thats fine.... 12.5 to 12.3 when you turn on the fridge etc...

If you mean it goes from 12.5 volts to 2 volts then please say so.
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Old 07-06-2011, 18:54   #9
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Re: Help: voltage drops under load

might have to turn everything off (disconnect) and do one circuit at a time with a volt meter. narrow it down one, by one
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Old 07-06-2011, 19:00   #10
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Re: Help: voltage drops under load

If the Bilge Pump is wired direct and does the same thing as the panel, I gotta believe you still have low battery charge or bad neg cable.... I mean the pump is totally separate form everything else right?....except the battery and ground cable?
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Old 07-06-2011, 19:15   #11
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Re: Help: voltage drops under load

Quote:
Originally Posted by Reklobe View Post
. . . I thought my batteries were toast so I got 2 new identical batts (2 westmarine 6 volts combined to 12 volts) and fully charged them. Same problem. I know the batts are hooked up right and work because they show 6 volts each and 12 total. . . .
If you new batteries test at 6.0 volts and 12.0 volts combined they are near dead. You need to invest in a digital voltmeter. A tenth of a volt makes a significant difference.
- - Also battery lugs/connectors need to be examined very carefully. Corrosion of the lead inside the connector to battery terminal can make an intermittent circuit - sometimes it works, sometimes it doesn't. Also the part of the battery lug where the battery cable connects can have internal corrosion giving the same problem.
- - At the other end of the battery cables, the same thing, dirty or corroded terminals and connections and even more importantly - loose connections will cause problems similar to yours.
- - An amp-meter in the main feed from the battery to the house distribution panel will show if there is any current flow when everything is turned off and an abnormal current flow as one by one items are turned on.
- - Connecting anything directly to the battery, bypassing the main battery switch is also an invitation to problems such as yours. Everything, and I mean everything should be downstream of the main battery switch with its own circuit breakers for wire/load protection. When you leave the boat, each non-essential item can be turned off by its own circuit breaker.
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Old 07-06-2011, 20:11   #12
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Re: Help: voltage drops under load

take a decent digital multimeter and measure the voltage right at the battery terminals, both with everything switched off and under load.

ciao!
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Old 07-06-2011, 20:52   #13
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Re: Help: voltage drops under load

The starter works, but the bilge pump doesn't?

Aren't starter normally grounded straight to the engine block, like on a car? Ground strap from the engine block to the battery?

Find the ground for the rest of the electrical system and clean it.
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Old 07-06-2011, 22:31   #14
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Re: Help: voltage drops under load

The voltage drops to 8ish volts with some of the devices on, and 0-2 volts with all of the devices on (obviously nothing runs).

I've cleaned all the leads on the brand new, fully charged batteries.

The only thing that happened differently that I can think of causing this is our unrelated problem of our engine dying around the same time this problem happened. We were cranking very hard on the starter motor to get things going...I imagine maybe the motor got hot and melted a lead there, causing a short (although that's the starter circuit which, in theory, is isolated from the house circuit where I am having problems). Which leads me back to the ground again I suppose? I triple verified the ground from the batteries to the engine block is good. The starter battery has a lead connected to the negative of the house bank which is then connected to the ground bolt on the engine block. All are cleaned/greased.
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Old 07-06-2011, 22:54   #15
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Re: Help: voltage drops under load

I am betting the starter motor is burned out or the starter solenoid is burned out maybe both. There is no such thing as a coincidence when electrical systems fail.
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