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  #1  
Old 09-22-2008
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Mysterious electrical issue

Greetings

I have a plain old 12 volt cigarette lighter plug at my nav station. When I plug my 12 volt laptop power supply and laptop into it, the cabin lights instantly become noticeably brighter. Any ideas as to what could be causing this? I disconnected the 12 volt socket thinking there was something wrong with it, but it made no difference.

Thanks in advance

Gary
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Old 09-22-2008
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Do you have a single or dual battery system? also are you running any auxiliary charging system (solar, wind, shore power etc)?
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Old 09-22-2008
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Sounds like your laptop battery is charging the boat battery- can you remove the laptop battery and run direct from the external supply and see if the lights are affected now?
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Old 09-22-2008
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Quote:
Originally Posted by NorthUp View Post
Sounds like your laptop battery is charging the boat battery- can you remove the laptop battery and run direct from the external supply and see if the lights are affected now?
Duh. I missed the obvious I think.

Try taking a volt meter and measuring the voltage at the socket. Then plug your 12V charger into your laptop and test the voltage at the cigarette lighter plug of the laptop charger (do not plug it into the boat's receptacle). if the voltage at the plug is higher than the voltage at the receptacle then you most likely ARE charging your boat battery with your laptop battery.

Also try plugging something else into the 12V receptacle that does not also have a battery (i.e. a fan or something with a heating element. These should make the lights dim as expected
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Old 09-22-2008
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Of course, if the laptop is discharging into the boat's 12 VDC system, I'd expect the laptop to have a much shorter run time. If that isn't the case, something else is going on
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Old 09-22-2008
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Additional info

OK, I haven't been out to the dock to test what happens with the laptop battery removed, but that's a good suggestion. That said, I can't see how the laptop battery would be discharging as I use it to run chartplotting software and it ran for about 6 hours without the engine running on Sunday, so it must be drawing power.
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Old 09-23-2008
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Gary, if it's discharging into your 12V system, how long the laptop will run for will depend entirely on the state of charge of both sets of batteries.

It's extremely unusual for this to happen (the power supply should have reverse polarity protection diodes in it which would stop this), but from the information you've given, to my finite mind, it can't be anything else..
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If the laptop is running for six hours, it ain't discharging into the 12VDC system, since NO LAPTOP I'VE HEARD OF can run for six hours while discharging the battery externally.
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Off the internal battery alone, no it wouldn't run for 6 hours. I'm guessing it starts charging from the internals at some point. However I doubt after 6 hours the laptop is still causing a voltage rise at the lights.

Then again, maybe gary has managed to come up with a self sustaining power source
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I don't know how many amp hours a laptop battery has, but it doesn't seem likely to me that one would contribute enough to a typical boat's electrical system to make a noticeable impact on what voltage the lights are seeing. And I'd think the two batteries, if connected that way, would equalize very quickly.
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