Electrical System Weirdness

Discussion in 'Electrical Stuff' started by Cosmo Knot, Sep 12, 2018.

  1. Cosmo Knot

    Cosmo Knot New Member

    Jul 14, 2018
    390 Motor Yacht 2005
    Cummins Mercruiser 480 CE
    First night on the hook in the "new" boat and woke up at 4 AM to a tapping/thumping sound. The frequency of the "taps" increased quickly until it was a steady hum. Traced it to the starboard cabin speaker. Stereo wasn't even on(!).

    Not quite sure, but I think I opened the port battery solenoid and the sound stopped. Looked like I drained the port battery bank pretty badly (was running a few lights and the fridges). I started the starboard engine. All is fine. Then closed the port solenoid and used the emergency start to start the port engine. The port dc volts (analog cockpit gauge) pegs high and I get a "High Volts" alarm. So I stop the port engine. Alarm does not clear. Port volts reading very low (~10 VDC).

    Try to start the generator to run the battery charger, but can't close the generator solenoid. Genny starts from port battery bank (of course). I don't see anything in the manuals or google about using the emergency start switch to start the generator, so I don't try.

    Can't find any info on a High Volts alarm. Keep searching and try the port engine again with the digital meters up on the display. Port engine starts (emerg start again) and the volts go up to 28 VDC. Shut off the engine again.

    Spent a couple hours trying to figure out WTF was happening, but to no avail. At one point, I did have both engines running with VDC at nominal and all alarms cleared, but eventually got the high volts alarm again.

    Decide to limp back to marina on one engine and managed to get it back in the slip (thanks bow thruster!). Hooked up to shore power and let the batteries charge all morning. Looked at the battery charger and it was pegged high at 60 amps (later was reading a more reasonable 20 amps).

    Went out that afternoon and everything seemed to be fine. I started the generator in the slip and kept it running the entire time, just to ensure the batteries stayed charged.

    The next weekend, I checked all four batteries and all tested good.

    At this point, I chalk it up to "extreme battery discharge." Any other ideas?
  2. Little Ducky

    Little Ducky Well-Known Member

    Jun 5, 2017
    Dickson, TN / Chattanooga, TN
    1998 290 Sundancer
    Twin EFI 5.0L w/Alphas
    Kohler 4kW
    Where you by chance running the port engine with the battery switch turned off?
  3. Cosmo Knot

    Cosmo Knot New Member

    Jul 14, 2018
    390 Motor Yacht 2005
    Cummins Mercruiser 480 CE
    No. The port battery solenoid was energized.
  4. Bt Doctur

    Bt Doctur Well-Known Member

    Aug 21, 2010
    New Jersey
    Ex SRV 240 Weekender twin
    in between
    you think so, very high voltage at a startup indicates a run away alt or no battery connected
    Do this rest
    Turn all battery switches off, try to start motors/generator with emerg switch
    What happens?
    Turn port battery switch on, attempt to start stbd motor with emerg switch What happens
    Turn stbd switch on, attempt to start port motor with emerg switch, What happens

    Another good reason for dedicated house, engine and generator batteries
  5. Cosmo Knot

    Cosmo Knot New Member

    Jul 14, 2018
    390 Motor Yacht 2005
    Cummins Mercruiser 480 CE
    I don't think this boat was meant to be out w/o the genny running.

    It has six batteries already. I would like to add to add two deep cycle house batteries, but there isn't any room.

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