Old Merc Outboard Electrics

Discussion in 'Electrical Systems' started by LP, Nov 2, 2012.

  1. LP
    Joined: Jul 2005
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    Location: 26 36.9 N, 82 07.3 W

    LP Flying Boatman

    I noticed on my recently wetted runabout build that the voltage on the electrical system is indicating 16V at about 5000 RPM. I'm afraid that this voltage is too high for the system, but other sources have indicated that they see this voltage level all of the time with their older outboards. This particular model is a '72 Mercury thunderbolt 500 if that helps anyone.

    I am worried about over charging the battery. Perhaps, I'm being overly concerned. I have heard of people adding the voltage regulator to the system to maintain proper voltage levels. Has anyone done this?
  2. Frosty

    Frosty Previous Member

    The battery itself is a regulator as its resistance to charge increases as its charged.

    Is this voltage at the battery with batt conected and the batt in good condition.

    Im not sure if its normal or not --I agree it is a little high. I dont like the idea of 5000 rpm unloaded in the drive way.

    Small motorcyles regulators are cheap and could be adapted.

    Depends if its a Dc or AC charging device.

    What is the voltage at 2000
  3. iceboater
    Joined: Jan 2011
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    Location: Iceland

    iceboater Junior Member

    If you are reading 16V from the dashboard meter, I would first check the voltage at the battery with good voltage meter. Voltage up to 14,7V is normal. Those dashboard meters tend to be inaccurate.

    If you are getting 16V at 5000 rpm and it drops to below 14,7V at your cruising rpm, I would not worry about the battery, just keep your eyes on the water level.

  4. michael pierzga
    Joined: Dec 2008
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    Location: spain

    michael pierzga Senior Member

    What ever you decide , you should consider that high voltage, low voltage cycles are harmfull to electronics. The classic way to protect dc electronics like fishfinders or gps is to filter the output of a alternator or battery thru a dc to dc converter.

    They arent expensive. Perhaps give a call to a supplier like Minnkota


  5. LP
    Joined: Jul 2005
    Posts: 1,417
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    Location: 26 36.9 N, 82 07.3 W

    LP Flying Boatman

    The battery is in good condition (new). I did have to use the starter a lot because of a self-induced carburetor problem. This is the second time out with the boat. The first time, IIRC it was holding steady at 13-14V. I was wondering if high battery usage could be a consideration. I would think though that a high charge rate, high draw by the battery, would tend to pull the voltage down rather than drive it up. I'm new to this type of electrical system so I'm still trying to figure thing out without damaging anything.

    The boat was in the water, in gear. It's the standard stator type charging device Mercury used on that era of motor. It is an A/C device that goes through a rectifier for DC conversion, but without a voltage regulator. (me thinks) I didn't made a note of the 2000 rpm setting, but at 3000 rpm, it was indicating 14V. Your comment on battery resistance makes a lot of sense though.

    I'd say 4-5K is my cruising rpm. 4800-5500 are recommended rpm limits an this engine. FWIW, the gauge is brand new.

    Thanks, for link. I'll have to check it out.

    I think I need to throw the battery on a charger and start fresh to see if the system voltage returns to previous levels with a fresh charge.
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