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Thread: 1985 K100RS Alternator

  1. #1
    Registered User Beemer01's Avatar
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    1985 K100RS Alternator

    I've been adding accessories to my (almost) vintage K, twin 50 watt driving lights, heated grips and an electric vest for those cold rides.

    Is the OE alternator adaquate, or should I move up to a 50 amp retrofit?

  2. #2
    Registered User kioolt's Avatar
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    I believe that you should upgrade to the 50 amp alternator. I did my 91K100LT back in 92 after I found out that the 33 amp alternator couldn't keep up with two 45 watt electric vests and trailer lights. Your description of what you are adding is more than that.
    2004 R1150RT 186,800 miles , 1991 K100LT 128,700 miles, 1982 R100RT 106,900 miles
    Total 422,400 BMW miles
    AMA,BMWRA,BMWMOA
    The cheapest thing on a BMW is the nut that connects the handlebars to the seat.

  3. #3
    Blocking the slow lane
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    I think you should be fine at highway speed. Poking around town would be a bit more dicey.

    House loads (headlight, fuel pump, rear brake light) are about 120 watts.
    Heated jacket and grips are 100 watts.
    Driving lights are 100 watts.

    The 33amp alternator puts out about 460 watts. Put a voltmeter across the terminals of your battery when everything is on, and see how many rpm it takes for the voltage regulator to kick in and limit the re-charging at the battery. Then you'll know what kind of engine rpm you need to maintain.
    Jon Diaz
    BMW K75/K12GT
    BMWMOA Ambassador

  4. #4
    Registered User Beemer01's Avatar
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    Thanks for the calculations

    Jon - Somewhere I had learned the calculations you outlined, but for the life of me could not remember them.


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