Question about 1989 BMW K 100

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Bad battery last winter I fixed a short I had going to my rear liscence plate lite. I also put a brand new battery in this april when I started riding again. Well today the battery died AGAIN. Brand new battery just dies. Last couple weeks I hav e noticed that it was starting hard. I drive 12 miles one way back and forth to work. I was able to get a jump from a friend. I rode to the gas station 11 miles away. Turned the bike off pulled out the key. Filled the bike up put key in and the bike started right up. So that tells me my charging system is working. SO does this mean I have a bad battery after only 2 months of riding? Or is my only riding 12 miles 1 way not enough to keep the battery charged???

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Don't rely on on whether the bike will start at a gas station to determine if the charging system is working. Get the multimeter out and do a proper test. Should be getting about 14.5 volts @ around 3K RPM. It's too easy of a test to to just skip. If it is charging then start looking for something else. Was the new battery properly charged to begin with? Loose battery connection? I suppose the GPS could be too much but you said that it was dying last november, so I would suspect something else. P.S. you need a properly charged battery to get an accurate reading for the charging test.

Posted on Nov 20, 2008

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The BMW/Marig batteries aren't what they used to be. Check the battery with a hydrometer after it has been completely charged. My guess that you will find a bad cell.

Regarding charging: The BMW voltage regulator only allows the charge voltage to rise to 13.8 volts (nominal) - if the voltage is slightly below this figure you may have an issue with the regulator. If the voltage fails to rise higher than 12.8 volts the charging system is defective.

Because the regulator only charges at 13.8 volts the battery will be slow to charge if it's unusually discharged.

Posted on Feb 09, 2009

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