'88 R100RSRapid Decline in Volt-Meter...

Just noticed in the last day that the voltmeter is now registering 11V after happily registering 13+V, since, like, always. Could this be a sign of a battery losing its Mojo and asking to have its twin brother take over for a couple of years?
Or could this be a sign that the alternator is going 'round the bend? Or perhaps the voltmeter is biting the big one...
Any thoughts?
Strangely coinciding with this discovevery is that the thisng is really idling fast. It is tuned at about 900RPM and today it was taching at 1500 at idle. Could these 2 events be related or are they random occurances?

Disconnect the battery and charge it fully. Check voltage at battery terminals while disconnected. Fully charged battery should read about 12.5 volts or better. If lower than 12 volts, battery is probably kaputsky. If battery is ok, other sources of trouble can be alternator, diode board, voltage regulator or corroded electrical connections causing voltage drop. Duane Asherman has a good website on BMW repairs. Go here: http://w6rec.com/duane/bmw/index.htm. Nothing occurs to me that would match up low voltage with high idle speed, though.

Little Egypt Airheads
1966 R50/2
1976 R75/6
1980 R100RT (named Rocinante)

Goin' to the AMA Vintage Rally....Gillette was too far away!

Some more "light" reading can be found here:

http://bmwmotorcycletech.info/trbleshootALT.htm

Have you put a known good voltmeter across the battery terminals and read the voltage there and compared to the dash voltmeter (which I assume where the other readings are coming from). This should be step 1...easy and fast to see what's going on.

What is the voltage across the battery at 3K RPM...and compared to the dash?

What does the voltage at the battery drop to when starting the bike?

How old is the battery? What type of battery? Have you measured the specific gravity of the battery fluid?
Maybe you can't do that if it's a sealed battery...

What's your driving style like...how many miles per day or per week? City driving or sustained highway riding?