Hi, Anonymous first perform the following tests:
1. Fill acid type batteries to proper levels.
2. Charge battery overnight at 1-2 amps you need 12.5 volts or better after charging.
3. Check battery terminals for damage or corrosion, check the battery cables at "BOTH" ends for loose, corroded, or broken connectors, "INSIDE" and outside the cable harness, perform connector wiggle test and check cables with an ohmmeter if necessary.
4. Hook up battery positive cable, then with your multimeter on the milliamp scale connect one lead to the negative battery post and the other lead to the ground cable. Meter should read 3 milliamps or less, 10 milliamps with a radio, 15 milliamps with radio and CB. If your meter reads higher you need to isolate the circuit by pulling fuses and circuit breakers one at a time and observe meter for a drop in amperage then get out your test light and track down the short in that circuit.
5. Hook up a voltmeter to the battery and start the engine, if meter falls below 9.0 volts while cranking you need to perform a proper load test on the battery and replace if necessary.
6. With the engine running at 3600 RPM, the battery should read 14.3-14.7 volts if not continue tests.
7. Unplug the voltage regulator from the alternator at crankcase by the front of the primary cover.
8. To test voltage regulator go to https://www.youtube.com/watch?v=F8EjV0IjW9Q
9. With ohm meter, one lead grounded, touch alternator pin meter should read infinity, if not replace the stator.
10. With ohm meter, both leads touching alternator pins meter should read 0.1 to 0.2 ohms on 1989 and later models. 0.2 to 0.4 ohms 1988 and earlier models, if not replace the stator.
11. With the voltmeter set on AC scale, both leads touching alternator pins meter should read 16 to 20 volts AC for every 1000 RPM'S 1989 and later and 19 to 26 volts AC for every 1000 RPMS. If not replace the rotor.
For more information about your issue please visit the websites below. Good luck and have a nice day.
05 06 07 K1200S Problems BMW com
Battery Charger For K1200S Word from Deltran Bikes com Excellence in Motion
https://www.tradebit.com/filesharing.php/search/0/bmw+k1200rs BMW Motorcycle Parts Fiche OEM BMW Motorcycle Parts online 1970 present BMW
Btw, I’m available to help over the phone in case u need at https://www.6ya.com/expert/gregg_c0ec1df182c7330e