Hi, Thomas8092 you need to 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 and it must be able to pass a proper "LOAD" test you may have cursory readings of 12.5 volts or more but little or zero amperage the battery is faulty and must be replaced, AGM batteries have this issue more so than lead-acid types.
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. The meter should read 3 milliamps or less, 10 milliamps with a radio, 15 milliamps with radio and CB. If your multimeter reads higher you need to isolate the circuit by pulling fuses and circuit breakers one at a time and observe multimeter for a drop in amperage then get out your test light and track down the short in that circuit.
5. Hook up the multimeter to the battery set it to DC volts and start the engine if multimeter 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 the multimeter set to the ohms scale, with one lead grounded, touch an alternator pin ohmmeter should read infinity, if not replace the stator.
10. With both leads touching alternator pins, multimeter 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 multimeter set on AC volts scale, both leads touching alternator pins multimeter should read 16 to 20 volts AC for every 1000 RPM'S 1989 and later and 19 to 26 volts AC for every 1000 RPM. If not replace the rotor.
12. Check for excessive wattage load from the headlight
For more information about your issue and valuable "FREE" downloads that you will need for viewing or printing please click on the blue links below. Good luck and have a wonderful day. https://www.kawasakimotorcycle.org/forum/mechanics-corner/116619-battery-drains.html How to diagnose fix parasitic draw on your motorcycle Kawasaki Concours 1000GTR Service Manual OEM Parts for Kawasaki Kawasaki 1000GTR Owner Manual
I don't know 100% for sure where the battery is but typically on Japanese bikes they're under the seat. You can charge it yourself using a motorcycle battery charger. Sears also sells a charger that's good for cars and motorcycles and will condition a battery that's sulfated. Changing the gas in the tank would be a good thing to do to help prevent the possibilities of fuel related problems rearing up when you show the bike. Other than that, a good going over before you put it on the market is a good idea.
There is a brake light switch for both fron and rear brakes. Just spray some WD-40 on it and work it back and forth a few times and it should be good. Could be a bad switch but you must find out which one is sticking
Compression, check with a compression tester
Fuel, is it clean and fresh. Is it getting to the spark plug, it is wet with gasoline?
A spark at the right time. New spark plug and engine correctly timed. A nice fat blue spark if you put the plug against the cylinder head?
Try looking at the main fuse (30amp) on the starter solenoid. Make sure handlebar run switch is to on position - we all do it! and also check battery connections are tight - especially the negative lead.