what is the voltage output at idle and again at 5k RPM?
it could be a bad battery...
That's right. Set your $10 digital voltmeter to DC volts and read the battery voltage without the engine running. Should be over 12 volts. If not charge the battery. Now read the voltage at 5K. Should be around 14. If over 16, the voltage regulator may be bad, but you'll have to see the specs for your bike. Basically, the voltage regulator changes excess voltage to heat, and the rectifier changes alternating current to direct current. If the voltage doesn't increase from idle to 5K, or worse goes down, the alternator, rectifier, battery, or wiring or all four could be bad.