Jim,
An alternator actually uses two different diodes. One set of diodes, the 6 diode array Tom mentioned, rectifies the ac output and changes it into dc for charging, lights, etc. When the motor is not running, a small voltage is fed thru the ignition switch and a red idiot light, and that's what initially powers the rotor and allows the alternator to put out current. Once the motor is running, this voltage is supplied by the alternator itself thru the second diode, and the idiot light goes out. If the idiot light burns out (it happens, but rarely) there won't be any voltage to start the alternator working and you won't get any output from it after you start the engine. That diode is also switched off and on at a high rate of speed by the voltage regulator to control the voltage output of the alternator, keeping it steady regardless of speed. If this regulator is not switching the diode on and off properly, the alternator can put out LOTS of voltage (100+), and fry the diode array and/or battery.
An alternator doesn't 'spike' by itself. That indicates a problem in the voltage regulator.
Make sure the idiot light is working, replace the voltage regulator (it could be an intermittent problem), and make sure all connections are corrosion free and tight. That should solve your problem and is a lot cheaper than a new bike. ;D