let me put it this way. A battery charger works like an ignition coil. It uses electromagnetic induction.

Now, an alternator works how?

Electromagnetic induction. The field inside the rotor becomes a magnet when its energized. The poles of the rotor spinning generate an AC signal into the stator. Since we cant use an AC signal in a car, we use a rectifier to cancel out the part of the sine wave we dont require. We just take the top part of the sine wave, then in an automobile alternator, we take off at 3 points so a minimum of six diodes are required in a 3 phase alternator. (some imports and new stuff have as many as 6 phases)

anyway, with the battery disconnected, the AC signal has no where to go.

I wonder what the threshold voltage of the rectifier diodes are??? I can tell you its been exceeded.

Guess what, if your spike didnt kill anything, you may very well have just killed one third of the rectifier in the alternator. It will still charge, but only capable of one two thirds its original output.

You may have killed it and didnt even know it!

and that goes for old or new! anything with an alternator.

does that make sense?