Alternator troubles.....

[robbo_yobbo]

Heres the leadup....

Im driving my car and over the period od 1 hour, I watch the voltage display drop from 12/13v down to below 10 when the car coffs and splutters.
I charge up the battery and the cars good to drive for another hour.

I figure, it must be the alternator, and as I was putting that motor in my new *(not so new) car on the weekend anyway, so swapped the alternator at the same time with another one.

Now I only get about 1/2 an hours worth of driving (on the same motor in different car with a different alternator) before theres not enough charge in the battery to run the car.

Is this just a huge coincidence and the new (used) alternator is screwed too, or could there be something else going on?

I took the motor, engine harnes, ecu, dash+dash loom, bem and transponder out from the old car and put it in the new car with the motor, so if its not the alternator it must be somerthing in here?

Or could it be I just havnt plugged something in???

Does the alternator run stand alone, or is it told when to switch on and off???

Cheers