The voltage regulator is built into the new alternator so normally surging voltage would be blamed on it, but if you have a digital instrument cluster, they use a wire to sense system voltage there because it is more critical there to prevent the display from flickering. There is likely to be a problem in the instrument cluster causing the surging voltage. And voltage that is too high CAN damage a battery.
Look at the small electrical connector on the side of the alternator. If there is only one wire in it, (red or brown; I can't remember), the voltage regulator senses system voltage internally. If system voltage is wrong, the new alternator is defective.
If there are two wires in the connector, as a test, your mechanic could disconnect the second one, (white as I recall, but I'm not sure). That will make the new alternator sense system voltage internally instead of from the instrument cluster. If that brings system voltage down to normal, that wire or the instrument cluster has a problem.
Normal system voltage when the engine is running is between 13.75 and 14.75 volts. Lower than 13.75 volts, the battery will not charge fully. Higher than 14.75 volts, and water will start to boil out of the battery. A little bit higher voltage, say up to around 15.25, will not cause a problem as long as it holds steady.
GM alternators since 1987 can cause voltage spikes too due to their design. Those spikes can wreak havoc on the dozens of computers on your car, and can confuse the voltage regulator. It is quite common to replace the alternators on GM vehicles four to six times in the life of the car. Most mechanics have found that to prevent repeat failures, it is best to replace the battery every time they replace the alternator.
Saturday, March 20th, 2010 AT 5:01 PM