Alternators are pretty rugged,
but can succumb to excessive heat and overwork. They can also be damaged
by sudden voltage overloads (as when someone attempts to jump start
a dead battery, crosses the jumper
connections or disconnects a battery cable from while the engine
is running.) Wasabi Racing has test equipment that can identify these
kinds of problems. If you suspect a weak alternator, you should have
it tested to see if it needs to be replaced.