On some vehicles I'm told that the charging bulb - if it blows - can cause this fault. The alternator gets its exciter current from the bulb circuit - it the bulb is blown the alternator does not generate much power.
If the bulb actually lights then it may well be the alternator.
A bulb is much cheaper than a new alternator...
http://www.howtomendit.com/answers.php?id=292574
Good luck...
Peccavi, January 2010
