Get one of these...
http://cgi.ebay.com/KH109-Snap-On-MT...item3a5807ed52
A Voltmeter does not an Ampmeter make...
Ampmeters are
the thing to see if your charging system is up to the job. You can lay this across your big wire from the Alt to see how much power it's producing, about the only thing your voltmeter dropping says is it's not enough.