battery issues

[Oehmig]

Hey forum. So I'm helping a friend get an 85' k100 running again. I've finally got the fuel system corrected and everything in functional shape but here's the problem I'm running into. The previous battery was low and continually fusing the contacts in the relay. Couple of whacks. Temporarily resolved. What I'm up against now is this... I put a fresh battery on and ran it for about five minutes. While running for a few minutes, the battery dies and the starter contacts get fused again. Is this symptomatic of a bad alternator? Seems like even with a fresh, known-good, battery, it runs for a while and then the battery dies and the starter contacts fuse again. I'm thinking alternator but don't want to pull it without some input... Thanks.

[kennybobby]

It could be alternator not charging, but to know for sure you need more data--measure the battery voltage while OFF, then while running at idle and at ~3k rpm. 

Also does the red battery indicator light on the cluster stay on or go off, or comes back on right before the engine dies?  If it is on while revved above idle then the battery voltage exceeds the alternator output and is not being charged. 

[Laitch]

In addition to checking into kennybobby's observations, have you cleaned and tightened the battery ground connection on the transmission and the frame grounds?