Battery drains quickly and rendered useless

[Dday62]

I have a 77 GT550. The bike will start and run but quickly drains the battery which is then rendered unusable: will not recharge. I did substantial work before this, finding multiple shorts in the wiring harness which were repaired. I check the regulator rectifier according to manual specs with my multimeter, though I am a novice. I had the bike running great (it has less than 5000 miles on it). During my last test, I measured 12 V across the battery terminals while the bike was running, but it only ran briefly until the battery drained off. I have gone through two batteries this way, an expensive experiment. Now I don't know exactly where to begin again.

[GT750Battleship]

Hi,it should be at least 13.5 volts,rising to 14 volts or so at a constant 3,000rpm or better,something definately wrong with the charging system,has the alternator got any brush material left,check regulator/rectifier condition with the meter. I'm not real good with electrics,but others "here" are...
Regards,
GT750Battleship.

[jabcb]

See this post for testing your alternator: viewtopic.php?f=29&t=9006&start=6

As GT750Battleship suggested, check the condition of the brushes.

As an additional check of the rotor, insulate the brushes from the rotor slip rings. (via either an air gap of an insulator)
Measure resistance between the slip rings & ground (metal part of rotor).
Should get open circuit.