I have a 1980 yamaha bobber that I unfortunaly wired myself. everything works great it has a head light, tail light, and a charging system and ignition. my problem is when I charge the battery with my battery tender the bike starts up just fine 12.5 volts at the battery. when I run it more than 20 miles the bike loses volts to the point that I have to kick start it. I ran it for another 15 miles and never lost power just not enough juice to run the electric start. Got back home and the battery was sitting at 11.5volts. my question is Ive checked my brushes "good". Checked the Alternator Ohm ed out good. Now what is next?