battery fault?

Further to my problem with charging, posted a couple of weeks ago,thanks to those who gave advice. The regulater seems ok. the voltage rises quickly to 14.5v. However if i connect the headlamp bulb (halogen H4 55w) it drops instantly to 12v , my friend steve borrowed a sportster with similer problems which was put down to the wrong bulb being fitted. Surely Harley dont have unique headlamp bulbs?
Thanks again
Chris

[Phil]

No they don`t, regular motorcycle bulb.
Check the charging AMPS, also check the battery cables from end to end.