ITs your alternator. (or wiring associated with alt)
At IDLE your battery should have 14.2-14.6 volts on it.
this is the voltage measured that is being MAD Eby the alternator, not that the battery is ACCEPTING.. (ie, even a BAD battery would have that voltage reading when running, it just simply wouldn't accept the voltage as a charge (due to a bad battery.)
assumes a good battery:
running: 14.2-14.6
off.. 12.0-12.4 (remove surface charge: key off, headlights and interiors on for 60 seconds)
Ray