IVE GOT A VOLT GUAGE FITTED IN MY CAR IF I HAVE THE STEREO AND AMPS RUNNING AS WELL AS HEATERS ETC.... THE GUAGE DROPS RIGHT DOWN!!
YOUR NOT PULLING TOO MUCH OUT OF THE BATTERY R U AND THE ALTERNATOR CAN'T KEEP UP?

ALSO CHECK THE BLUE WIRE TO THE ALTERNATOR IS CONNECTED AS THIS IS WHAT PUTS YOUR DASH LIGHT ON IDF THERES A PROBLEM!