I noticed the eye on my battery is black also and that the battery is not sealed like a maintenance free battery should be. My next battery will be an Optima or an Interstate.
Also when the engine is at idle most cars do not produce enough power for the electrical load, thus you do run off you battery at idle. Also if your battery was bad and causing a problem with your alternator then the computer would give an indication. OBDII systems are nice.
Hoen Lights, New Era Intake, YELLOW interior badge
Now: Running with the Devil!
YELLOW calipers and rotor hats with pads (brand undecided)