that is self explanatory, isn't it? you need 12-14 volts to start a motor, you don't need that much to run a radio, lights, wipers or anything else. if the alternator isn't charging the battery, the voltage is dropping to just below 12-ish while driving, due to headlights being on, using turn signals, computer, abs, auto tranny (maybe?), etc. could just be bad connection, too, but, if not, sounds exactly like a bad alternator.

95 Sportage w/welded and 5.89 geared dana 44's on 38.5" TSL's.- SADLY SOLD
CURRENTLY - 2000 sportage, 5.38 gears, welded diffs, 35" tires, 5.5" lift

http://www.cardomain.com/ride/341410