Broken down college kid

is the battery fully charged ? Do you have some accessory running that takes lots of juice ? Needle going up to positive means it charging , I get that if my battery is need of a charge ,like after car died and hazards were on for 2 hours , but after 1/2 hour of cruising it settles down to middle of gauge .
Is the pulley on the alternator same size as old one ? If it is larger it might not charge enough at idle .