Slippage rarely happens with a serpentine belt, even if it's worn. Reading 13.68 volts at idle isn't necessarily a problem.

You may have worn alternator bearings, a weak belt tensioner, or a bad idler pulley. Where I would start is by removing the belt. While it's off, check the idler wheel for trueness and make sure the bearing turns smoothly without wobbling. Idler pulleys tend to wear into a cone shape, so inspect for a tapered rolling surface. Also make sure the tensioner has good tension. Spin the alternator pulley by hand, and feel for roughness. Finally, check the a/c compressor and power steering pulleys for smooth operation, as the belt ties all of this together. If you can't find any other problem, install a new belt.