you should be pushing about 14v or higher at about 2500 rpm. If you still only get 13.5 or so - before you condemn the alternator or regulator - check your battery. Here's how. Let the car sit overnight to let the battery rest and the internal voltage to stabilize and eliminate "false readings". Then, the next morning - DON'T START THE CAR! Just lift the hood and read the voltage across the battery - It should be 12.5- 12.9 volts. If it is at 12v or lower - you have a bad cell that will suck the juice out of your alternator causing it to put out a lower voltage with the current needed for charging. If the voltage on the battery after an overnight rest is 12.5v or higher - then suspect your alternator or regulator. Just some time saving advice from someone who has been down that path several times.