Turn key on lights come on tutn key to crank engine lights go out

I just replaced both the alternator and the battery. Now the charging system warning light continuously lights up for a few seconds the turns off while driving or idling. The voltage gauge reads 14 volts. I've driven the van several miles with the A/C and headamps on. The new alternator seems to keep the battery charged. With the engine off, the voltage gauge reads approx 12 volts.
Any ideas as to why the charging light is doing what it does?