New alternator and battery idle is low and battery light keeps coming on 2003 windstar.?

Replaced the alternator and battery recently, followed the procedure to get the computer to relearn it's idle and have driven 100kms. When the rpm gets below 1500rpm the battery light comes on. When sitting at an idle it is only at about 800rpm. Any idea what is going on, or does it just need to be driven more for the computer to relearn it's idle?