2000 mustang battery light

I have a 2000 base model mustang and it
died going down the road. I put a new battery
and alternator on it but the light stayed on. All
connections are good and belt is not slipping
nor does it show any signs of wear. If I jump
the car it will start but will die eventually and
after a while won’t run at all without the
jumper box connected or recharging the
battery completely. I’m not sure what the
problem is but for some reason the alternator
isn’t charging the battery as I drive. I though
it may be a bad alternator so I took it back
and exchanged it but the problem is still
happening. Any and all help would be
appreciated. I don’t want to have to put the
car in a shop.