Battery won't stay charged

I have a 1998 Ford F-150 4.6L. I put a new
battery, solenoid and alternator in the
truck. The battery light is still on and won't
keep the battery charged up. I tested the
the hot wire on the alternator and it only
reads .04v to .05v. The battery is testing
normal at 12.1 to 12.4 when the truck isn't
running. With the truck running it tests the
same except it will start to drop the longer
its running. When I text the hot wires
coming out of the inline fuse between the
solenoid and alternator I get the same
reading on as the alternator when I test
that side and the same as the battery when
I test that side of it. I have checked all
fuses in the cab and under the hood and
they are all good. The reason I replaced it
was that my truck was at my dads and he
thought the battery was that was in it was
bad cause it was about 7 years old so he
took it out and put a new one in it that had
at his house. Well when he hooked it up he
didn't notice that the new one he was
putting in that the terminals where
opposite fopprom the one that was in it. So
he put the cables one backwards. Well he
only touched the last cable to hook up for
a second because it sparked off on him.
Then he noticed it and switched it out with
the new one I went and bought. So I hope
that didn't do something else that I can't
find.