I just read your post closer. Are you saying that the engine will crank
fine and fast 1 out of 10 times. That it wont crank at all the other 9?

The engine starts off the battery and runs off the altenator. Sounds like
your problem is a starting problem,not a charging problem. Sounds to me
like the starter pilot bushing($2 at your favorite vendor).A worn bushing
causes the armature of the starter to bind.Maybe the starter solinoid is
sticking.Beat it with a hammer.Clean the terminals-all 3 or 4 of them on
the starter,or just pry them a bit with a screw driver a bit to break the
corrosion.Should help. Bye againnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnn