Well, watching all the episodes over again (only on ep. 026 from restarting) I noticed that in the first few episodes, there was a fight between this little kid who referred to himself has a samurai and ash. Ash sent out his Pidgeotto only to notice that it was worn out from the last battle Ash had with it. Misty had said something like "Ash, don't you know you have to let your pokemon rest before calling them back out again!?". That's the closest to "healing" in the pokeball as I have seen or heard. This is a very good question though.