Kingdom Hearts 1 ps2: Do i have 2 pick up the little green experience balls?

[kellydakitkat]

or do i automatically get them? In Kingdom Hearts 1 do i have to run around and pick up the green experience balls during battle? like, do the disapear fast? or can i wait till after battle? and if i dont pick 1 up will it automatically be added to me or will i of lost it? plz answer ! :}