“Actually, the key to simple battery is touching someone against their will.”
She touched Trump, Trump withdrew from her, which showed it was against his will, so she is guilty of assault also.
Forgetting, for the moment, she breached security protocol, and no one had any idea what was in her mind.
For the touching to be battery there has to be an intention of harming the person. The only intention by Lewandowski was to clear the space for Trump. Once the space was clear, which took about 1 second the interaction with Fields was over. No malice intended. The only intent was to clear the space and remove the possible threat from a candidate. A battery charge is a stretch and seems to be politically motivated. Fields is a political activist.