How to add temporary AC to an item as charges.

I am creating a homebrew item that as a reaction you can increase a creatures AC for one round by using charges from the shield. But i cant seem to find where i would add that. It already has the correct number of charges but when I try to add the condition of spending a charge to increase AC by 1 it just adds it automatically, changing the bonus from +5 to +6. But the additional +1 AC should only be added as charges are spent. 

Pretty sure that's not something you can do with the homebrew system at the moment. In particular, temporary effects aren't supported.