Say you have two first aid kits (FAKs), one with 30 charges and one with 47 charges. Put both into a simple storage chest... they will show a stack of two FAKs (30 or 47 charges). Take one and put it in your inventory. It is now 200 charges again, and the one in the storage is still 30 charges (or 47).
I just did this with about 12 FAKs of various charges and I ended up with 12 FAKs all with 200 charges.
- tested with basic FAKs