Cant heal when holding weapon

I don't get it, you're not supposed to be able to hold your gun and use a consumable. You have to put away your gun and then use one, be a shield cell, battery, medkit, syringe, etc. etc. Or, if you're holding your gun then press whatever button you have set for consumables (4 by default for PC players) the game will put your gun away for you. 

As for Sentinel, if you have a shield battery, you can charge up the Sentinel rifle by pressing B (once again on PC, whatever key is displayed on the bottom right is what button you're supposed to press.) then you can charge up the weapon. But you do expend the shield battery.