Hmmm, I'm unsure. I don't think hidden information is always detrimental to gameplay. I also think that if you're paying attention to the shield visuals it is obvious that opening a single valve to one shield charges it so much quicker than if you've got all the valves open charging all the shields. Maybe the potential player uncertainity you mentioned would be mitigated by adding the UI elements I mentioned in my other response which should make the charge level of a shield and its rate of change more readable.