It'll be a few times during the game. Basically, we're probably going to have a single pool of energy to power all special attacks, weapons, etc. We want to have a few parts in the game where you can increase your maximum energy. I came up with a work-around that would take two extra items: empty batteries (1 gained whenever we want to increase the maximum energy) that can be changed into charged batteries at the cost of a full bar of energy. Not a whole lot more work involved, but it does seem a bit messier than our original idea...
If there's no way other than having seperate items for every new max, we'll just go with this.