Hi all, I have the feeling I'm missing something big. Consider first a capacitor having +9V on both terminals, in this case nothing happens. Now I set the negative terminal to 0V, and make sure a current is able to flow; the capacitor will now charge up to 9V, right? Ok, now I make sure the capacitor is unable to discharge in any way; so it has its positive terminal always 9V higher than the lower, right? Here's the thing: If I now connect the negative terminal to a +9V potential again, then the positive terminal has +18V (of course, all relative to 0V ground). I understand this has to be the case in terms of the above boldface line, but I don't see how this works, physically. I have the feeling I'm almost there; in terms of energy stored per coulomb etc. The battery is still at a +9V relative to ground, but the same capacitor connected to it is now at +18V relative to ground..?