ICOT: with a capacitor there is no fire created, according to the currently accepted understanding that a current is not “fire” only a spark. The issue of closing a circuit would only be relevant according to the Chazon Ish’s understanding of boneh, which is not widely accepted. Even according to the CI closing a circuit is only boneh when it accomplishes something significant. He learns this concept from the Chaye Adam who says that winding up a clock is make b’patish because it causes and inanimate object to function, and the CI equates the 2 melachos. Therefore even according to the CI, if you close a circuit and nothing happens, as is the case with a capacitor if it doesn’t reach it’s capacity, you have not done boneh.
A technical question which may or may not have relevance in Halacha. If the capacitor does not reach its full capacity, does it store the remaining charge and need less time to reach capacity the next time, or does the electricity dissipate?