buying Monster Batteries w/ Parasitic Tentacles
This boardgamegeek thread has a poster who appealed to iello's site and got this response


  "The energy cubes that were already on Monster Batteries go back into the bank pile, and the purchaser of Monster Batteries may then use the effect upon purchase."


So it seems like whatever cubes are left on there are simply lost to the bank, meaning the longer you can keep that card once you activate it, the better (before someone resets it with Parasitic Tentacles.