Turns out batteries are best left to bunnies, not mice (Energizer joke). Two varieties of mobile smartphone back-up batteries are being recalled in Japan, one featuring Pikachu and the other a generic Pokéball image. Buyers reported that the units became uncomfortably hot and some even saw smoke.
If you came into possession of one of these, please see the page linked above for refund instructions or contact the seller! For the rest of us, let this be a lesson that Pikachu’s power cannot be harnessed so easily by human technology!