When a Tamagotchi's batteries run out, they should be replaced. The batteries are hidden behind a lid on the back of most Tamagotchis. The batteries can be removed from their recess using screwdriver or a sharp, thin utensil such as a toothpick, open paperclip, or plastic needle.
Classic Tamagotchis use CR2032 batteries. In modern Japanese Tamagotchis, such as the Tamagotchi Plus Color or Tamagotchi iD L, AAA batteries (単4 tan 4) are needed, although American batteries work fine as well.
It is recommended that the Tamagotchi owner does not use mixed brands of batteries or old and new batteries.