2nd generation ring doorbell battery help

Hello I have just purchased a 2nd generation doorbell for my parents. Tried to keep it easy for them but I’m confused myself, as there doesn’t seem to be a battery I can buy to have as a replacement whilst the normal one is charging. I’m not keen on leaving my parents without the doorbell for 8-10 hours whilst it charges. Surely there’s a way round this. Any help would be greatly appreciated. Many thanks