Battery notification while on sleep mode

How to turn off “battery will be fully charged at 4am”. This notification comes through even while on sleep mode and comes in the middle of the night. I know the common response is to put phone on silent, but it’s for my elderly grandmother who really struggles with tech and it’s not an option for her. I was hoping the sleep mode would help her but it’s not the case is there another way to either turn that off or atleast silence it? Thanks for your help