These cameras are not rechargeable, they require a special lithium battery to work, otherwise you will not get effective use.
To address your first comments about lasting 3 weeks, this is dependant in a lot of variables, how strong the WiFi signal is, recording quality, recording length, how many times you live view, and how many times you capture throughout the day, I.e. high traffic areas that are triggering.
Having said that, some batterys don’t seem to be the most efficient from the box, given experience on comments from this forum.
My advice would be change the battery for the right type (2 x Energiser lithium AA) and monitor use again to see what length of battery life you get