Ring doorbell battery

I have the original doorbell I assume 1. It is not hard wired as I do not have that possibility at the moment, only battery option. The battery does not seem to be lasting for as long as it used to. Do these batteries reduce in efficiency after a number of years. If I’m only getting 3-4 weeks use now does that mean I need to upgrade the battery it seems a very expensive option at £118 Uk Sterling to buy a new door bell. Does the Ring door bell three come with the option of a removeable replaceable battery and will it fit on the same bracket that I use for the current door bell.