For anyone working with old Macs, the first step is often to "recap" the board and deal with the PRAM battery.
Getting Started With Your Vintage Macintosh – Garrett Fuller
While modern computers have straightforward BIOS batteries, the PRAM battery in vintage Macs was often hidden deep in the analog board or soldered, encouraging a "no user-serviceable parts" ethos. However, the community has kept these machines alive by finding modern replacements, like this Keystone Electronics 108 1/2AA Battery Holder. 4. Exceptions to the Rule