All you need is a regular CR2016 lithium battery. It's not some wacky battery, like the SEGACD FAQ seems to suggest. I just did it to mine (SEGACD2) and to a working dupe. It's just like changing a cart battery, except a bit trickier to get inside. When popping the spot welds take care, as the posts are not that tightly secured to the SEGACD. It's far safer if you have a soldering iron handy, if you need to re-attach a post.
Take your time, and have some needlenose pliers, a small thin prying instrument, and patience. I simply attached the new battery with electrical tape and cardboard (for pressure).
It works great! No more RAM INITIALIZATION FAILED PLEASE FORMAT messages. The games just play!!!
Watch next week in buying and selling for my Ebay auction for a sweet restored Genny 2, SEGACD2 (with new battery installed!), and 32X set, with assorted games of each type. =)