Sounds like it, battery stores all CMOS data. I had it happen before. It's a CR2032 battery in the PS2, iirc - unsure if the PS2 slims differed, as I don't have one of those.
Good guide for taking the console apart, along with some general maintenance/testing:
http://www.tweak3d.net/articles/ps2repair/3.shtmlGood luck!
