Dreamcast does not have a battery back up at all, just enough RAM to remember the time and date. It just uses the AC Cord to support that Memory....but once its unpluged for 24 hours, that memory is erased...but if left plugged in, then it won't forget it
The VMU Memory Unit on the other hand uses the CR2032 watch battery (2 of them) which is the exact same battery used in the Saturn's backup ram....and the same battery used in the Neo Geo Pocket Color