CR2032

If your friends and family ask you from time to time for service for their computers (or you have older PCs running yourself), then you should stock up on some CR2032 batteries. Next time you see a "BIOS checksum error" or some such, chances are the battery on the MF motherboard is dead you just need to replace it to get the computer running again. And you may need to reset the BIOS settings though and see if the default values are OK. And set date and time.