They are the CPU batteries. If you have the "blue" mpu type, they are CR2032's. If you have the "green" mpu type they are BR2335, although CR2335 will also work. The difference is in the the way the batteries discharge over time. The BR type are more resistant to higher temperature applications and their voltage stays evenly throughout the life of the battery, they are better suited for backup nvram. The CR version starts at a somewhat higher voltage and over time, the voltage drops below the 3 volts, they are meant to be used on higher drain situations. So BR is preferred, but for home use where loosing the accounting information and having to setup the game again is not a big deal, the more common CR variant will work. I have been using the CR type for a few years now and have had no issues. They have typically lasted 3 years with the machine only powered up around 5% of the time.