Battery replacement
The original battery for the backup RAM and RTC leaks over time and can cause damage. It can either be a NiCd battery or a coin cell, in both cases rechargeable.
Replacing the battery will probably cause the backup RAM to be corrupt. Forget about your hi-scores and settings if you don't provide 3V on BATVCC while replacing it.
The battery can be replaced with an equivalent 3.6V NiCd battery or 3.7V rechargeable coin cell (LIR2032 or ML2032), but the 470 ohms (yellow-purple-brown) charging resistor must be removed if a non-rechargeable battery (such as a common CR2032) is installed.
The resistor can be added back if a rechargeable solution is required later on.