The Dead Battery Society
http://www.arcadecollecting.com/dead/dead.html
"Several arcade game manufacturers decided it would be a good idea to put a battery on their game boards that, when they die (and they will die), disable the game. Why they did this isn't exactly clear. Usually the way suicide batteries work is, the batteries supply power to a bit of RAM that holds a decryption table. This table is the key to decrypting the encrypted program stored in the board's ROMs. When the battery dies, this table goes away and the program code can no longer be decrypted. The CPU no longer has valid code to execute. The board stops working."