The Play and Charge Kit is sold separately from the Xbox 360 Wireless Controller. It contains a rechargable battery pack and a USB cable which connects the controller to the console.
The battery pack that comes with the Play amd Charge Kit contains 2 standard rechargable AA batteries soldered to a control circuit board with a 6-pin header.
The pinout for the header is (left to right): Temp, P-/GND, SCK, SCA, P, VCC
The SCK, SCA, GND, and VCC pins are connected to an AT24C04M 2-wire serial EEPROM chip. Presumably the controller checks the EEPROM to verify that the battery pack has been charged fewer than a factory-set number of times and stops charging if that number is exceeded.