3.6V (half AA type) Lithium PRAM/Clock Battery for Mac

This small battery is known as the PRAM (Parameter RAM) battery or clock battery. It is used to maintain the state of a small amount of memory which contains certain system settings including the system clock.

The standard symptom of a failed PRAM battery is that your machine's date and time will be incorrect when you boot your Mac - it will generally correct itself shortly after boot when your Mac checks an internet time server. In some Macs you will also see power management related issues if you have a failed PRAM battery.

NB: It is important to install the clock battery the correct way round - make sure you match up the positive and negative markings on the logic board with the battery.