A lithium CMOS battery for DVR (Digital Video Recorder) is a small, coin-shaped battery that is used to power the CMOS memory chip on the DVR's motherboard. The CR1220 battery is a specific type of lithium coin cell battery that has a diameter of 12mm and a thickness of 2mm. It is commonly used in small electronic devices such as watches, calculators, and remote controls, as well as in DVRs and other digital recording equipment.