What is a CMOS battery? What are its functions? - Quora
Amazon.com: Synergy Digital CMOS/BIOS Battery, Works with Dell Precision M4500 CMOS/BIOS, (Li-Ion, 3V, 200 mAh) Ultra High Capacity, Compatible with Acer 23.22207.041 Battery : Electronics
Asus X551CA CMOS Battery Replacement - iFixit Repair Guide