1.5 volt button cell battery , It has a 6.8mm diameter and a 2.1mm height.
Alkaline Equivalent Names: AG1, LR60, 164, LR621
Silver Oxide Equivalent Names: SG1, SR60, 364, SR621SW
Difference Between Alkaline and Silver Oxide Versions
Most brand name batteries such as Energizer ,Duracell and Maxell will sell only silver oxide batteries in this size. Silver oxide batteries will last longer as their capacity is about 20 mAh, 53% more. If you do not mind switching batteries more often, generic alkaline batteries are cheaper to buy. You can actually find a replacement battery in every Home/Hardware store .
Battery Repair Video Tutorial