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This is a handy little non-rechargeable CR2016 Lithium Coin Cell Battery!
Sitting at 20mm diameter and is 1.6mm thick it's small but packs a punch with a nominal voltage output of 3V and a capacity of 90mAh. For other drain rates the capacity may be reduced, check the datasheet! Note that these are often "stacked" to make a 6V CR2032-sized pack of batteries.