Adafruit CR1220 12mm Diameter - 3V Lithium Coin Cell Battery - CR1220
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These are the highest quality & capacity batteries, the same as shipped with the iCufflinks, iNecklace, Datalogging and GPS Shields, GPS HAT, etc. One battery per order (you'll want one battery per cufflink or pendant.)
Brand may vary but all battery brands are verified to have the same nominal voltage of 3.0V and nominal capacity of 40mAh