Synergy Digital Smartwatch Battery, Works with FitBit Blaze Smartwatch, (Li-Pol, 3.7V, 120 mAh) Ultra Hi-Capacity, Compatible with FitBit Blaze Battery
Fitbit Charge 2 - Review - Full specification - Where to buy?
Fitbit Fit Bit Charge 2 Battery LSSP411415 SW-24 60mAh | eBay
Amazon.com: Synergy Digital Smartwatch Battery, Works with FitBit Charge 2 Smartwatch, (Li-Pol, 3.7V, 70 mAh) Ultra Hi-Capacity, Compatible with FitBit Charge 2 Battery : Sports & Outdoors