I can confirm that it is a T3 Torx screw in the Fitbit Charge (NOT CHARGE HR). Here is a replacement battery with all of the necessary tools (including T3 Torx Screwdriver) for the repair:
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[http://www.mpfproducts.com/LSSP031420AB-Battery-for-Fitbit-Charge-HR-Smart-Fitness-Watch-FB405|Battery for Fitbit Charge]
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[http://www.mpfproducts.com/50mAh-Battery-for-Fitbit-Charge-Activity-and-Fitness-Watch-FB404|Battery for Fitbit Charge]
I can confirm that it is a T3 Torx screw in the Fitbit Charge (NOT CHARGE HR). Here is a replacement battery with all of the necessary tools (including T3 Torx Screwdriver) for the repair:
[http://www.mpfproducts.com/LSSP031420AB-Battery-for-Fitbit-Charge-HR-Smart-Fitness-Watch-FB405|Battery for Fitbit Charge]