The batteries in a Fitbit Flex are built into the product and thus not replaceable. The product’s manual contains instructions for charging and caring for the battery of a Fitbit Flex. The Fitbit website publishes the manual, as of 2015.
Fitbit recommends using only the charger that shipped with the product to charge its battery. Users charge the Fitbit Flex with certified computers, power hubs or power supplies. Fitbit strongly cautions against attempting to force open the built-in battery for replacement, as this could permanently damage the product.
The Fitbit Flex features a rechargeable lithium-polymer battery that should last about five days between charges. An owner logs into Fitbit.com to check the battery level of a Fitbit Flex.