Hello @Ishu23, welcome to the Community Forums!
Let me share with you that the battery of our Charge 2 device was designed to last 5 days but this also depends on how frequently you use it’s features. That said, I recommend you charging your Fitbit device with a USB port from a computer or a UL-certified outlet into the wall and then turn off all the features that you don’t use that much. After doing so, please this help article for more tips and recommendations. Such article includes as restart process that will refresh device device's performance.
It's not possible to only replace the battery of your device, however, if after taking into consideration the information above you continue experiencing difficulties with the battery life, don't hesitate to contact me back so we can move forward. For any warranty-related inquiry, please check our policies here.
Keep me posted, I’ll be right here if you need anything else.
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