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The battery level that shows on the app is the level it was when it last synced so, as you're having sync problems, it will be out of date. That should sort itself out when you get it syncing again.
I would try restarting your phone and then restarting your fitbit using this procedure:
Give this a few tries as it doesn't always work. If it still won't sync after that then there are some more ideas to try in this help file: https://help.fitbit.com/articles/en_US/Help_article/1866/