The rechargeable battery inside your Sonicare 9900 Prestige toothbrush is not replaceable. If you are discarding your Sonicare 9900 Prestige toothbrush, a qualified professional can remove the battery to recycle it. For safety reasons, we do not advise attempting to remove the battery yourself at home. 

Instructions for qualified professionals on how to safely remove the built-in battery can be found here.