The BHFEA420J is an integral vacuum and the battery is not replaceable. 

If your unit is malfunctioning, please contact our support team at:

Phone: 1 (800) 544-6986
Website to start chat: https://www.blackanddecker.com/pages/what-is-the-warranty-for-my-black-decker-tool

Our customer service team will validate the warranty and replace the unit.