Some Black and Decker vacuums have a battery that is a part of the handle. While you can simply eject and replace these batteries, other Black & Decker vacuums have internal batteries that require more work to replace. With a Phillips screwdriver, you can replace internal batteries in less than an hour.
Run down the battery
Remove the vacuum from the charger. Turn on the vacuum, and allow it to run until the motor completely stops.
Remove the housing screws
Locate the screws holding the housing unit together. Remove the screws, and pull the housing unit apart.
Locate the internal battery
Although the internal battery is usually in the handle portion of the housing unit, in some models it is behind the motor. If the battery is behind the motor, remove the screw that holds the motor in the housing. Remove the lead wires that connect the battery to the vacuum or motor, and lift the battery from the case.
Insert new batteries, and reassemble the unit
Connect the lead wires of the new battery, and attach the battery to the motor if necessary. Return the motor to the housing, and replace the screws that hold the housing together. Allow the battery to fully charge before using the vacuum.