Dell Latitude E5520M (Early 2011) - Manual (Page 7)

Battery Slice
5
Undocking Your Computer From the Battery Slice
1 Disconnect all the cables connected to the computer.
2 Flip the computer on its back.
3 Press the release lever and lift the battery slice from the computer.
Charging Your Battery Slice
You can recharge your battery slice independent of the computer using the
AC adapter that shipped with your computer. A docked computer will also
charge the battery slice.