The Dell Active Pen PN557W uses two tiny button cell batteries. They are hard to install and the magnet in the end often flips them around as you go to screw it
Dell Premium Active Pen (PN579X) Review: Intuitive and Versatile
Dell Active Stylus pen for laptops and tablets PN556W (5056, 7275, 9250, 5175 and 5179, 5855 models) - YouTube