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
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Stylus Pen Pn557w For Dell Latitude 3198 5175 5179 5285 5289 7275 7285 7389 5290 7390 Xps13-9365 9265 Venue 10 Pro 5055 5056 - Tablet Pen - AliExpress