The Deggy Steel Pen uses a 3V lithium battery, size 123.
The battery is expected to last 2 years, depending on use.
How to change the battery
Main Battery - Steel Pen
Part Number: DGA232006Manufacturer/Model: Panasonic CR123 (or equivalent)Technology: LithiumVoltage: 3 V
Lenght: 1.36" (34.5 mm)Diameter: 0.67" (17 mm)