The battery in the Thru-axle is rechargeable. A fully charged battery will give you 3 to 6 months of use between charges, an equivalent of at least 10.000 shifts. The actual battery performance can be impacted by several external factors such as temperature.
For dropbars (using the handlebar unit): The transmitter module in the handlebar has a coin cell (CR1632) and will last at least one full year before needing replacement. Replacing this battery is possible without having to remove your handlebar tape.
For flatbars (using the ring shifter): You will not have to replace batteries, the battery is rechargeable with a proprietary cable included in the Powershift package. The ring shifter battery lasts bewteen 4 and 12 months, an equivalent of at least 20.000 shifts.