Though this recently-discovered info has not been confirmed by Nintendo as part for the final product, FCC documents show the Nintendo Switch’s removable battery won’t be upgradeable.
People familiar with the Wii U game pad already know about the need for a high-capacity battery pack. The game pad, with its own battery, lasts about three hours, while the high-capacity one extends the use of the game pad to 8 hours.
The FCC filing doesn’t mean Nintendo will for sure make the Switch without a removable battery, but the FCC received a prototype similar to what is supposed to be the finished product. There may be some small changes in the finished product still.
Will Nintendo include a removable battery in the Switch?
[Sources]: Endgadget: You won’t be able to upgrade the Nintendo Switch’s battery; FCC: FCC ID BKEHAC001.