Teardown
Notes
The main chip on the PCB is a TI BQ2425:
2A Single-Input I2C, Stand-Alone Switched-Mode Li-Ion Battery Charger With Power-Path Management
This is the same chip used in the Xbox Adaptive Controller, Xbox Elite Series 2 Controller, and the Xbox One Rechargeable Battery Pack.
RDC posted pictures and schematics of the XB1 Rechargeable Battery Pack on this Acidmods thread. His battery pack (Model 1556) is very similar my Xbox Rechargeable Battery (Model 1727), though not identical.