Chord Poly Battery LP113452 2100mAh 3.7 V. The battery is identical to the one in the Chord Poly, from the same producer.

Easy replacement, you only need a 1.5mm Metric hex key.

Remove the SD card. Remove the four screws. Gently lift up on the bottom of the case separating the two pieces. The battery is attached with double sided tape that is weak and is easily removed. You can replace it with a fresh piece of double sided tape if you wish as shown on the picture. Battery connectors are easily removed from the board and replaced. Then you put it back together.