Look this video, it’s really not difficult. Music 100% OP1
It’s not really a technical video, but it will give you a good idea of what to do.
https://www.youtube.com/watch?v=nlat3XRY8x4
I can only assume the large black thing is the battery. Sadly that dashes my desire to add a higher capacity battery just because I can because it seems to fill its area.
Thanks for posting.