There is no soldering needed. I designed it to work with a triple A battery in lieu of having to solder a bit and using a different one. You will need to cut a few things. Specifics can be found in the google doc which details all necessary steps for v1. The update is largely similar. All cuts and pictures can be found there.

https://docs.google.com/document/d/1P0pfgshO0wcZc8t_GU1f0yHcCa1ZkxvJk6GtM2CcyB4
https://imgur.com/a/sKQRMLg

VIDEO GOING OVER FILES AND UPDATE:
https://youtu.be/oc_TLun5hRk

I don't own an Ultralight 2 anymore and as such do not know if the rechargeable battery setup has room to sit at the necessary angle due to how cramped the UL2 is. The normal AAA setup still works of course and I've still provided the updates necessary to fit the rechargeable option but it may not fit.

Follow-up video on charging (added after original mod):
https://youtu.be/vPNx4j_xXlk