If anyone is still looking for help I decided to do some digging as well. I didn't want to rig up a bank of 18650 cells and resisters etc lol. So I found a battery that was similar in dimensions and hoped would fit. I got the battery and it does barely fit. It is only 4400mah (8800mah @ 3.7V) which is almost the clamed OEM battery in the Xtreme.
I wired the colors up the same and left one red off from the Xtreme. So far so good. I'm getting roughly 10-12 hours of playtime and its charging fully and shutting off automatically. My Xtreme used to charge forever and never stop and it also would only last an hour off of a charge. Anyways, I love JBL but when I contacted them they basically told me there's nothing they could do as I'm from Canada which is absurd.
Anyways, here's a link of the battery I bought and it works well.
[https://www.amazon.ca/gp/product/B00FYWM7PA/ref=oh_aui_detailpage_o04_s00?ie=UTF8&psc=1]
Hope this helps for anyone still needing it.