I love my wireless Sennheiser XSW pedalboard system but have noticed lately that I needed to recharge the transmitters more and more often, which is a drag even when you have spares. I feared I'd have to buy new transmitters but searched the web and found this very instructive post on a photographers' forum.

https://www.eoshd.com/comments/topic/30340-sennheiser-xsw-d-batteries/

Of course I tried to put a new battery in one of my unit and IT WORKS! You just have to be cautious when soldering. The factory battery is a 850 mAh but I got myself some 1500 mAh batteries just to check if I can play longer before having to recharge.

Stay tuned. I'll give you the result of this experiment in a few days.