Maybe the charge circuit in the Sena is fucked?

If you wanna go full rat but have amazing batt life but lose any weather proofing:

Buy a lithium battery charger for 18650s. Xtar VC4 is good for cheap:

https://www.amazon.com/XTAR-Battery-Charger-Display-Multi-Functional/dp/B07HDRZYPS

Buy some 18650s, at least one: https://www.amazon.com/SOOCOOL-Authentic-Samsung25R-Rechargeable-Battery/dp/B09CQFCKLZ or similar

Glue one of these to your helmet and clip the leads for your broken battery, and connect them to the leads from this thing:

https://www.amazon.com/LBTODH-Battery-Holder-Storage-Plastic/dp/B092TGQ7PB

Boom, external rechargable Sena that will prob last 2-4 days of talk time per charge..

The real answer is just buy a new Sena, or even better, a Cardo, but that would be a fun project.