Sena will tell you that once the battery goes dead, you're out of luck unless the unit is still under warranty, which typically it's not.
I just replaced the battery in each of my Sena 20S with this https://www.amazon.com/gp/product/B00YZM8D5I/ref=ppx_yo_dt_b_asin_title_o05_s00?ie=UTF8&psc=1
following the instructions in this video; https://youtu.be/vzzTNr96BWA
The hardest part is finding a T-8 bit to remove and replace the 5 tiny screws.
I just replaced the battery in each of my Sena 20S with this https://www.amazon.com/gp/product/B00YZM8D5I/ref=ppx_yo_dt_b_asin_title_o05_s00?ie=UTF8&psc=1
following the instructions in this video; https://youtu.be/vzzTNr96BWA
The hardest part is finding a T-8 bit to remove and replace the 5 tiny screws.