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/B0...?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.