Amazon.com: Replacement for Sennheiser BA300 Battery - Compatible with Sennheiser RS 4200 RR 4200 Set 830-TV RI 830 Set 830 S RI 830 S is 410 RI 410 Headphone Battery : Electronics
Battery For Ba300 Hc-ba300 Ba-300 Lithium Ion Battery 3.7v 150mah Headset Phone Battery - Buy Battery,Battery For Ba300,Battery Pack For Ba300 Product on Alibaba.com
AliExpress
BA 300 Rechargeable Battery for Sennheiser – Battery World
Amazon.com: Sennheiser BA300 Lithium Polymer Rechargeable Battery For Sennheiser Set 900, Set 840, Ri 900 : Electronics