Synergy Digital Headset Battery Compatible For Plantronics W440 Wireless Headset Battery Li-Pol, 3.7 Volt, 140 mAh - Ultra Hi-Capacity - Works With Plantronics 82742-01 Rechargeable Battery Telephone Accessories - Newegg.com
Replacement 84598-01 82742-01 Battery for Plantronics Savi W740, W740/M & W745 | eBay