18650 lithium ion battery pack 11.1V 3200mAh is rechargeable and safe
G&G 11.1v Li-Po 800mAh Stick Battery – Airsoft Atlanta
Amazon.com: Evike Airsoft - Matrix High Performance 11.1V Stick Type Airsoft LiPo Battery (Model: 1000mAh - 20C / Small Tam.) : Sports & Outdoors
Amazon.com: Crazepony Airsoft Battery 11.1V Rechargeable LiPo 1400mAh 30C Hobby Battery with T Plug to Mini Tamiya Cable for Airsoft Model : Sports & Outdoors
11.1 Volt Lithium Ion Battery Pack (3500 mAh) with Protection IC