The 
NUPROL Li-Ion Battery 1500mAh 11.1v 20c (STK-Deans)
 is a high-performance lithium-ion battery designed for airsoft replicas with slim or stick-type battery compartments. It offers a balance of modern cell technology and efficient power delivery via a pre-installed Deans connector. 
 
Technical Specifications
According to nuprol.com, this specific stick (STK) configuration features the following characteristics: 
Chemistry: Lithium Ion (Li-Ion), which provides a high energy-to-weight ratio and a long lifespan of up to 1,000 charge cycles.
Voltage: 11.1v (3-Cell/3S), offering a faster rate of fire and quicker trigger response than lower-voltage batteries.
Capacity: 1500mAh with a total energy of 16.65Wh.
Discharge Rate: 20C constant (30A) and 40C burst (60A).
Physical Specs: Weighs 136g and measures 18 x 20 x 198 mm.
Connectors: Pre-wired with a Deans (T-Connector) and a JST-XH balance connector.