The 
NUPROL LiPo Battery 1200mAh 7.4v 20c
 is a slim, lightweight lithium polymer battery designed for airsoft electric guns (AEGs) with limited battery space, such as M4 stock tubes and AK upper receivers. It offers a good capacity-to-size ratio and high discharge rates for improved performance. 
 
Key Specifications
According to the manufacturer NUPROL and various retailers, the battery features the following specifications: 
Type: LiPo (Lithium Polymer)
Voltage: 7.4v
Capacity: 1200mAh
Continuous Discharge Rate: 20C (24A)
Burst Discharge Rate: 40C (48A)
Configuration: AK Slim Stick
Dimensions: Approximately 12mm x 17mm x 185mm (Note: a slightly different, wider "stick" version also exists at 13mm x 23mm x 128mm)
Connector: Small Tamiya (also available in Deans/T-connector version)