The 
NUPROL LiPo 7.4V 1000mAh 20C AK Stick is a super-slim lithium polymer battery designed for airsoft guns with limited battery space, such as AK upper receivers and some M4 stock tubes. It provides high energy density and a consistent power output for improved airsoft gun performance.
 
the key specifications for the NUPROL Power 7.4v 1000mAh LiPo Super Slim Stick Battery are as follows: 
Type: LiPo - Lithium Polymer
Voltage: 7.4V
Capacity: 1000mAh
Continuous Discharge Rate: 20C (at 20 Amps)
Burst Discharge Rate: 40C (at 40 Amps)
Configuration: AK Slim Stick
Dimensions: 169 x 15 x 10 mm
Connector: Mini Tamiya (also known as Small Tamiya)
Balance Connector: JST-XH
Weight: According to Amazon UK, the battery weighs 120 grams. 
Usage and Safety
Compatibility: Its slim dimensions make it ideal for fitting into tight spaces, including AK battery compartments and M4 stock tubes.
Performance: The high discharge rate helps provide a better trigger response and a consistent rate of fire in airsoft electric guns (AEGs).
Charging and Storage:
Must be charged with a compatible LiPo balance charger.
Always charge and store LiPo batteries in a fire-retardant charging bag.
Do not fully discharge the battery, as this can cause permanent damage.
Stop using the battery immediately if it shows signs of swelling or expansion.