The 
Nuprol Power 7.4v 1550mAh 20C LiPo Battery
 is a stick-configuration airsoft battery that provides good power delivery and is designed to fit in tight spaces like buffer tubes and handguards. It is available from several online retailers with both Mini Tamiya and Deans connector options.
 
Key Specifications
Attribute 
Detail
Chemistry
LiPo (Lithium Polymer)
Capacity
1550mAh
Voltage
7.4v (2S)
Constant Discharge
20C @ 31A
Burst Discharge
40C @ 62A
Configuration
Stick
Dimensions
16 x 18 x 95 mm
Weight
57g
Connectors
Small Tamiya or Deans (JST-XH balance connector)
 
Reviews and Usage Information
Performance: Reviewers on Amazon UK and other airsoft retailers state the battery provides a good trigger response and reliable performance. Customers have reported it lasting a full day of airsoft gameplay with no significant performance reduction.
Fitment: The slim "stick" configuration is specifically designed to fit in tight airsoft gun compartments such as M4/AR-15 buffer tubes and certain handguards, where larger batteries may not fit.
Safety Precautions: LiPo batteries require specific care. According to Nuprol, they must be charged with a LiPo balance charger and stored/charged in a fire-retardant safety bag. The battery should not be run completely flat, as this can cause damage.
Quality: The batteries are produced at an ISO 9000 Quality Control accredited manufacturing plant and use high-quality silicone leads.
Potential Issues: One user on Amazon UK reported a battery arriving damaged and nearly exploding when charged, while another noted a battery swelling. Such product failures can occur, but overall customer feedback indicates good quality and performance from the brand.