The 
Nuprol Power 9.6V 1600mAh NiMH "double stick" battery
 is a popular rechargeable battery primarily designed for airsoft guns using a Mini-Tamiya connector and a crane stock or other split battery compartments. It is known for its durability and providing a noticeable improvement in an airsoft gun's firing rate and trigger response compared to 8.4V batteries.
 
Key Features and Specifications
Voltage: 9.6V
Capacity: 1600mAh
Chemistry: Nickel-Metal Hydride (NiMH)
Configuration: "Double stick" or "nunchuck" (crane) style, consisting of 8 individual 1.2V cells
Connector: Mini-Tamiya female connector
Dimensions: The individual sticks measure approximately 18 x 118mm + 18 x 118mm. A single "small block" version is also available with dimensions of 18 x 35 x 122mm.
Recommended Use: Primarily for airsoft guns (especially those with crane stocks or front wiring with long, thin compartments like AK variants), but also suitable for some remote control vehicles and drones. 
 
Usage and Maintenance
Charging: Must only be charged using a smart charger designed for NiMH batteries (with an automatic power cut-off function). It is recommended to use a fire-retardant charging bag for safety.
Discharge: NiMH batteries should be fully discharged before recharging to prevent damage to the overall capacity.
Storage: For storage, they should be kept in a charged condition and charged at least once every six months to avoid capacity loss or failure.
Performance: Users generally find they provide a full day of airsoft play, though very trigger-happy users might need a spare.