This is the E-Flite Blade 7.4 Volt 800mAh 2-Cell Li-Po battery pack with the JST and balance connectors for the E-Flite Blade CX and Blade CX2 helicopters.  It is a good idea to have a spare battery so that you can have less downtime between flights, or  you can just have them both charged up so you are ready to go for the quick flight on your lunch break, or whenever else you feel the urge!