This is the 8.4V NiMH Power Series 3 Battery Pack from Traxxas.

FEATURES: Stick configuration
          High current Traxxas female connector

INCLUDES: One 8.4V 7-cell NiMH Power Series 3 Stick Battery Pack

REQUIRES: A peak-detecting charger rated for NiMH batteries

SPECS:    Length: 160mm (6.3")
          Width: 45mm (1.8")
          Height: 23mm (0.91")
          Charge Rate: 3A
          No mAh rate stated