Amazon has the GoPro Battery BacPac, model no. ABPAK-304, for $36.99 (49.99 – 13 instant savings) with Free Shipping for a limited time. B&H Photo also has it for the same price with Free Shipping for a limited time.

The GoPro Battery BacPac easily and seamlessly attaches to the back of your GoPro, increasing battery life. The LCD window displays battery level or charging status. The LED light displays recording status so you don’t have to struggle to see it. Totally ideal for long-duration Time Lapse photo sessions, endurance motor sports racing or any application where you need extended battery life. Charge it with the GoPro Auto or Wall Charger, or other USB charger OR your very own computer. The GoPro Battery BacPac is compatible with the HERO3+, HERO3, and HD HERO 2.