Funny you asked but I got a GTW680 already and I am about to use with the RavPower
RayPower10400mAh battery that I just received. Do keep in mind that another member in the posts earlier said an external battery may not be built to take the heat in the summer, especially in the car and using an external battery may not be recommended.
However for the short times that I am out, I think an external battery may work for me. I will test it to see how long the battery lasts when the camera on all the time.