Qn:

Im using a canon 40D.

Are we able to chose the battery source at any point of time if I connect a battery grip?
1. from battery grip
2. from camera
3. from battery grip and camera.

Last but not least, is there a way to charge all the 3 batteries at 1 go? Or I must charge the batteries individually?

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The Phottix Battery Grip (Premium Series) allow you to insert 2 batteries at one time. It is modelled very similar to the original grip. There's no charging option at all for the grip (Similar to original grip).

The power source is directly from the grip too. To see a installation of a grip in action, do consider Youtube videos such as http://www.youtube.com/watch?v=p_306uUz2tE.