This works for the BG-E2 and BG-E7 (may or may not work for other models/clones).
If you are working with a gripped camera, opening the battery compartment (even if batteries are fully charged) shuts off the camera. There is a method to bypass this 'feature', but you need to know in advance that you are going to need this. This would be helpful for really long shooting sequences (night-time star trails, &c.) where you may very well need to change out the batteries but don't want to lose any of the camera settings, nor stop the long-exposure shot.
Before starting to shoot, open the battery compartment door. In the upper right-hand corner, just above the right-hand battery release, there is a small square hole. Using a small object (I used a straightened paper-clip) push on the left-hand side of the hole. You should hear a click as the switch returns to the left. Now, with the battery compartment door open, you should be able to power-up the camera.
Leave the battery compartment door open throughout your shooting sequence. As batteries become discharged, change out one - then the other - and continue shooting.
When you are finished, closing the battery compartment door will reset everything to normal.
