I have a Canon 6d that gets about 4 hours of battery life on a single batery when shooting time lapse. I purchased a battery grip that holds two batteries and the grip is toggled into the on position. However, I noticed that after about 4 hours of operation, the camera was displaying the battery as almost 90% depleted. Is this just showing the capacity of one battery or is this the total capacity left for the battery grip. I was under the impression that it drew from both batteries in parallel at half the rate of when it pulls from one battery, in which case the display should be for the total capacity of both batteries.
As per my instruction manual: "Be sure to attach the battery grip to the camera before inserting the batteries. The results of the battery check may not be displayed properly if you attach the battery grip to the camera with the batteries already inside."
Also the battery grip power button is specifically for the additional camera controls on the battery grip. I keep mine off unless I'm shooting vertically but I don't think it will make any noticeable difference to the battery life.
Note: I'm using a 3rd party battery grip on my 6D