I can't find anything that tells me how to make the battery in the grip take over from, or support the one in the camera. Yesterday I went out with the camera showing 45% battery, that's with the grip attached. Only about 50/60 shots in it flashed up empty and died. So I removed the grip and the battery in the camera then read 25%. I switched it with the one that had been in the grip and that said 95%. Anyone know what's going on?