I recently purchased a used D90 and MB-D80 grip. I charged both batteries (1 nikon/1promaster) before use. I assumed that once one battery was dead, the next battery would kick in. Battery 1 died today. The camera acted like I had no battery power when in fact that Nikon battery still had about full power. In order to use my camera, I had to remove the promaster battery. Once the camera was on, I checked the battery info in menu. I had already taken about 45 pictures on the nikon battery. So why all of a sudden act like the batteries are dead?? Is this how this grip works? Kinda sucks if it is. Seems like that just makes it as effective as carrying an extra battery in my bag to put in when one dies.