I am using a D300 with the battery grip MB-D10. For the moment, I don't have the big battery so when I want 8 fps I use 8 AA batteries. The problem is that using Duracell brand new batteries, the D300 does not recognize that I have the battery pack and it does not give me the 8 fps I want. I have selected NiMH in the menu. However, when using the regular battery the D300 and D200 use, the D300 does recognize the battery and calculates the % of use.
first, I'd recommend getting Pearstone 2900mah rechargable batteries from BH, they are amazing and will save you money in the long run. Also, for the duracells, have you selected Alkaline? (im pretty sure that was an option)
I highly recommend Sanyo Eneloop (or the MAHA Powerex Imedion equivalents) over any other higher capacity battery. The much slower self discharge much outweighs the larger capacity for most uses. (I have had my batteries in the grip for two months and still get 8 fps when I pull it out now and then).
Make sure you are selecting the right battery type in the custom functions. I have tried alkalines and had no problem getting 8fps. But, I only got around 250 shots on a brand new set of alkalines. I have to give another vote for Sany Eneloops. I've gotten ~1000 shots on a charge with no problem. They also hold their charge well during periods of non-use. BTW, any AA solution will result in no % battery life meter, just a 2 bar battery symbol.
Actually you need to make sure your battery pack is working okay. I used regular AA's in mine for a few weeks getting ready for a trip to Alaska. But when I was in Alaska, the grip all of a sudden won't recogonize the battery pack of AA's... I cleaned the contacts, then it worked again for a short time, then it wasn't seen again... so atleast with mine there is an issue with the contacts, I haven't had a chance to take it into the service center yet.