After reading about the higher voltage of the rechargeable NiZn batteries I decided to give tham a go in my K-7 battery grip. Their typical charged up voltage is in the order of 1.83V compared to NiMh's 1.3V(from memory) and Lithium AA's 1.63V.
After charging up the batteries, I checked the voltage and they were indeed 1.83V as advertised and so I tried them in my K-7 Battery grip and altered the battery selection type to AA Lithium (for higher voltage).
Unfortunately, when I pressed the shutter, the camera seemed to do either a double exposure/double shutter release or some other strange thing and the exposure was way overexposed. I also noted that the AF was off in that it misfocused. I even tried the "Auto Battery" select but it made no difference.
I have been advised that the voltage of the NiZn's will quickly drop back to the level of the AA Lithiums of about 1.63V after some use, but I am not happy about using them even so.
Interestingly, over at the "other forum", there have been a few who have used the NiZn's in their K-x's and have had no issue.
Anyway, just a post to warn people about using the NiZn's in their K-7 battery grip as it may not be the right battery type to use. Everything is working fine on my K-7 now after replacing the NiZn's with the propietry DLi90 bateery.