Originally posted by Apapukas IMO, just get the grip + AA's and forget about battery life.
For a price of one 3rd party Li-ion battery you can get 6 decent AA's that will last at least two time more. Also, having spare AA's are always handy, because you can use them in your flashes or just everyday devices, if needed
Also, the new AA's are much better in cold weather than they used to be, so again, one more reason to use them instead of Li-ions.
My Li-ion (that came with camera) is in a battery tray, deep in camera bag, waiting for emergency
Hah, and you can strap some wrist weights on while you're taking pictures, too! You must have strong forearms.
I carry the insert with AAs in my camera bag for emergencies, but that's more weight than I want to carry inside the camera if I can avoid it (plus you lose the weather-safe spare SD card storage).
I picked up 2 cheap 3rd party batteries on Amazon, plus the original with the camera. With 1 in the body and 1 in the grip, I've never run out (though I came close a few nights ago because I had forgotten to charge them after the last two times I went out taking photos), and if I did, I still have the third before I have to use the AA grip insert.
Of course, this is for my K5, and maybe the K3 doesn't handle third party batteries well?