I have not personally used the rechargeable but several posters elsewhere have reported very reduced life, down to six rolls, with them. Very brand variable it seems and depends on the lenses used. If you shoot big heavy zooms then the autofocus demands are very draining, same with standard CR123A, adding VR lenses also adds to the power drain, so reports of long life may be from manual focus users. Reading about CR123A generally not F6 specific there are many reports of poor quality and low life with voltage differences it seems to be one type not easily matched to the originals for some reason.
The MB-40 grip with the EN battery lasts seemingly forever, and gives 8fps if that is of interest, some like the bulk and stabilising weight others abhor it.
You should check your serial, before around #18754 ISTR requires updating if not done a firmware update fixed a battery use issue and minor data writing bugs.