I meant to post this in "Canon Accessories" and mistakenly put it here --- please forgive.
When I got my first battery grip some years ago, it came with the usual accessory six AA battery cartridge.
My immediate thoughts were "that would be a big joke" ; and, I had since always disregarded the idea of using AA Rechargeables in a Battery Grip; I never even tried it, not even once.
Then, a few weeks ago, I read in another thread here of someone who swore by the abilities of AA Eneloop rechargeables and considered them even better than a pair of LP-E6N.
I had plenty of Eneloop and even a more plentiful supply of Tenergy (the premium grade Tenergy).
I decided to give it a go in my 7DMkII.
I first tried six Eneloop and I was surprised and impressed; I have no scientific means of true comparison; but, my seat-of-the-pants impression was that the AA vastly outperformed a pair of new quality LP-E6.
Then, with a two day event coming up, and the fact that my 7DMkII goes through batteries like a Poland China hog goes through slop; and, considering that I have gotten in the habit of simply always leaving the GPS receiver ON (so I won't take half-a-days photos before remembering to turn it on), I had topped off all of my batteries; and, wanting them to be as fresh as possible, I loaded the AA cartridge with six Tenergy and put it in the grip the night before.
The next morning, I still seemed to have plenty of battery power, so I left the AA cartridge in the camera to see how it would fare.
When the battery warning finally started flashing, I would have already been through two pair of LP-E6; by then, I was beginning to be a believer.
I have several LP-E6 chargers; I DVM check the voltage of every battery that comes off a charger; and, the most I can show is 8.41 volts, with 8.37 volts being more like the average.
I just now checked a fresh load of Tenergy AA and they are showing 8.61 volts.
When I removed the same Tenergy from the camera to top them off, the low battery warning had not yet began and the six-pack showed 7.26 volts.
Nominal voltage of a rechargeable AA is 1.2 volts; six of these in series is 7.2 volts.
Divide my 8.61 volts by six = 1.435 volts per battery.
My impression is that the AA rechargeables are definitely superior to the LP-E6; and, I can buy a 24-pack of the AA for less money than a pair of quality aftermarket LP-E6.
So far, I only see a couple of drawbacks to the AA cartridge; the battery power indicator in the camera seems to show the AA at 100% right up until seconds before the low battery warning starts flashing; so, you can't trust the battery gauge when using the AAs; also, loading the AA cartridge is very fiddly; very easy to drop a battery (or all six of them); and, the AA cartridge is somewhat flimsy and could easily be broken.
The best scenario would be to have several AA cartridges already loaded with batteries and just swap out a spent cartridge for a fresh one instead of fiddling around trying to load the individual batteries.
