I have no idea how aftermarket battery grips work. I don't actually have any Nikon MDB-xx's either, but I am pretty sure that they use the batteries sequentially.
The 7.4v should not be a problem, but I don't have one of these so I'm not really so sure. The camera almost certainly has an internal voltage regulator, though.
Generally speaking, third party battery experience has not be very good, with the very notable exception being the batteries for the D1/D1h/D1x, which are by all accounts vastly superior to the Nikon ones. Otherwise, it seems to be at best a 50/50 proposition. My impression from reading Nikonians over 8+ years is that you're a lot more likely to be satisfied with Nikon batteries. I realize that this makes no sense, as battery technology is not an ultra-secret kind of thing - but go back and read the dozens or even hundreds of threads. I would guess that even if you successfully replace this set with another set of third party batteries, you're probably exceeding the price of the Nikon ones in the first place, so probably this is a case of false economy.
I do think that leaving them sitting for several months is inviting trouble, although I'm surprised that they were down so far after just two months. And hopefully you didn't mean that you got a total of 80 frames on the three batteries? If you really mean 80 total, you might consider working without your grip for a while, to see if it is the main issue with battery drain. No Nikon DSLR I've ever used has consumed anywhere near that much current - you should be able to do 80 frames on just a single stock battery even if you're using LiveView and flash. I literally don't think I've ever run out of battery in a day, on any of the Nikon batteries - even the old EN-EL3 in the D100.
If it were me, I'd have gotten one additional EN-EL9a and been done with it. Swap them when one gets low and charge the low one.
Note that I'm hardly a Nikon-only guy. I use lots of third party stuff, but batteries seem to be one area where this just isn't a great idea.
_____ Brian... a bicoastal Nikonian and Team Member
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