Mike, you might find the single battery in the 5D runs long enough? I haven't used a 5D so wouldn't know, but I am surprised by my 7D which will get a full night of astrophotography (8pm - 2am) out of one battery. If doing time lapse at 20 sec intervals it'll get 6pm - 3am approx before stopping. So at most I'd change the battery once in the night if I go longer than that.
I used to love battery grips (the feel, stability, verticle grip, just so nice..) but have found with heavier DSLR's like the 7D with a medium weight lens on it, it's just too heavy for me, and too bulky for my preferred camera bag, so do without for now.
Roger.
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