I have a 40D and the 580EX II external flash. I'm considering getting the BG-E2N battery grip for it. I find it really inconvenient to have to carry AA batteries for the flash (given that I have to carry the extra charger for them too). Does anyone know whether the grip can actually provide power to the flash? I'd rather carry extra batteries for the 40D insteaf of AAs. At least I will only need one type of charger.