A friend of mine uses an external battery to recharge his iPhone. It is called "Just Mobile Gum PRO Mobile Power Pack 440", has an USB output 5V / 1000 mA and a capacity of 4.400 mAh. I think that combined with a USB charging cable (Nokia CA-100 or compatible from EBay) this can be an interesting alternative to using multiple batteries for the NIT, as it can also be used for other devices. My friend has told me that this Power Pack has a weight of 150 gr, which would be not much for carrying it around.