You obviously didn't pay much attention to the movie when Heather Donahue states "we have enough battery power to power a small country for a week....". She states this in the car on the journey at the beginning of the movie.
The directors obviously already thought of this and made sure that it was covered in the script.
err...I hope u know that u can turn cameras OFF while they are not in use, the battery is only used up when the camera is in use, which from the movie, u can say was ON for around 3 hours, which isnt much.
(The marginal power disipation of rechargable batteries is small enough to be ignored in this case)
When my daughter went to camp this summer, I bought her two packs of double AA's for her CD player. They were the extra long life kind. She ended up only using four the WHOLE summer yet she used the CD player non-stop each and every day for 4 months. Also, I'd like to add, how do we know that they didn't have extra batteries with them anyway.