This is exactly what I came here to say. Just bought one of these for my sister today, it's down to $50, plus another $14 off with the promo code AASTROE5. 15,000mAh is enough to charge just about anything short of a tablet several times over.

http://www.amazon.com/15000mAh-Portable-Ultra-High-External-Motorola/dp/B00D5T3QK4

http://www.amazon.com/gp/aw/d/B00D5T3QK4/ref=mp_s_a_1_1?qid=1397333660&sr=8-1&pi=SY200_QL40 correction I meant 15,000 mah

I used a coupon code last week to bring down the 15000mAh version down to $35. Not sure if it's still active but I can look up the code if anyone wants to try.

Edit: AASTROE5 http://www.amazon.com/gp/product/B00D5T3QK4/ref=oh_details_o00_s00_i00?ie=UTF8&psc=

Edit2: I doubt it's still active. Just noticed the regular price is now $40. It was $47 or so when I used the coupon.

you can use your car's battery to charge stuff. note that getting into the fest is basically a 2 hour traffic jam for 2 miles, so filling up your car's gas tank before entering Double JJ is a good idea so you don't drain your car's battery/run out of gas before the fest is over.

You can also use something like this