high-capacity external backup battery to charge 2 ipads simultaneously

I'll be traveling with my family in April, and we have 2 iPads, 4 iPhones, and 1 iPod Touch going with us. I'm looking for a high-capacity external backup battery that can do several tasks. Most importantly, charge the iPads at the same time. The only one I've found so far is the HyperJuice 15,600mAh Plug. Its a great looking product, but cost $159!! Plus, it only has 2 USB inputs which restricts me to only charging 2 total devices. I can probably get past the latter, if the price was lower! It also needs to be able to charge multiple devices while the actual "charger" is plugged in and charging itself. Naturally, the capacity needs to be large, it should be compact and light weight, the battery should not overheat and become a fire-hazard, etc., etc.