External Battery for Iphone 3g?

[dontshoot80]

Hey guys, My wife and I do a lot of traveling and we use our iphones as a major source of in air entertainment. After a recent trip i was thinking how great it would be to have a sweet external battery to bring along and allow us to get through a 12 hour flight...... That would require recharging about 3 - 4 times each..... Anyone have any ideas on how to turn a stack of 18650's into an external battery.... Ideas on max battery voltage for iphone / the connector etc....

any help would greatly appreciated,

 

[gorn]

Input voltage for the iPhone is 5v.