iPhone/ iPad External Battery

Given the current issues with iphone battery life (even the 4s it seems), anyone have experience with external batteries for their iPhone?  I'd prefer something that could also at least run my iPad if not charge.  Not interested in a charging case since it would only be for occasional use.  A couple extra hours for a long day will come in handy.

I almost picked up a Minty Boost a couple times and there seem to be a few really cheap Chinese iphone batteries on Amazon.  I don't want anything could damage phone or charges only when you hold it a certain way every other Tuesday.