I am looking out for a battery pack which i can use to conveniently charge my smartphone, my wife's iPhone 4, and my HP touchpad tablet. Apart from this it should be able to charge MP3 players and basic cell phones that support USB based charging. I am not interested in the dedicated iPhone cases but need something generic which can be used with multiple devices, phone and tablet alike. My budget is 50$ and am looking for the best bang for the buck product. Suggestions and recommendations appreciated.