The IMP 500 by IMAX Power is the best protable battery you can buy.  I got one and it malfunctioned and the company quickly shipped me a replacement.  The customer support and service is top notch. I would not hesitate to buy anything from them.  The battery charges my Motorola Droid in about 1.5 hours.  Unbelievable. The IMP 500 still has hours of charges left after charging my Droid and iPod Touch.  It is so easy to use and looks great too.  Compact and easy to carry. The cable and the connectors are excellent too. If you need a portable battery, this is the one to buy.