I've tried external battery chargers that take 2 AA batteries, but with the latest batch of electronics they just don't seem to have enough current to charge these devices.  I started looking for replacement chargers and came upon this particular model.  It seemed to have a small enough profile to allow it to travel almost anywhere I go without adding additional weight but still packing enough of a punch to recharge my phone or other electronic devices that are on the verge of dying.  This device has saved me a few times while I have been out on the trail where my phone is constantly searching for a cell signal which in turn drains my battery fast.