So I decided to purchase the iWalk 5400 instead of a second Droid battery. The idea here is that you literally charge as you need it. It's capacity is 5400mah and comes with a few universal adapters. Unfortunately one of them isn't a Micro-USB that the Droid uses. However you can use the Micro-USB cable that came with your Droid and plug it right into the standard USB port of the iWalk. In addition to a large capacity battery another advantage is you don't need to turn off your phone just as you do with replacing a secondary battery. I'm thoroughly enjoying this accessory as I can use it for my Droid or my wife's phone. Here is the specs and a of the iWalk that someone made for their phone.