I purchased an Imp500 battery charger from Imaxpower.com after trying to decide on a second battery for my Droid. They also sell on Amazon. It holds a 5000 Mah charge. I was able to get 5 full charges off the pack from a 40-50% battery remaining on the droid. I think I made the right decision, rather than getting a second battery. I don't need to shut down to charge and the form factor of the external battery is small enough to fit in my coat pocket. They are marketed for the iphone, but work great on both my Droid and my SO' s Eris. It came stock with a mini usb adapter and, of course, an iphone adapter. I had to request the micro usb adapter for Droid compatability. I've even used it to charge my e cig battery. Cost was a bit over fifty bucks. I have jury duty coming up in February and expect to need it in the waiting room. It gets boring sitting there waiting to see if you get called.