There are plenty of high capacity battery chargers available to buy and they are certainly a good idea. You don't want to be caught with a dead phone during a power outage. However rather than buy one of these devices why not take advantage of the high capacity batteries that most of us have in our garages already? The battery packs for my Ryobi brand cordless tools are 18V devices. With a few inexpensive parts we can easily turn that into the 5V needed by your cell phone.
1/4 inch plywood
1 UBEC (universal battery eliminator circuit) DC/DC step-down converter - 5V@3A outut (available from AdaFruit Industries http://www.adafruit.com/products/1385)
1 USB shell connector, type A socket
1 22k Ohm resistor
1 20k Ohm resistor
1 6.2k Ohm resistor
2 picture hanger hooks
Step 1: Build the Enclosure
Carefully measure your battery and cut the plywood to the correct dimensions to fit over the battery connector clip. Using a razor blade, cut out slots for the battery's spring clips to fit into. Assemble your enclosure using some wood glue.
Step 2: Make Battery Contacts
I had some brass hooks for hanging pictures in my toolbox already. They can be bent and trimmed down to make good battery contacts for our enclosure. We want to make the contacts have some spring so they make good contact and allow for a snug fit onto the battery.
Solder the UBEC onto your contacts paying careful attention to the polarity. Make sure the negative side of the UBEC (black wire) goes to the negative terminal of the battery pack and that the positive side (red wire) goes to the positive terminal.
Cut and glue some pieces of plywood to secure the clips in the proper location.
Step 3: Build Voltage Divider
While the UBEC gives us the 5V needed to charge our USB devices, many devices want to see around 3V on the USB data lines in order to charge. To go from 5V down to 3V we need to make a simple voltage divider circuit.
Note that I made two separate voltage dividers (one for each data line) but this shouldn't be necessary.
Cut a hole in your enclosure for the USB connector and then solder the voltage divider to the connector and the UBEC.
Step 4: Finishing Touches
Add some holes for cooling (or just for looks). Throw some finish on the outside of the enclosure to help keep it clean from grubby fingers. Plug in your phone or other USB device and happy charging!