When I go on long trips, I like to be prepared. I also like to listen to music (when awake on that trip.).
Recently, I got a Android smartphone, and like all smartphones; the battery won't last more than eight hours.
This summer (2013) I am going on a 14 hour bus ride. So there's my problem: Too little power, no job, and real external battery packs cost upwards of $50.
So, I made my own; with a bit of help from the fellow evil scientists of the internet.
Step 1: Parts.
-A brain, some logic, and common sense.
- A "size D" four-battery case.
-A pack of Diodes.
- A USB cable to your device.
- A multimeter.
- Extra wiring (just in case).
- Electrical tape.
-And of course, size D batteries.
All of these things (besides the first) can be found at a hobby store (such as Radioshack).
If you feel a little uncomfortable jumping into this, check out this Instructable on some basics.