How to Make a External Cell Phone Battery for Under $10

The intro will just explain one thing: why I decided to make this.
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 internet.

Step 1: Parts.

To build this pack, you will need:
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-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.
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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.

Step 2: USB Cable and Wiring.

The first thing is to make the adapter for your device. If it's a iPhone 5 or an Optimus G, the process shown here applies. So, take the cord for your device and cut it about half way. You can always cut it shorter later on, so it's a good starting point.

Once you have cut your wire, strip each smaller wire inside of it. Proceed to next step to find which are the power cables.

Step 3: I HAVE THE POWER!!!

Now, with the USB male half of the cable plugged into a USB female power source, test two of the wires at a time with your multimeter to find the Negative and Positive wires.
I actually recommend doing this from a USB outlet adapter, because if you end up shorting this cable...you don't want to have to buy a new laptop.

Generally, Red is always positive (+) and black is always negative (-), but it's a good idea to test and not just assume.

Step 4: The Other Part of This Trinity.

I couldn't get a good photo of this part, sorry.
If you look close at the diode you (hopefully) have, you will see that there is a silver line on one side.

That is very important, because the negative (-) wire from the the battery compartment gets taped to the side of the diode nearest to the silver line. The other side of the diode is then taped (or however you're connecting it) to the negative wire of the phone cable.

The positive wire (+) (from the battery compartment) then gets directly connected to positive wire (+) of the phone cable.

Step 5: Final Notes.

Some final thoughts on this:
This project is about on par with a cheap charger, and you know what they say, you get what you pay for.

Although a $50 battery would give you almost bullet-proof safety with your phone, this is still safe enough for real use; plus, you just made this yourself.

Also, if you didn't, make sure to read the notes embedded in the photos, they contain important bits of information. Not deadly important, but still on the must read.

And finally, all constructive comments and criticism are welcomed.