How to make a external cell phone battery for under $10

Picture of 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 fellow evil scientists of the internet.
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Step 1: Parts.

Picture of Parts.
To build this pack, you will need:
-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.

Step 2: USB cable and wiring.

Picture of 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.


Picture of 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 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.
some phones need these wires connected with resisters to indicate to the phone it is pluged into a charger
The Dark Lord (author)  dasimpson19812 years ago
Hmmm...I didn't know that. Point taken, although this setup works for my relatively new phone.
lucky break then
diychaser20 days ago
What about using a usb dc charge booster they can step up 1v to the 5v u need there about a dollar a piece on ebay. You could eliminate the need for a wire. Good project overall, keep on chugging.
hfdjgl2 years ago
you should use a power source closer to 5 volts

4 D cells @ 1.5 volts each in series = 6.0 volts - 0.7 volts from the diode = 5.3 volts. Voltage should be acceptable.

Typical alkaline D cell capacity is around 10,000 mAh. My cell phone battery capacity is 1540 mAh. 10000/1540 = 6.49. Theoretically, I could recharge about 6.5 times, but in real usage it won't recharge that much.

The Dark Lord (author)  hfdjgl2 years ago
I should, it would be ideal; but this was the best I could find in Radioshack. I didn't have the time to buy online either.
djsarnaldo1 year ago

great proj. i'll make something like this soon.

please tell me how much charges usually take to dry the batteries.

They have 50,000 mah external batteries on eBay for $20. I'm going on a school trip to DC in the spring and am going to charge peoples phones for money, I don't expect to make much but hopefully I can make some extra cash.

How did that business venture work out for you, anyway?


Gilik1 year ago
Nice instructable! which app do you use for the notification of battery charging?
The Dark Lord (author)  Gilik1 year ago
Sorry for the late response, but to answer your question;
"Battery Widget Reborn".
ah you need the resistors on the usb data lines most new phones do now
The Dark Lord (author)  dasimpson19812 years ago
This charger was made for the sole purpose of charging the phone. The wires for data transfer inside my USB cable are completely ignored and cut away.
why do you have the diodes
The Dark Lord (author)  dasimpson19812 years ago
My phone won't recognize it's charging without them; most likely because it wasn't without them.
I only had it plugged in for less than 30 seconds, but I didn't want to to risk a $500 phone over a simple experiment (turned instructable).