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.

Teacher Notes

Teachers! Did you use this instructable in your classroom?
Add a Teacher Note to share how you incorporated it into your lesson.

Step 1: 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.

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.


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.

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 China 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 in America.

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.

Kit Contest

Participated in the
Kit Contest

Battery Powered Contest

Participated in the
Battery Powered Contest

Be the First to Share


    • Made with Math Contest

      Made with Math Contest
    • Multi-Discipline Contest

      Multi-Discipline Contest
    • Robotics Contest

      Robotics Contest

    19 Discussions

    country survivor

    6 years ago

    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.

    3 replies

    6 years ago on Step 5

    Nice instructable! which app do you use for the notification of battery charging?

    2 replies
    The Dark LordGilik

    Reply 6 years ago on Step 5

    Sorry for the late response, but to answer your question;
    "Battery Widget Reborn".

    techboy411The Dark Lord

    Reply 3 years ago

    I use that widget too! I used the free version for a long time until i upgraded to the full version.


    4 years 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.


    Reply 4 years ago on Introduction

    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 Lordhfdjgl

    Reply 6 years ago on Introduction

    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.


    5 years ago

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

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

    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.

    The Dark Lorddasimpson1981

    Reply 6 years ago on Step 5

    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).