Cheap DIY iPhone External Battery

The beauty of this design is that you can take a box of AA batteries with you on a trip and not have to worry about your phone dying and you can buy AA batteries just about anywhere.

Use your existing docking cable to connect the iPhone to the battery. I prefer this method to the one where the battery attaches to the phone directly. The battery can be in your pocket or purse while you use your phone normally with just the cable connected to it.

Caveat: I did this and tested it on my first generation iPhone!!! Should work on new 3G iPhone but I haven’t even seen one yet.

Radio shack sells a 4 AA battery holder with on/off switch for $1.99.
Radio Shack 4 AA battery holder

Radio Shack 4 AA battery holder

I prefer rechargeable batteries but any AA will do.

Type A female USB connector

Type A female USB connector

You will need a female Type A USB connector. I had one laying around that came with the last motherboard I bought.
USB A Male to A Female Extension Cable

USB A Male to A Female Extension Cable

You could also buy  a cable from or and cut off the male end. (OUCH)

Computer end of USB cable

Computer end of USB cable

I cut off the end that was intended for the motherboard.
Type A female USB connector

Type A female USB connector

Now you would think that all I needed to do was connect the Red (VCC) and Black (GND) to the  battery and it would be done. I tried that and it didn’t work.

To make a long story short, the iPhone wants to see some voltage on the data pins or it will not start charging. I created a voltage divider with a couple of huge resistors (100K) and tied both data pins to the junction which yields half of the battery voltage which seems to satisfy the iPhone. The current drawn by those two 100K resistor is 30 microamps at 6V.

Wiring Diagram

Wiring Diagram - pin colors = wire colors

Resistor divider under shrink tubing

Resistor divider under shrink tubing

I forgot to take a picture of the resistors before I put the shrink tubing on but believe me, the resistors are in there.
You can tie the shield to the GND wire but I didn’t bother since there is no data being transmitted over this cable.
External iPhone battery for less than $10.

External iPhone battery for less than $10.

Here is the finished product. I pasted the diagram on the box because 2 weeks from now I won’t remember what the hell I did to make it work!

I suggest you turn off the off the switch on the battery holder when not in use to prevent the resistor divider from draining the the batteries. (Do not despair if you don’t have an on/off switch, it will take a long time to drain the batteries with a 30 microamp load.)

WARNING: There are no guarantees and I will not be held responsible for you damaging your phone.  I only tried this with my phone using 4 NiMH batteries that were putting out about 5.2 volts.

I would love to hear from you if you found this article useful or if I made a glaring misteak!?!

I just ordered a car charger for the iPhone for a nickel plus $2.98 shipping. It has the iPhone specific connector that I can use instead of the female USB connector. I will update this blog when I have tried it.

iPhone car charger

I just received the above charger and although it works it is not what I was looking for.

Charger circuitry

The board contains 4 capacitors, 5 resistors, 1 diode, 1LED and the regulator IC.
Two wires connected to Firewire power pins.

Two wires connected to Firewire power pins.

Unfortunately the cable only has two wires in it and the are attached to pins 19 & 20 (Red, Firewire power +12 VDC) and pins 29 & 30 (Black, Firewire ground).
This charger supplies approximately 9 VDC to the Firewire power pins and it seems to do the job.
I also ordered the following cable for 33 cents plus $2.98 shipping.
Retractable USB to iPhone cable

Retractable USB to iPhone cable

This one should do the trick. When I receive it I will cut of the USB end and wire it directly to the battery pack.


21 comments so far

  1. Nano on

    Built one this morning, works on Iphone V1 and Ipod touch.

    Flying this weekend and I was worried about being without a functional media player after a few hours of video and mp3.

    Thanks a lot!


  2. Lars on

    I just verified that it DOES work with the new 3g iPhone, however new batteries will give over 6v and the phone will not recognize the charger. If you take one battery out and short the socket, you’ll gt closer to 5v (on brand new batteries). Basicaly the 3g is picky about the input voltage (or the voltage at the data terminals) and won’t charge unless it sees 5v across 1 and 4 and half that on 2 &3. To summarize, add a voltage regulator to this project and you should be 3g ready!

  3. V42 on

    Ahhh, I think you forgot to square the voltage.

    Power (watts) is equal to V^2/R. Resistance in series is additive (so you got that right). That makes it a 180 micro amp draw (6^2/200000), but then who’s counting!

  4. V42 on

    Ahhh, sorry I misread you’re original post and confused watts and amps (as well as mistyped amps for watts in my post) 😦

    You are indeed correct… a 30 micro amp draw. Which in turn, consumes 0.00018 watts of parasitic power too 🙂

    So with 2500 milliamp hour batteries in series, for example, it would take 9.5 years for it to drain unused due to simple shelf storage. Since the rechargeables self-discharge at a much faster rate, a pretty practical arrangement I’d say.

  5. Dennis on

    Actually, I found that you don’t even need the resistors. Just tie the two leads together (D+ and D-) on the iPhone side. Basically the iPhone will talk to itself. Don’t tie them to the voltage leads at all.

    This minor improvement was brought about by necessity – I found myself without power and without a car charger after Hurricane Ike, but with a USB cigarette lighter adapter. The regular USB to dock cables wouldn’t work. I remembered seeing this before and thought that that might be the key, but I didn’t have any resistors handy (nor would I have had a way to solder them anyway).

  6. Andy on

    How many charges does a fresh set of nimh’s provide you on an iPhone with a 4 aa setup?

  7. Doc on

    Ahh 100KOhm resistors eh? Well, now my little adaptor I made is working. Thanks!

  8. Bradley on

    For the first topic about a pocket battery do I have to solder anything

  9. Bradley on

    Oh yea and If I use alkiline batteries what will happen

    • fastdad on

      Yes, you need to solder the resistors.

      If the total voltage exceeds 5.9 volts, the iPhone will not recognize the device as a legitimate charging source. Since alkalines are at least 1.5 volts when fresh, 4 of them equals 6 volts. Rechargeables generally on ly reach 1.4 volts when fully charged.

  10. michael goins on

    i bought a griffin external battery for my iphone. but it will not fit with my otter box case on so i bought a dock extender but it still wouldnt work the battery has 10 pins and the dock extender oinly has 4 how would i go about making thois work?

  11. dave henry on

    This is a great gadget. Thanks for outlining the design.
    I have a related question though — that someone may be able to throw some light on. I bought some $5 Apple iPhone chargers on eBay, and they charge, but make the touch screen twitchy. It will do things on its own, and not open the same app that you press the icon for. Is this because the data voltages are wrong? And how do you test these voltages? D+ and D- seem to be .5 volts on the USB. On the 30 pin, how do you know which ground to use when testing different voltages? Any better way than using needles for probes?
    Love the 4AA backup charger idea!

  12. Russell on

    Does the 4AA battery pack or modified cigarette lighter adapter work with iPad?

  13. Sky1er on

    would 3.5 volts work? I have a 3.5 volt lithium battery from an old charging case that broke.

  14. iphone hdmi cable to Tv on

    I relish, cause I discovered exactly what I used to be having a look for.
    You have ended my 4 day long hunt! God Bless you man.
    Have a great day. Bye

  15. direct loans on

    Way cool! Some extremely valid points! I appreciate you writing this write-up plus the rest of
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  16. bhatla on

    With 4 alkaline AA batteries ( 6 V) just add two diodes in series with the power leads from the batteries, will drop the voltage about 0.6 volt each, thats 1.2 volts.The iphone now sees 4.8 volts, should work fine.

  17. justin Russell on

    How long does it last?

    • fastdad on

      Depends on your batteries. The purpose of this design is to be able to take a box of AA batteries with you on a trip and not have to be near and AC outlet. You can purchase AA batteries almost anywhere in the world.

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