Picture of Emergency charge a phone with a spare USB cable.
The other day my phone, which is on its last legs ran out of battery while I was expecting a few important calls and texts that would need replies fast, I was in tech with no phone charger and wouldn't be home for another six hours so I did this... 

*WARNINGS* The course of action in this instructable is potentially dangerous, may damage your phone or its battery, will certainly void the warranty and has the potential to cause fire, damage and personal injury, I cannot and will not be held responsible for any injuries or damages incurred, consider this course of action a last resort and use your own common sense and initiative to try and solve the problem without resorting to such methods of phone charging. 

On a side note these methods can work for any device like an MP3 player or the like. 

Also if you can use the devices own ports, as any device that can be powered and charged via USB cable will have a built in system for controlling the battery charging, negating a lot of the risks of charging a lithium battery without proper safety measures...

Plus you should be able to do this with any phone charger or the like, however USB cables are cheaper and easier to find...
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Step 1: Find a USB cable.

Picture of Find a USB cable.
Find a USB cable.

Now cut the cable and separate the black and red wires inside, strip the ends of the black and red cables. 

Step 2: Determine the pinouts on your device.

Picture of Determine the pinouts on your device.
Googling "circuit diagram for ********** connector" should provide you with results, sites such as and its english counterpart are excellent places to look.

I'm using a sony ericsson in this 'ible - There are two +5v ins, one is charge and one is USB, we need the charge one in this case, along with negative, the image below has them detailed.

This method is relatively safe, since the device will usually have circuitry in place to safely charge the battery.  

Step 3: OK, you can't?

Picture of OK, you can't?
For some reason you can't charge your device,  maybe it has an awkward port like an old nokia or some such. 

Take out the battery.
sqeeek4 years ago
o.O yeah, be VERY careful doing this. Usually the phone will charge it at less than the battery's output charge, less than 3.6v in this case, and with much less amperage... Look up 'overcharge li-polymer battery' on youtube sometimes, these things can turn into a blowtorch pretty fast. My guess is the battery might be damaged, even if it did charge. Still good for emergencies though, if you have to :) Interesting instructable.
I'm pretty sure the phone has the battery charging circuitry inside. its why you can charge it via usb if you need to, think how the ipod or iphone works. :)
killerjackalope (author)  sqeeek4 years ago
I covered the safety issue on lipos at the intro, though they are capable of exploding they're not that dangerous as long s you use some common sense when charging this way and don't get them too hot it'll be fine
ElvenChild4 years ago
yeah im really gonna mod my iphone usb charger to mod my iphone
killerjackalope (author)  ElvenChild4 years ago
It's for when you're missing the cable, but have a random USB cable about, try reading the whole thing properly.
sqeeek4 years ago
Ah, didn't see that. Cool :D Nice instructable.
killerjackalope (author)  sqeeek4 years ago
amando964 years ago
 For your own sake, don't do this, buy an extra charger, lol
killerjackalope (author)  amando964 years ago
 First off, using the contacts on the phone is perfectly safe, as long as you use the right ones, any phone with USB connectivity charges at +5v from the charger too. 

Admittedly there is a danger with the direct charging of a lithium battery with the wrong voltage, however it's stressed plenty in the instructable and doesn't need repeated so many damned times.
 Yeah, the battery directly seems like a bad idea, could it ''pull'' too many amps from the usb port and ruin the usb hub?
well technically speaking, USB specs say that USB ports are designed to withstand a short circuit for an indefinite period of time. Also, USB ports are only supposed to supply a maximum of 500ma and only after the device has requested the extra power. But most ports and devices don't bother asking before they attempt to draw 500ma. I say most because RIM Blackberries are the only phone, that i know of, that will wait for confirmation from the USB host before drawing the 500ma and charging the battery.
killerjackalope (author)  amando964 years ago
 Well that was a completely depleted battery and it didn't pull enough to set off the power surge warning, all USB hubs are limited safely by the computer's OS so it can't exceed 500ma. The computer automatically disables power to the port until you remove the device and then choose to reactivate the port.
wow quick replies
Rainh2o4 years ago
I've directly pulsed charged LiPo, Nicads and NMH batteries from the car battery before, you just have to understand the danger and be careful.  Just used a couple of wires from the cig lighter and pulsed it to the terminals of the battery.  A few mins of this usually gives you plenty of time to talk in an emergency.
killerjackalope (author)  Rainh2o4 years ago
Thanks for the anecdotal backup... Everyone's rather excitable about exploding LiPo batteries...  
caledonian4 years ago
Alternative option. Some hotels have like a box full of chargers that have been left behind accidentally by guests. If you pass by one, they may be accommodating if they have one that fits.
killerjackalope (author)  caledonian4 years ago
That's a great tip for when you're in a hotel... Hmm, I'll remember that... 
Sorry but this is really dangerous. if you don't have the right circuits to limit the amperage going to the battery it could explode, and trust me lithium polymer batteries are not fun when they do.
killerjackalope (author)  bobmcbob_3454 years ago
I think I explained that there's a serious risk of that happening, no need to beat a dead horse... 
If you connect a 5V supply to your Lithium Polymer battery directly, this likely happen : 

Be careful.