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...
Step 1: 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.
Googling "circuit diagram for ********** connector" should provide you with results, sites such as pinouts.ru 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?
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.
Step 4: Use the Battery Terminals Directly.
This one is potentially dangerous, you'll want to keep the battery in your hand and stop any time it becomes hotter than normal charging makes it.
Basically you touch the terminals against the corresponding wires - with lithiums especially make absolutely sure you have the polarities correct and do not touch the middle terminal with the wires, as this can potentially damage the battery's functionality with the device and stop it working even if it's not broken.
Step 5: Be Smarter.
Don't leave the charger at home when you can bring it, remember to charge your devices for when you need them...
When it's needed this stopgap measure is very useful for saving a situation but overall can't be considered an effective charging method. Also never leave the device or battery unattended while doing this, since it's possible to cause a fire quite easily.