Charging your car's battery with an old computer power supply

What would you do if your car's battery is dead and you don't have anybody around to give you a jump? Need to have it working in time for the morning commute? This is what I did when I was in this situation. This is a quick tutorial on how to charge you car's battery using a power supply from a PC.

One evening I found myself in quite a pickle. I had forgotten a device plugged in to my 12 V cigarette lighter/power utility port overnight and through the next day while it was in my garage. When I went to get the device I had noticed that the car's battery was completely drained. I had no other car around to jump start it, and driving to a store was not an option as this was 10:30 at night and I need to drive to work in the morning.

Being a techie geek, I started rummaging through old stuff I had, looking for any possible way to charge a car battery. I had a collection of old broken PC's that I pulled parts out of occasionally when I needed to repair a computer, some of which still had the really old style AT power supplies that has a hard toggle switch and didn't require a motherboard to switch on. There are some useful voltage sources that you can get from a PC power supply, if you are a weekend electronic hobbyist and need a quick and not so dirty (PC power supplies deliver very precise, clean voltages that computer electronics require) source of power. When you measure the power leads coming from a power supply, you should find the following voltages, and the colors should be pretty standard:

Black: Earth Ground or 0V
Red: +5V
Yellow: +12V
Purple: -12V

These are the voltages present if using black, or earth ground as a reference. if you need a 24 volt source, use purple as your ground (-) and yellow for your source (+).

For my purposes, I used black and yellow. I had a 12V car cigarette/whatever plug from an old cell phone charger that I wasn't using anymore, so I didn't feel bad about butchering it. using a VOM/multimeter, I made sure that I matched which wire was for the +, which is the nipple of the 12V plug, and which one is ground. This is important because if you get them switched around you could short the car's systems and/or cause a fire, and you will be worse off than when you started. Once I wired the 12V plug to the PC power supply leads, I ran a 110V extension cord out to the car and managed to get it in the car and still be able to close the door.

**NOTE - Be careful doing this. If you sever the extension cord it or crimp it too much, it could short and ruin the cord/ make smoke/ pop the circuit breakers/ all of the above.

If you made it this far, you are doing rather well. Your car should have an 'always on' 12V port. Some cars have a few of them, some of which require the ignition key being in the other-than-off position. Don't use that one. Instead, use the always on port. Plug it in, turn the key on and your dash should come to life. Don't get to greedy and try to start the car, if the battery is low enough all it will do is overload the PC power supply. Don't try it, you've make it this far with the whole thing just to start over and you will soon run out of power supplies. By morning you car should start with little or no difficulty, as mine did. remember to unplug the 12V plug before starting the car. Why? Read the above paragraph.

**Update -
Most modern power supplies (atx and newer) need to be 'triggered', meaning that voltage is not present immediately when the supply is plugged in. To activate the power supply, short, or use a jumper between the green wire and ground (black). This will "turn on" the power supply.

Hope you learned something. Enjoy.