Fixing Laptop Power Problems

Intro

Understanding how laptop power works can be very useful in determining what the problem is. I will first explain how it works, then I will explain how we can troubleshoot.

AC power adapter

How the AC adapter works

The most common type of AC power adapter involves a small box, with one wire to go to the computer and another wire to go to the wall. There is usually an LED light on it, to tell you that it is on. (Is it light on?)  Apple notebook power adapters often have an LED that is actually built into the side of the computer, not into the adapter box, so obviously it won't light up when it's not connected to the computer.
The power adapter box does several things. It lowers the voltage from the wall's 240 volts AC in the uk and 120 volts AC in the usa to typically 19 volts DC (it will be labeled with the exact voltage). It includes some power noise filtering. It often includes an automatic circuit breaker or overload detection or thermal fuse  If this gets tripped, you can generally reset it by unplug it from everything for a few minutes.
You can measure the voltages on your AC adapter with a multimeter. When it is unplugged from the computer but still plugged in to the wall, it is normal to find that the voltage may be 1 to 3 volts higher than the printed output rating.

Troubleshooting: AC Power adapter LED is off when you plug the adapter into the wall but not into the computer

Has it been overloaded? Is the power strip turned on / is the outlet working? Is the cord from the adapter to the wall fully plugged in on both ends (try wiggling)? Your adapter may be fried- test the output voltage try borrowing an identical adapter from a friend and seeing if that one will work in it's place

Troubleshooting: Check output voltages with a voltmeter/multimeter

For those of you with circular connectors, your task is easy. Measure the voltage between the inside and the outside. A diagram on the adapter's label will tell you which should be positive and negative
For those of you with the 3-pin Dell power connectors, I don't have a diagram for you yet, sorry. If you try measuring each of the three possible pairs of pins, you should get 20 volts between one of the pairs.

Power distribution inside the laptop

How laptop power distribution works

Okay, the power comes into the plug on the back of the computer. This connects it to metal traces inside the mainboard of the computer. These will be connected to voltage regulators which often output 5 volts, 12 volts, 3.3 volts, and the CPU voltage; they sometimes look like this image.
These will be connected to cylindrical capacitors distributed throughout the mainboard. Capacitors act like tiny, fast batteries, supplying extra power where needed to maintain a steady voltage during high demand.
Finally, power goes to all the devices that need it.
Power for the LCD display is usually done separately  Generally, LCD backlights require high voltage to operate. A component that might be called a power inverter will step the voltage up to what is needed for the backlight. Sometimes this inverter is a discrete and replaceable component, and sometimes just a chip on your mainboard. It is also involved in the brightness control.
The battery is an important part. It supplies power to the input-side of the voltage regulators, just like the power from the AC adapter. Additionally, there is a charging circuit, which uses the power from the AC adapter prior to the voltage regulators, to charge the battery. (By the way, this is why the AC adapter voltage is always rated higher than the battery's rated voltage-- you need higher voltage for charging.)

Short circuits

Anywhere in your laptop, physical or electrical damage can cause a short circuit. A short circuit will consume all available power, causing your laptop to not turn on. If your power adapter LED comes on when you plug it into the wall, but then goes off when you plug in the laptop, you probably have a short circuit.

Troubleshooting short circuits

An ohm-meter / multimeter will read less than ~3 ohms when you measure the resistance between sides of power going into a short circuit. When measuring between then power pins going into the back of your computer, consider the 2 pins where you expect voltage to be applied by the AC adapter. If the resistance indicates a short circuit, this is bad.
A common trick to help isolate possible problems is to try removing anything that might be a short circuit. Take out the battery, the hard drive, the DVD/CD drive, the floppy drive, PCMCIA cards, USB devices, miniPCI cards. See if the computer will then turn on.
You might have luck with a thermal imager to find a hot spot caused by the short circuit.

Spilled liquid

Laptops exposed to small amounts of spilled liquid are repairable in some cases. Liquid can cause the following types of damage:

• Short circuit due to wet liquid (problem goes away after liquid evaporates)
• Short circuit due to dried liquid residue
• Corrosion (especially bad with soda such as Coke or Pepsi)
• Electronic components damaged by above short circuits

Immediately when a spill occurs, you typically want to turn off the laptop, turn the laptop upside down (so the liquid goes out the same way it came in), remove the power and battery, and allow to dry at least overnight.
To remove dried liquid residue: Try removing the keyboard and cleaning any components that have dried liquid, by using distilled water and Q-tips.

Computer turns off randomly

Your computer can turn off due to: power overload in the AC adapter, overheated processor / clogged fan, overheated battery, pushing the power button, Windows telling it to, the BIOS telling it to, loose wires (especially the power cord), or intermittent short circuits.
The most common problem is cooling. Try going somewhere air conditioned. Notice if the fan is broken or clogged with dust (common older HP Laptops).