Charge a laptop via USB power (500mA)?

How many USB power sources do you have? Do you have a multimeter? Do you have laptop's original power source available or at least its detailed picture? How good are you with soldering iron? What's your budget and most importantly, do you have a plug which goes into your laptop's power port? Also, how much USB extension cables and hubs do you have?

I'm electric engineering student (first year though, so be warned!) and I have an idea which might work.

I was thinking about serially connecting several USB power sources and using them to charge the battery. There are two ways to accomplish this: to use external charger which will charge battery outside of the laptop or to use laptop's charger.

In my opinion, if you have correct plug for the laptop's power connector, you could use it to power internal charger.

Here's an idea: I'll assume that your laptop's external power source provides 15V (It could very easily happen that it uses around 20V. Mine does and it uses around 15V for battery.). You would need 3 USB power sources to provide 15V (3*5=15, if it's 20, then use 4). Then you would need to take some USB extension cables and cut off USB connectors. Then take the 5V lines and serially connect them. Each USB cable has 4 wires inside. Here are pin-outs for some common connectors: http://pinouts.ru/SerialPortsCables/usb_cable_pinout.shtml You need to establish using a voltmeter which cable inside the USB cable is GND and which is USB Vcc. Then you need to connect GND of one cable to others USB Vcc. So this is what you vould get in the end: USB Vcc GND-USB Vcc GND-USB Vcc GND The potentials difference between the right-most GND and leftmost USB Vcc should then be 15V. Now you would need to connect the ends to the plug and connect the plug to the laptop. You might be able to charge the laptop using that contraption. Of course, if you do connect this to laptop, do not try to turn it on. If you do, it will most certainly overload the power source and possibly damage the USB ports which provide the power.

Another interesting option which would increase the charging speed would be to repeat the procedure and then parallelly connect the 15V cables. Once you got the 15V cable, connect it to another 15 volt cable to that + and + cables are connected and - and - cables are connected and then connect it to the plug. This way, you would get more amperes so laptop would be able to use more power.

I gained a bit more experience, so I now know that this most likely wouldn't work. Another way would be to make a boot converter which will boot the 5 V to 15 V. I can't find a good device to recommend, but for start NCP1403 looks interesting. Unfortunately, it can provide only 50 mA, so charging will take quite a while. Another problem is that the circuit required for its (and similar device's) operation is a bit complicated and requires some skills to make it work. The good point is that it will only take one USB port o work.

I also read somewhere that if overloaded, USB ports in computers will limit power to safe level and notify operating system about the surge.

On the other hand if you don't have necessary equipment, it would probably be cheaper to just buy compatible charger. Also, I wouldn't try to use for this experiment any expensive source of USB ports. Some broken down old computer which can be sacrificed or some externally powered old hubs which can be sacrificed would be best. As I said before, I'm still first year, and I don't have much experience with this kind of things.

Also, here's this link: http://www.edaboard.com/ftopic147112.html These guys, unlike me, seem to actually know what they are talking about.

Once I get some free time, I'll upload some illustrations for this post.

If you really want to do this you need to join Hackers Anonymous. :0) Sure, you could do it, but it would cost you more in parts at the electronics store than a new charger. Plus it would probably not be safe to use unless you're already an electronics engineer and not just a guy with a hot soldering iron and a dead battery.

If your laptop's charger is no longer available you should look for one (new or used) that puts out a similar voltage and amperage and has the right plug size and polarity. A volt or so higher or lower than 15 is probably still usable since in all probability it's going to be reduced anyway inside your laptop. Likewise the amperage could be a bit high or low and still be useful. Plug size and polarity of course have to be exactly the same though if you're still itchin' to break in that new soldering iron this could be your chance. You will of course void any warranty on your new/old charger and probably your laptop in the process but I think you knew this. Ebay/Craigslist is one route to this end.

Sadly no. USB just does not have the power requirements to charge a laptop. And any DIY that could exist would either be unstable or would be on the dangerous side (IE overcharge). Just amazon/ebay/craigslist the correct AC adapter.