# Charge a laptop via USB power (500mA)? [closed]

My laptop is almost dead and I do not have the charger. Is there anyway I can DIY a charger for a laptop. Even if it takes 20 hours. To make it clear, I want to send power from a usb port on another computer, or a usb charger to my DC socket. Thanks.

What equipment/steps would I need to accomplish it?

The battery on my laptop says, 5400 mA 15v. The math turns out to be that a usb port could in theory charge my battery in (5 600 / 500) * 1.2 * 2.2 = 29.56800 hours. if i concider an 80% efficiency. I know that my phone usb charger is 10 times more powerfull then the standard, that means 9 hours, if I pair a couple, then I could have something feaseable.

I have an Asus UL30A

EDIT: after doing the math, i think what I need is something that gives repetitive pulses of electricity powerful enough to charge my battery, given usb obviously does not have the power to do at hand. Think of the theory as filling a bucket with water and then throwing it at someone to make them wake up and not just flinch.

http://en.wikipedia.org/wiki/Battery_charger#Pulse

Note this is not the same as Can I charge a computer through its USB port?; here we're interested in converting USB power from one computer to the laptop's power input.

• you're welcome to edit your posts, but do not remove the automatically added duplicate link. this question was closed as a duplicate and that link tells us which duplicate. (to be fair, i think you were making an unrelated edit when it was closed, so you didn't mean to remove it. but if in the future you're tempted to do so, don't.) Jun 30, 2010 at 1:04
• Sorry about that, we were edditing at the same time it appears and my cache of the version I was editing overwrote yours. small issue. beyond that. please consider my comment below. Jun 30, 2010 at 1:06
• @quack: I don't wish to dump on any mods, particularly those working on SU (which I know is rough territory), but I'm inclined to agree with @Oldarney: I don't think these are dupes. Where the post marked in the closure is about charging via the USB port, this seems to be about building a DIY adapter that draws power from a USB source and delivers it to the laptop's power socket. (Although I admit that's not 100% clear from the text, the comment on this answer indicates such.) It may still be off-topic, though. :) Jun 30, 2010 at 1:09
• @john, @oldarney: after seeing the comments on the answer below i'm inclined to agree, but it's unclear. please clarify exactly what you're trying to accomplish. (if it's as @john suspects, to go from {other computer USB} > {DIY adapter} > {laptop power input}, i think that's clearly a different question. just make it clear for us.) Jun 30, 2010 at 1:16
• You know you can buy a used charger for that laptop on Ebay for \$10: cgi.ebay.com/… Jun 30, 2010 at 3:04

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.

• That can't work. The 50mA wouldn't even be enough to initialize the laptop's battery manager, much less have any leftover power to charge the battery. So it wouldn't ever charge at all. (The power jack on a laptop is not connected to the battery, it's connected to the laptop's internal power supply that runs everything, including the charger. But the charger is controlled by a battery manager. You'd never get the manager running with so little current.) Apr 21, 2012 at 21:55

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.

• what is hackers anonymous? or what if its a figure of speech what does it entail? it would surprise me to find that such a device would run me over a hundred bucks though. -from my phone. Jun 30, 2010 at 2:13
• Hackers Anonymous is a fictional(?) association of crazy old men who cut S-100 motherboards in half with a hacksaw in order to make two computers out of one. (mea culpa) Hotei PS: My last course in EE was a LONG time ago... the problem is you have to step up the voltage from USB to get it to 15 volts for your DC input on laptop. This typically requires a transformer...(HEAT is not your friend) You'll lose much more than 20% going up and back with homemade (gut feeling). Stepping UP DC is much harder than stepping DOWN. Jun 30, 2010 at 2:33
• cgi.ebay.com/… Im thinking stepping up the voltage on one line and connecting the other in parallel would give me 15 volts. now its the matter of ordering and putting the pieces together in a neat box? Jun 30, 2010 at 20:31
• @oldamey : The devices you linked to are rated at max of 2 watts. You'd need quite a few to equal a modern charger (my HP laptop charger is rated 90 watts). They also would only get your 5v USB voltage up to 10 v since they are locked at 2X. I suppose you could put them in series but then you'd get 20 VDC, not 15. Maybe your laptop would take 20v ok - maybe not. You could easily end up needing many more ICs (at \$10 each) than the cost of my original suggestion... Jun 30, 2010 at 23:00
• How would you void your warranty on your computer by using a similar charger with the same voltage, amperage output? I don't believe that would be legal, unless you were under some contract to use only their brand of power adapter. Nov 21, 2016 at 0:00

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.

• more specifically, laptops are not (to my knowledge) designed to be charged via USB -- there's no connection between the USB to the battery-charging circuits. most USB hosts are designed to provide power, not consume it. see the dupe. Jun 30, 2010 at 0:36
• The question is What equipment/steps would I need to accomplish a diy charger. I know the ports wouldn't help. Jun 30, 2010 at 0:53
• @quack there is no link to the dupe Jun 30, 2010 at 0:58
• @quack my post asks what equipment do I need to DIY a charger that uses usb power, not that charges through the usb port of the computer, which is kinda funny. Jun 30, 2010 at 1:04
• @mrstatic: it was removed by an edit but was still visible in the revision history. Jun 30, 2010 at 1:07

If you wanted to seriously attempt this, you'd need to calculate the requirements of your laptop. There is good information here: http://www.girr.org/mac_stuff/usb_stuff.html

• 98.5599991 hours to charge. if i consider an 80% efficiency. 9 hours with my 10x AC to USB phone charger, which are becoming more and more common. Jun 30, 2010 at 0:40
• The dell axim hard drive seems to consume 840ma when running. If a computer can handle two of these than it can handle 1.7A! which would keep my CULV laptop on perpetually during webbrowsing; this is since my laptop has a 12 hour battery life and such a usb solution would charge it in about 8. Jun 30, 2010 at 0:51

It's not possible without removing the battery. Modern laptops have battery managers that control the charging of the battery. To get power to the battery manager, you need to hold the power jack at the correct voltage. But to do that, you need to supply enough power to run all the hardware that's one the same bus as the battery manager. With a 5W input, you wouldn't have enough power to do that. The laptop would never enable the circuitry to get a charge to the battery.

The only theoretical possibility would be to use the power from the USB port to charge, say, a 24V battery (likely made from two 12V sealed lead acid batteries). You'd need a DC-to-DC converter to boost the 5V to 24V. Then, once the 24V battery was charged, you could use that 24V battery to charge the laptop through a DC regulator.