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Whenever my phone can be plugged (via a USB cable) into my laptop computer, my laptop will charge my phone's battery. But how can I reverse this charging, to command my phone's battery to charge my laptop?

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USB ports generally put out 5V at 0.5-1A. This power is taken from the battery or power supply and put through transformers to lower the voltage and control the current.

Power through USB port is output-only. You cannot reverse this flow. Even if you could, you would be trying to charge a 12V+ battery with 5V. There are ways to output a voltage higher than the input voltage, but because of how electricity works, the output will have a much lower current than the input.

The situation is impossible in the first place because USB ports on a laptop shouldn't accept a voltage input. If they could, you still wouldn't be able to charge the battery because that's not how the circuitry works. Even if the laptop had a specific module to accept voltage from the USB ports to charge the battery, the input would need to have quite a high current to charge the battery. A phone cannot provide this current.

TL;DR (without the repitition): it's impossible to do because it's too impractical to implement.

TheWanderer
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    I wonder if this is the case with USB-C as well, since there are laptops that charge via that port. Newer Nexus phones can charge other USB-C devices as well. I'd be interested to see what would happen if you plugged a Nexus 6P into a dead USB-C powered laptop. – Chris Powell Jun 27 '16 at 18:40
  • @ChrisPowell the manufacturer would've had to build in something to accept power from the port, then convert it to the correct charging voltage and current for the battery. The phone would probably drain in a matter of minutes. – TheWanderer Jun 27 '16 at 18:42
  • Oh yeah, no doubt. I'm not saying it would work well. But it could theoretically give the laptop a minute or two of power. You'd be much better off using a large USB-C power brick (which would still drain rather quickly.) – Chris Powell Jun 28 '16 at 13:17