Can you charge your laptop's battery by plugging a phone via USB? [duplicate]

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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1 Answer

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.

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