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I would like to power my raspberry pi 3 from an OTG supporting smartphone. I can get my hands on a Galaxy S3 for this purpose. My raspberry only uses the built-in wifi module, runs a loop and no usb nor hdmi devices are attached, so the pi is not drawing that much power.

From what I could find out, the raspberry should be drawing around 400 to 600mah.

Using google I couldn't find anyone who is trying to achieve the same.. did anyone try and achieve this? Any other advice?

edit: thanks for both answers, very useful. I will investigate more and might run a test. I will keep this question updated with any progress.

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Check the specifications - http://www.usb.org/developers/onthego/otg1_0.pdf. However, I didnt find there a maximum current, there is 100mA threshold for A-device on page 33, that tells, that when the device-A current rating is over 100mA, it must fullfill some USB-2.0 specifications.

Conclusion - check the specification of your phone, if it is less than 100mA, dont connect. If it is more, the phone could survive your test (see page 33). Although, there are some negative hints https://forum.xda-developers.com/galaxy-s3/help/usb-otg-portable-hard-drive-size-limit-t1761951 but myself, I didnt find galaxy S3 relevant usb-otg information.

Friend of mine connected an usb-DVD to a tablet and the tablet got a permanent damage.

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An RPi doesn't act like a real USB device when connected to micro USB plug: it won't "enumerate" (a fancy USB slang meaning "to configure a connection") and thus will not negotiate the power it will draw. By default, USB spec limits unconfigured devices to 100 mA, and USB OTG is no exception. However, regular USB hosts (like laptops) usually have loads of energy to share and in practice will provide at least 500 mA to any device. A smartphone often doesn't have that much, so it may actually stick to the spec and provide only the bare minimum (which is plenty for typical OTG applications like keyboards or flash drives).

Technically, if your smartphone respects the USB spec (which I'm willing to bet is the case of well-known brands), you lose nothing by trying it out. Worst case, your RPi won't start, or will keep rebooting. However, that last line from jaromrax's answer is important even if the odds are small: if your smartphone happen to lack any kind of overcurrent protection, you may damage it by connecting the RPi to it.

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