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The modern USB charging circuitry of mobile phones is pretty sophisticated. For example they can negotiate higher charging voltages etc.

This question, inspired by a discussion on worldbuilding, is simpler: I suppose I can charge any phone by simply providing 5V DC on the VCC/GND pins of the receptacle without even connecting the other pins, let alone communicating with the device. Think of a replica of Volta's first battery or a potato battery.

If this is correct, the actual question is which voltage range the charging electronics typically support on the two pins without being damaged. We will roughly know the voltage of one battery cell and can stack them in order to get in the vicinity of 5V but we cannot actually measure it and may end up half a Volt or so high.

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  • \$\begingroup\$ For a sense of scale, here is a 500lb potato battery array that can generate 4mA at 5V. At minimum, a USB device charger would need to supply at least 500mA, so that would seem to suggest you would need around 30 tons of potatoes to get the job done. \$\endgroup\$ – J... Oct 22 '19 at 16:30
  • \$\begingroup\$ @J... Nice hack although I suspect the number is wrong. Could you hear anything from a 25mW "sound system"? A BBC report quotes a research team explicitly claiming that you could use a potato battery with cooked potatoes for cell phones etc. \$\endgroup\$ – Peter - Reinstate Monica Oct 23 '19 at 5:50
  • \$\begingroup\$ I presume the 'sound system' was probably powering a small set of headphones and not a loudspeaker. 25mW would be plenty to drive a set of earbuds. \$\endgroup\$ – J... Oct 23 '19 at 16:27
  • \$\begingroup\$ Also, Related \$\endgroup\$ – J... Oct 23 '19 at 16:28
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Think of a replica of Volta's first battery or a potato battery.

In theory: yes

In practice: no

The current required to charge a phone's battery usually exceeds what a Volta type or potato battery can provide. Unless you'd make the battery really large.

What voltage a phone can suffer without damage on its charging port depends on the design. The USB specification ( usb.org/documents ) says 5V +/- 5% but most phones are designed to take up to 5.5 V or higher in order not to be damaged by cheap chargers that output 6V.

Current is not a big issue

It will be an issue, if the phone draws more current than the battery can deliver at 5 V, the voltage will drop and the phone will decide that its connected to a crappy charger and stop charging. For example, you will be unable to charge a phone with 5 V, 10 mA. It will not simply take longer, it will not charge at all!

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    \$\begingroup\$ Also, if you're simply supplying 5V to the USB power pins the phone will limit itself to drawing 500mA (adhering to the USB standard). Devices that incorporate a fast-charging feature require a specialized smart power supply that can identify itself as having a higher current supply capability. The device itself will not attempt to draw more than 500mA until it can successfully negotiate a higher current with the power supply, so developing a charger capable of fast charging requires not only a 5V supply with sufficient power, but also smart circuitry able to communicate that to the device. \$\endgroup\$ – J... Oct 22 '19 at 16:25
  • \$\begingroup\$ @J... : which phone does that? i've charged about 15 phones on cheap (no qc, no usbpd, no nothing) chargers and none draw fewer than 720ma to charge... \$\endgroup\$ – dandavis Oct 22 '19 at 19:19
  • \$\begingroup\$ @dandavis All phones that can fast charge do it. If the data lines are not connected the phone will take 500mA and no more. I can't speak to your experience, I have no idea what phones or chargers you're talking about. If not QC and not USB-PD then most likely the other scheme being used is divider resistors providing indicator voltage to the data pins that correspond to higher available charge current. If you want fast charge, you need to do something with the data pins other than leaving them open. \$\endgroup\$ – J... Oct 22 '19 at 19:43
  • \$\begingroup\$ @dandavis Now, I'll admit that the divider scheme is probably not what we might call 'smart' circuitry, but it does require more than just a potato and wire. \$\endgroup\$ – J... Oct 22 '19 at 19:49
  • \$\begingroup\$ @J... You're correct in stating that for more than 500 mA (actually the USB specification allows for only 100 mA) some negotiation should take place. In practice though, many devices "do not follow the rules". My Nexus 5X for example will draw up to 1.4A from a 5V as long as the voltage stays high enough. Even if the USB data lines aren't connecting to anything it can and will draw such a high current. Other devices charge slowly at less then 500 mA unless the USB data lines are shorted or kept at certain voltages. \$\endgroup\$ – Bimpelrekkie Oct 23 '19 at 6:38

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