What's stopping the ability of charging a cell phone from another cell phone? If I were to create a crossover/patch USB cable and connect it between two phones or tablets, would there by any way to have one of their USB ports provide a charge to the other?
Most phones are USB Peripherals (Slaves). They do not provide power on the USB VCC pin. So it won't work at all that way.
Some phones have USB OTG capability. They can provide power over the usb pins, like a host. Some need to be hacked/modded/rooted to do this. Even then, some phones need power injector cables because they can only do OTG data, not provide power.
Mostly, it's very impractical to rely on. Just get a usb portable battery pack.
Most phones are not designed to output power on the USB power pin. Even if the USB power pin on the donor cell phone has *some connection*** to the battery, most phones use single cell Lithium battery, which works around +3.7V. USB power is +5V. The recipient cell phone expects +5V.
** through body diodes of the MOSFETs in the charger, for example
To provide +5V to USB OTG (see the post by @Passerby in this thread), the cell phone would need a step-up converter to boost from +3.7V to +5V. It's possible to have a +3.7V to +5V boost converter in the "transfusion cable", though.
In any event, the success of the "transfusion" would depend heavily on the design of the battery charge controller in each particular cell phone. Such charge controllers are not obligated to follow a standard (such as USB). There is no guarantee that the USB power pin on the donor cell phone will be "back-powered" from the battery.
It depends on the connection cable.
I coupled following two OTG enabled devices to test charging one phone with other.
- Xiomi RedMi 4 (Android v6.0.1 Smartphone, 4100mAh Li-Polymer battery)
- Lenovo A3300-HV (Android v4.2 Tablet, 3500mAh Li-ion battery).
OTG of both devices works fine with SanDisk 16GB pendrive.
I used one micro USB male to OTG female converter (Which we use to connect pen-drive with phone, I'll say it OTG cable) and one USB male to micro USB male converter (Ordinary data cable, I'll say it Data Cable). Any one of the android device was connected to OTG cable and the other was connected to Data cable. The female socket of OTG cable was connected to male socket of Data cable.
I found the device connected to Data Cable is always sucking power from the device connected to OTG.
When The Lenovo tablet is connected to OTG Cable and Redmi 4 is connected to Data cable, I found the Redmi 4 is sucking charge. While the same Redmi 4 is connected to OTG and Lenovo one is with Data Cable, the Lenovo is sucking charge from Redmi 4.
So, my conclusion is the microUSB to USB connection is not one-to-one. It do some voltage and signal conversion to make one device charge source and other chargeable. I'm not really very clear about this point.
I can't say what happen in case both the device are directly connected using a microUSB male to microUSB male cable. But I expect such cable must have some functionality to mark devices connected to one side as source and other side as peer.