what happens if you plug a non QC 2.0 device into a QC 2.0 charger? So I recently got a external battery pack for my phone that has two power outputs - a Quick Charge 2.0 output and a 5V / 3A Max output. It is my understanding that Quick Charging only works if the device supports (eg. a Samsung Galaxy S7) it so what happens if you plug in a phone that doesn't support it (eg. a Samsung Galaxy S4)?
From Qualcomm FAQ :


  .... Quick Charge is compatible and interoperable, a certified adapter can be used with a non-Quick Charge device, though the fast charging benefits of Quick Charge will not be available


(Emphasis supplied)

QC chip is integrated within the device in enabled devices, and negotiates with the QC charger to vary the voltage and or current. In devices which are not enabled with QC, this negotiation cannot happen and normal charging takes place

If a cable/ charger that does not support fast charging is used, there is no harm caused -  charging will be normal charging instead of fast charging. In the case of cable, it is because it may not be designed to carry adequate fast charging current, which is usually higher