iphone won't charge with fast charger but will still charge with standard charger

To identify counterfeit or uncertified cables and accessories, look carefully at the accessory's packaging and at the accessory itself.

Identify counterfeit or uncertified Lightning connector ...

Unfortunately there are a wide range of devices out there.

Some claiming “fast” - while possibly legit - use proprietary signaling which only performs on “some” devices.

Cables are another notorious point of possible incompatibility.

Then there are the cheap knockoffs and outright counterfeits …

Apple devices are indeed “pricy” … don’t cheap-out on the only things between your nice, shiny, low-voltage DC-powered Apple device and AC Mains voltage.

If interested in “tech” … you might also find this piece interesting:

http://www.righto.com/2014/05/a-look-inside-ipad-chargers-pricey.html?m=1