Will a fake iPhone charger ruin your battery?
Short anwer. No. Long answer. It depends. If it’s got 5 volts to pump into your iPhone with 1 or 2 amperes, it is okay. But is it?
Apparently, fake chargers pose no threat to your iPhone.
The charge control circuitry for an iPhone is inside the iPhone. The “charger” is just a regulated DC power supply which provides power to the phone for operation and charging, but it doesn’t make any decisions about how fast or how long to charge the battery. As long as it provides approximately 5 V at the appropriate current (1, 2, or 2.4 A), the phone should be happy with it, and charge the battery normally.
However, the real risks are…
The main risk with “fake” chargers is that they may not provide sufficient electrical isolation between the AC line and the 5 V output. With a good charger, you should be able to stand with your feet in a grounded bath tub while you hold the bare metal of your phone in your hand, while at the same time the phone is connected to the charger and the charger is plugged into the wall. This should do you no harm, because the 5 V output is completely electrically isolated from the 120/240 V input from the wall.
Electrocution could also happen to you.
Sadly, several people have been electrocuted by fake chargers that were poorly built and did not provide this isolation. Current flowed from wall outlet through charger to phone and then through the person to ground, electrocuting the person. Don’t try this just to prove that your iPhone charger is fake or not.