Does the wireless charging speed differ depending on the exact place we put the iPhone on the charging mat?
I mean, if I don't put the iPhone correctly on the charger, it won't start the charge. But as long as the charge is ok and starts, will the speed only depends on the power of the charger, or may it depend on the exact position of the iPhone? Charging faster if the phone is exactly on the good location and slower if it is a bit away from the center location of the wireless charger.
The Qi standard (that the iPhone uses for wireless charging) allows for multiple types of chargers regarding phone placement.
Some chargers require you to place the phone at a very specific place in order to charge. They will generally not be able to charge at all if you place the phone incorrectly.
Most chargers allows so-called free placement, where you can place the phone any place on the charger (within reason) and get full speed charging. This is done by actually having more coils than really necessary within the charging surface. Those directly underneath the phone are then used and activated, whereas the other are not. This allows the charger to work across the full surface independently of phone placement.
The standard also allows for chargers where the coils actually move to place them underneath the phone. I haven't seen such charger available for ordinary home users though.
