The iPad and iPhone share the same connector; however, their power adapters are distinct in size and wattage rating. Suppose you are frequently on the move and would like not to carry many adapters. Can the higher-wattage adaptor be used with both devices?
The brief answer is affirmative. And we advocate rapid charging if you desire it. Apple expressly permits the use of its chargers with all Lightning-compatible devices. Modern electronics are equipped with systems that ensure they draw the correct amount of power—enough to charge quickly without causing damage to the gadget. Certainly not immediately.
Anecdotally (and as confirmed by Apple), iOS devices charge significantly faster with a higher wattage power brick. If you have an iPad Pro and use the 29-watt MacBook USB-C charger to refuel it, it will do so in half the time compared to the iPad's 12-watt charger.
The difference between either of these chargers and the 5-watt charger that is included with an iPhone will be just as significant and noticeable. This is due to the fact that the rated amperage is what differentiates the adapters for the iPhone and the iPad. The larger iPad charging brick is rated to take 2.1 amps, whereas the iPhone charger is only rated for 1 amp due to its smaller size.
However, the amperage rating is not the only factor in determining the charger's maximum capacity. The load is what determines the real amperage of the circuit (i.e., the iPad or iPhone). Modern battery-powered electronics have a lot of complexity between the charger and the battery, according to Steve Sandler, founder, and chief technical officer of the electronics analysis company AEi Systems. This complexity includes battery-charging circuits within the device as well as battery-protection circuits in the lithium-ion battery itself.
These circuits are designed to manage the flow of electricity to the battery, and if the circuits inside the iPhone were designed to tolerate 1 amp, but are routinely exposed to 2 amps, this could stress the system over time. The circuits are designed to manage the flow of electricity to the battery.
Since Apple maintains that the charger for the iPad and the iPhone are compatible with one another, our recommendation is that you sign up for AppleCare (which is worth it for a variety of reasons) and charge all of your devices using the largest wall chargers you own. Make Apple replace your battery after the first year if you see a significant decrease in its performance.