Yes, Apple's official website about the iPad charger page, also explained.
iPad charger parameters are 2.1A\/12W, iPhone charger parameters are 1A\/5W. When using the iPad charger to charge the iPhone, if the Phone can only receive 1A current, the iPad charger will only provide 1A current output.
However, starting from the models after the iPhone 6, the charging profile of the iPhone is the same as that of the iPad, which means that the later devices can also use 2.1A current charging, and the maximum support is not more than 12W power, and the iPad charger can shorten the charging time of the iPhone to a certain extent.
The iPad has a much larger battery capacity than the iPhone, so it is designed to charge at a current of 2.1A for the best charging time.
If the iPhone charger is used to charge the iPad, since the maximum current output of the iPhone charger can only be 1A, the entire charging time will be about 2.1 times that of the original. Due to the design of Apple on the USB interface, the iPad will recognize the iPhone charger at this time. Charging current of more than 1A is not required, and it will not overload the iPhone charger and cause damage.
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iphone30W USB? C Power Adapter Overview
Whether at home, in the office or on the road, Apple 30W USB? C power adapters allow you to charge quickly and efficiently.
Is this power adapter compatible with any USB support? C's device, but for ideal charging performance, Apple recommends pairing it with a 12-inch MacBook.
You can also pair it with the iPhone 8, iPhone 8 Plus, iPhone X, iPhone XS, iPhone XS Max, iPhone XR, or select iPad Pro models to take advantage of the fast charging feature.
Apple official website: 30W USB? C Power adapter
Can not charge, because the charging speed is too slow iPhone phone charger can not charge the iPad computer, because the charging speed is too slow, so it can not achieve the purpose of normal charging, which will lead to the loss of power of the iPad battery