I used Apple's original 5W charger and Anker PowerPort Nano 18W PD charger to charge the iPhone XS Max, and reached the conclusion
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It can be seen that the gap between fast charge and slow charge is particularly obvious at the charging stage of 0% to 50%. It takes about 90 minutes to fully charge the iPhone XS Max with Apple's original 5W charger. While using the Anker PowerPort Nano 18W PD charger takes only 30 minutes, less than 1 hour is the biggest significance of fast charging.
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Because from the 0% to 100% fully full situation, 18W fast charge also takes nearly 140 minutes, and is not much faster than the 5W slow charge that took 200 minutes, and the reason for this phenomenon is mainly because of the mechanism of lithium batteries: when the battery power reaches about 90%, it will enter the... Trickle charging; Mode, when the charging speed will be slowed down to protect the safety of battery use.