Why does the fast charging of electric vehicles become slower with increasing charging time?

The essence of “fast charging”: dynamic collaboration between car and charging pile
Fast charging is not a unilateral "output" of the charging pile, but a coordinated cooperation between the vehicle and the charging pile. The whole process can be divided into the following key steps:
Physical connection: After plugging in the gun, the charging pile provides low-voltage power to the vehicle and wakes up the on-board BMS (battery management system).
Parameter negotiation: BMS exchanges "identity information" with the charging pile, including the maximum required power of the vehicle, battery voltage range, etc.
Dynamic adjustment: During the charging process, BMS monitors the battery status in real time and sends current and voltage demand instructions to the charging pile, and the pile end adjusts the output accordingly.
It can be figuratively compared: the vehicle is a person drinking water, and the charging pile is a faucet. The speed of drinking water depends not only on the amount of water output from the faucet, but also on the thirst and swallowing ability of the person drinking water. The higher the charge, the slower the charging? The key lies in the battery characteristics
Many car owners have found that when the battery power reaches 80%, the charging power will drop significantly. This is not because the charging pile is "lazy", but is determined by the physical characteristics of the battery:
Charging curve law:
0%-50% stage: battery activity gradually increases, charging power rises rapidly, and the speed is the fastest.
50%-80% stage: to avoid battery overheating or overvoltage, BMS gradually reduces the current and the power decreases slowly.
Above 80% stage: enter "trickle charging", use small current to balance the battery cell voltage and protect battery life.
During the first 20 minutes of charging, the battery is in the preheating stage, the battery activity gradually increases, the charging power required by the battery continues to increase, and the charging speed continues to increase. When the SOC reaches about 50% (the results vary for different brands and models), the charging power will gradually begin to decrease. When the SOC reaches 80%, the charging speed will further decrease and enter the "trickle charging" stage until it is close to full.
The golden rule to extend battery life: shallow charge and shallow discharge
Daily use: keep the battery power between 20%-90%, charge as needed, and avoid deep discharge.
Long-distance travel: Lithium iron phosphate batteries can be fully charged once a week to calibrate the power display, and other types of batteries do not need to be fully charged deliberately.
Temperature influence: Low temperature will reduce battery activity. It is recommended to preheat the battery before charging in winter (some models support this function).

In general, we can follow the principle of "charge as you use, shallow charge and shallow discharge", especially at public charging stations, and charge to 80%-90% each time, and keep the SOC at no less than 20% daily. Doing so can fully protect charging safety and reduce charging waiting time. Even for lithium iron phosphate batteries, it is sufficient to fully charge once a week to correct the range.
The battery life is determined by the "number of full cycles" (i.e. 0%-100% charge and discharge), not the single charge amount. Shallow charging and shallow discharge can reduce the number of full cycles and extend the life.