Why use a USB cable to charge the phone battery for a long time?

There are two ways mobile phone chargers can charge mobile phones: Wireless charging and direct charging. Wireless charging is to charge the phone without a data cable and phone charger. The direct charging directly connects the charger to the mobile phone for charging. USB charging refers to the use of USB devices to charge mobile phones. Currently, it refers specifically to PCs and laptops. Generally, USB charging is much longer than charging time of the charger. In order to explain the difference between USB charging and DC charging, it is necessary to first understand the charging principle of the lithium battery.

The battery’s charging method is constant current, and it usually passes through three charging stages:

(1) Pre-charging, at this time the voltage is less than 3V;

(2) Constant-current charging, after the voltage reaches 3V, it enters the fast charging stage, and the current is relatively High, constant current charging continues until the voltage rises to 4.2V;

(3) Constant voltage charging. When the voltage rises to 4.2V, it is changed to a small current charge, and the voltage is unchanged at 4.2V. Normally, the current at this time is 1/10 of the constant current phase.

The direct charging method will impact the lithium battery and the mobile phone and cause damage in the long term. The USB output current is smaller than the above two, which is why the USB charging time is longer. This small current is shown in the constant current phase. The purpose of constant current charging is to compensate for the capacity loss caused by self-discharge of a lithium-ion battery after it is fully charged.

At present, the USB output voltage of the computer is 5V, the output current is 500mA, and the charging current of the charger is 1000mA. Because the USB output current is small, the current during the constant voltage phase is even smaller.

Therefore, charging time with USB is longer, and it is more time spent in the constant voltage stage. If you can’t wait, the process of charging the phone with USB sooner will end, because there is still a part of the supplemental power is not filled. This is actually a current problem rather than other problems that consumers worry about such as leakage. For lithium batteries, they can be charged at any time as long as they are not overcharged. This is different from NiMH batteries. (Related articles: how to charge your mobile phone battery)

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