Why does the phone lose power quickly when charging with computer USB?

Why does the battery drain faster after charging with a computer USB? The answer is simple, because you may not have fully charged it. The USB output voltage of a computer is 5V, which is generally very accurate. The reason why people think that the USB cannot be fully charged is because the USB output current is small, and the final stage of charging requires trickle charging. Trickle charging is used to make up for the capacity loss caused by self-discharge of the battery after it is fully charged. After the battery is fully charged, the capacity lost due to self-discharge is about 5% of the nominal capacity.

Theoretically, trickle charging should be at 1/500 of the charging current. The USB current is only 500mA, so the trickle current is too small, so the process is very slow. However, we often unplug the cable in advance. The result is that 5% of the power is gone in an instant, even if the phone shows that the power is still 100%, this is just a problem of power calculation.

How mobile phone charging works

At present, almost all mobile phones use lithium batteries. From the perspective of charging methods, the charging method of lithium batteries is voltage-limited constant current, which is controlled by an IC chip. Specifically, the voltage of the battery to be charged is detected first. If the voltage is lower than 3V, the mobile phone will pre-charge first, and the charging current is 1/10 of the set current. When the voltage rises to 3V, the mobile phone will enter the standard charging process.

The standard charging process is: constant current charging with a set current. When the battery voltage rises to 4.20 V, the phone will switch to constant voltage charging to maintain the charging voltage at 4.20 V. At this time, the charging current will gradually decrease, and the charging will be completely completed when the current drops to 1/10 of the set charging current.

Is it safe to charge your phone using a computer USB?

Many friends have mentioned this issue, saying that many mobile phones will have leakage problems when charged with computer USB. In fact, this is not the case. The lithium batteries of mobile digital products have protection boards that limit the minimum voltage and charging current. Moreover, whether the mobile phone is using a charger or a computer USB, to put it bluntly, it has to use the same USB cable. The only difference is the charging current. At the other end of the cable, as long as it is not a knockoff power supply with contact problems, the difference between a regular power supply and a computer USB is only the charging speed.

Previously, some media conducted tests, using the USB of a laptop to charge the phone. Compared with charging with the original charger, the charging voltage was slightly higher, about 5V and 4.2V, and the charging current was slightly lower, about 190mA and 220mA. However, when using the USB of a desktop computer, the charging voltage was still 5V, and the charging current increased to 230mA.

So we can conclude that the only difference between charging a mobile phone with a computer USB and charging it with the original charger is the difference in current and charging speed, which has nothing to do with safety. Even if you touch the phone during charging, your hand will feel numb, but it is just induced electricity. This is related to the air humidity and the clothes you wear, similar to static electricity. If you really feel uncomfortable, the solution is very simple. Just put the host on the ground.

As a winner of Toutiao's Qingyun Plan and Baijiahao's Bai+ Plan, the 2019 Baidu Digital Author of the Year, the Baijiahao's Most Popular Author in the Technology Field, the 2019 Sogou Technology and Culture Author, and the 2021 Baijiahao Quarterly Influential Creator, he has won many awards, including the 2013 Sohu Best Industry Media Person, the 2015 China New Media Entrepreneurship Competition Beijing Third Place, the 2015 Guangmang Experience Award, the 2015 China New Media Entrepreneurship Competition Finals Third Place, and the 2018 Baidu Dynamic Annual Powerful Celebrity.