Why is the last 1% of a mobile phone's battery durable sometimes?

In fact, the reason is very simple, because the remaining power is difficult to estimate.

Mobile phones now basically use lithium batteries, let's talk about lithium batteries. Lithium batteries are metaphysics, like a child. I've shown it to you. You have a low temperature. In order to take care of this baby, countless engineers racked their brains and found a babysitter called the Battery Management System (BMS). This BMS is specifically responsible for taking good care of the battery, don't overcharge, don't overdischarge, heat up when cold, and cool down when hot.

Let the battery work comfortably. Naturally, it is also its task to estimate how much power is left in the battery. In the junior high school physics class, the teacher will give us a multimeter to measure the battery voltage. If the voltage is low, it will be dead. So, the idea came. I checked the voltage in real time. Can I know how much power is left in the battery? So young children naively think that the lithium battery is 4.2v at full power, 3.0v without power, and the midpoint of 3.6v is half of the remaining power.
Naive!
There is something called a discharge curve. With so many colorful lines, let's look at the red one first. The vertical axis is the voltage, and the horizontal axis is the amount of electricity that has been discharged. see it? At 3.6v, the red one has already put 1500mAh, and the remaining power is not 50%, but 25%.

The discharge current also affects how much the battery can discharge. The lines of different colors in the figure correspond to the discharge current. The larger the discharge current, the faster the voltage drop. This picture is actually drawn during constant current discharge. The mobile phone has various working states, such as standby, talking, microblogging, and playing games. Under different working conditions, the discharge current varies greatly. The battery will age. After a period of time, the curve will change.

I have some friends to say, can I not look at the voltage? Can you detect the current in real time and make an integral, can you know how much it has been released? In fact, many high-end mobile phones are already doing this, but it still cannot be accurately predicted. Because the battery will age, the capacity of the aging battery will decrease, but the extent to which the battery ages is a metaphysics that can only be estimated. Temperature also affects the battery. Low temperature will reduce the amount of electricity that can be released. You have read the news about electric cars in winter. This is the truth. The battery also has a self-discharge phenomenon, if you don't use it, it is slowly discharging itself. When batteries are produced, there will be slight differences, causing their aging, discharge curves and other parameters to be different. We use the word consistency to describe the size of the difference between individual batteries, the smaller the difference, the better the consistency.

Battery manufacturers emphasize that different batches of batteries cannot be mixed because the consistency between different batches of batteries is poor. Look, how can it be possible to measure things that are affected by so many factors together? The engineer’s hair fell out and there was nothing to do, so the product and UI classmates came up. Engineer: It is estimated that there is still 10% of the electricity. Now the outside world is quite cold, and I am not sure how much electricity is left in the battery.

Product: Let users quickly recharge
UI: I immediately let it show that there is only 1% left
User: I rely on, 1%, it looks perfect when charging, but you can use it even if you don't charge it, but it will damage the battery.

Sometimes the power will be cut off suddenly, and sometimes it will last for a period of time. How long it can last depends on the mood of the battery. However, the product and UI students tend to tell you to charge as soon as possible, and the buffer period is sufficient. You feel that 1% of the battery is very durable. Secretly tell you that the aging degree of a mobile phone battery is actually calculated based on the power-on time. The cumulative power-on time for one year tells you that the battery should be replaced.

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