Super class (214): why a good data cable has a great influence on charging efficiency?

I believe everyone has had the experience of charging a mobile phone from 10% to 100% in less than an hour with the original charger and data cable. However, after switching to a third-party data cable, the charging speed of the mobile phone becomes very slow, or even can not be charged at all. It is obvious that this third-party data cable does not match the charging demand of the mobile phone, It means that you need to find out the original data cable, or replace it with a data cable that can meet your needs. Many users have doubts about this. Isn't the structure of data cables unified? Since the physical structure is the same, why is there such a compatibility problem?

 

In fact, such a problem is not caused by the physical structure of the data cable, but more by the materials used in the data cable. As we all know, at present, the data cables used by both mainstream and high-end mobile phone products are basically universal, and there is no "exclusive data cable" in the real sense. Even if you want to use your own data cable for fast charging, just like the one plus mobile phone, it is only limited to the fast charging handshake. When it is used with the data cable of ordinary style, The basic charging and data transmission functions will not be affected. Therefore, when you switch to a third-party data cable, the charging speed becomes obviously slower. After eliminating the reason that "handshake fast charging protocol must use original wire", the rest is naturally the problem of "data cable quality".

 

Don't underestimate the "data cable quality". In fact, when we open the pages of jd.com or Taobao, we can see that the price of similar wires ranges from a few yuan to a hundred yuan. Obviously, it can't be explained by a simple "brand premium", The function of data cable and charging efficiency are the key factors. I believe you can understand the differences in functions, such as the differences between USB 2.0 wire and USB 3.0 / 3.1 wire, the differences between ordinary USB type-C interface and lightning 3 interface, and so on. How to understand the charging efficiency? That's what we're going to talk about today. Why does voltage drop have a significant effect on charging efficiency?

 

Before we start talking about the charging efficiency, let's call a key word "voltage drop". Voltage drop refers to the voltage difference between the two ends of the wire. For example, if a 5V power supply is connected to the input end of the wire, but only 4.8V voltage is detected at the output end, the voltage drop of the wire is 0.2V. So how does the pressure drop come about? In fact, although the material we use for the data cable is a good electrical conductor, it is not a superconductor after all. There is resistance inside. Therefore, when we connect the charger and mobile phone with the data cable, it is equivalent to connecting a resistance in series in a circuit. After the charging circuit is formed, there will be current passing through the data cable, and there will be resistance and current, A voltage is naturally generated at both ends of the wire, and the value of this voltage is the voltage drop value. The voltage drop of 5A current wire is only 0.3V, which is equivalent to 0.9W loss

iPhone 12 charger

Why is voltage drop a key word to judge charging efficiency? That's because in the charging process, the input voltage of the terminal equipment is processed by the "voltage drop" of the data cable. For a simple example, when the output voltage of the charger is 5V and the current of the charging circuit is 2a, using the data cable with a voltage drop of 0.2V means that the input voltage of the terminal equipment is 4.8V and the total input power is 9.6w; When using a data cable with a voltage drop of 0.4V, it means that the input power of the terminal equipment is only 2A * 4.6V = 9.2w, and the wire brings an additional 0.4W loss. The lower the input power is, the slower the charging speed is, which is the main reason why the voltage drop of wire can affect the charging efficiency.

 

The voltage drop of wire supporting 3A current is 0.6V, equivalent to 1.8W loss

iPhone 12 charger

Moreover, the above is often just a theoretical calculation. In fact, many terminal devices have minimum charging voltage requirements. For example, a certain device supports 5V ± 5% of the charging voltage, that is, 4.75V to 5.25V. When you use a wire with a voltage drop of 0.4V under 2A current, you may have to reduce the charging current to reduce the voltage drop because the charging input voltage is only 4.6V, or even stop charging directly. You can recover after replacing a data cable with a lower voltage. Theoretically, when the voltage drop of a data cable reaches 0.4V under 2A current, if the voltage drop of the data cable is 0.25V, then the current must drop to 1.25A. At this time, the input power of the terminal equipment is only equivalent to 4.75V * 1.25A ≈ 5.94w, which is significantly lower than the original theoretical 5V * 2A = 10W.

 

What causes the pressure drop? The line resistance plays a decisive role in the voltage drop

 Apple Charger Cable

Since the "voltage drop" has a significant impact on the charging efficiency of the data cable, which attribute of the data cable will have a significant impact on the "voltage drop"? In fact, according to the calculation formula of voltage drop "voltage = current * resistance", we can know that the resistance of wire rod will have a significant impact on the voltage drop. The resistance of wire is what we often call "wire resistance". According to the calculation formula of "resistance = resistivity * length / cross-sectional area", when the material of the cable is the same, that is, the resistivity of the wire is directly proportional to the length, but inversely proportional to the cross-sectional area. Therefore, we want to reduce the resistance of the cable, shorten the length, and increase the resistance Increasing the cross-sectional area is the most direct way.