Why should the battery wire be broken in automobile welding?

Many auto mechanics know that the negative wire of the battery must be unplugged when welding in the car, otherwise it may puncture the battery or burn out the car computer. But what is the principle of doing this, many people can't understand. Today, I will give you an analysis of why the battery wire should be disconnected when welding in the car.

As we all know, the characteristics of automobile electrical system are low voltage, DC and negative grounding. Low voltage means that the voltage of the system is only 12 or 24 volts. Even for electric vehicles, the voltage of the power system is as high as several hundred volts, and the voltage of the control system is 12 volts. Direct current means that all electrical components in the system use direct current. Although the generator generates alternating current, it is also DC after rectification, and so is the battery. Negative connection means that the negative pole of the power supply is directly connected to the car body, and the negative poles of various electrical equipment are also directly connected to the car body. Because the car body is made of metal and has electrical conductivity, it can be used as the anode and cathode of the electrical system, which can greatly save the number of wires in the electrical system.

This is a closed, safe and reliable electrical system. The generator is the main power supply, the battery is the auxiliary power supply (also the electrical equipment), and other lamps, starters, stereos, sensors and actuators are all electrical equipment. All kinds of electrical equipment are connected in parallel and do not interfere with each other when working. Even if one component is damaged, it will not affect the work of other equipment, let alone cause damage to other equipment.

So what if we add an electrical system to this electrical system? Such as electric welding.

Let me briefly talk about the working principle of electric welding. Welding machine can be simply regarded as a transformer, its output voltage is 50~70 volts, and its working voltage is about 30 volts. When welding on the workpiece, first connect the negative electrode of the welding machine with the workpiece reliably, and then clamp the workpiece with the electrode holder. At this time, the welding part will be short-circuited instantly, forming a huge current to ionize the air between the electrode holder and the workpiece, forming a conductive arc, and generating a high temperature at the welding part to melt the metal, and then cooling the welding to combine the metals. The voltage of electric welding machine is about 30 volts, and the current can be as high as several hundred amperes.

Let's simplify this circuit. Welding machine can be regarded as power supply, negative electrode wire and electrode holder wire are wires, and workpiece and arc can be regarded as loads. The simplified circuit diagram is as follows.

If we weld in the car, it is equivalent to connecting the welded electrical system to the electrical system of the car. At this time, the body becomes a workpiece, and the welding part is the entry point. In this case, let's analyze the influence of electric welding on automobile electrical system.

As we all know, when welding, the welder's negative electrode can't be directly connected to the welding point, but must have a distance. For example, if we weld a car door, the welding point is on the car door, and the negative electrode of the welding machine may be connected to the frame, so that the car body becomes a part of the wire of the electric welding system. However, the negative pole of the automobile electrical system is grounded, and the car body is also the negative pole of the electrical system, which can also be regarded as a part of the conductor. After such analysis, you will find that in the welding operation on the car, the car body actually becomes the public conductor of the welding system and the automobile electrical system. The influence of electric welding on automobile electrical system also appears here.

If the negative electrode of the welding machine is well grounded, the connection point with the car body is firm and reliable, the distance between the welding point and the negative electrode grounding point of the welding machine is short, the car body has good conductivity, and the resistance can be regarded as zero. In this case, electric welding has no effect on the automobile electrical system. Because all components of the automobile electrical system have resistance, the resistance of the welding machine system can be regarded as zero. As we all know, the characteristic of all circuits is that the current takes precedence over the nonresistance circuit, so at this time, although the current of the welding machine system is large, it directly returns to the negative electrode of the welding machine without passing through the automobile electrical system. The welding system and the automobile electrical system can still be regarded as two independent systems, only a section of wire is used, but they do not interfere with each other.

What will happen if the distance between the welding point and the negative grounding point of the welding machine is long, or if the resistance of this car body is large due to corrosion and segmentation? Let's continue to analyze.

As mentioned above, the characteristic of all circuits is that current takes precedence over nonresistance circuits. If a certain section of the circuit has a large resistance, then the current will "find another way" and automatically find a circuit with a smaller resistance, or flow together on two sections of the circuit. When welding in an automobile, if the resistance between the welding point and the negative grounding point of the welding machine is large, then the huge welding current will also look for other lines with smaller resistance, and danger will occur in this case.

All kinds of electrical equipment in the car are very precise, especially some sensors, actuators, electronic control units and so on. Their resistance is very small, generally only a few ohms or even a few tenths of ohms, and the internal resistance of the battery is only about 0. 1 ohm. The voltage they can withstand is also relatively small, and some components can't even withstand the voltage of 12 volts that comes with the system, so they need to be reduced to 5 volts before they can be used. In addition, there are many diodes, triodes, resistors, capacitors, chips and so on. Among these extremely sophisticated electronic components. If excessive current or voltage is applied to them, they will be damaged immediately.

Suppose that due to corrosion, excessive distance and other factors, a resistance of 0.3 ohm is generated between the welding point of the welder and the negative grounding point, and the welding current is 100A. Then, during the welding operation, the welding current does not completely pass through the car body between the electrode holder and the negative grounding point of the welding machine, but will automatically find other circuits to form a parallel circuit with the car body between the electrode holder and the negative grounding point of the welding machine, and the current simultaneously passes through these two lines.

Assuming that the resistance of a circuit in the automobile electrical system is also 0.3 ohm, then 50A current will be shunted to this circuit. This kind of current will enter the automobile electrical system and form a circulating current in the system, which poses a great threat to the precise electronic components (such as electronic control units, batteries, etc.) in the automobile electrical system. ), even burned or broken down by it, which is what we call burning computers and bad batteries. It can also be said that the closed and safe electrical system of the car itself was "invaded by foreign enemies", and some fragile components were directly "killed" and the system was paralyzed. This situation is particularly easy to occur at the moment of arc starting and arc extinguishing. At this time, surge voltage will be generated in the welding system, which will have a great impact on the automobile electrical system.

What if we disconnect the battery ground at this time? When the battery ground wire is disconnected, the electrical system of the automobile is disconnected from the body, that is, the negative poles of all electrical equipment are disconnected, and a loop cannot be formed. This is equivalent to closing the city gate, so that other foreign intruders can't enter, and they are no longer interfered by any external system. At this time, when welding on the car, even if the body resistance between the welding point and the negative grounding point of the welding machine is large, there is no other loop for the current to go, and it can only pass through here and will not enter the electrical system of the car. In the words of a celebrity, my feet are off the ground, the passage of virus invasion is closed, and intelligent IQ has occupied the highlands.

Therefore, when welding in an automobile, if the body resistance between the negative welding spot of the welder and the grounding point cannot be guaranteed to be zero, the battery negative electrode of the automobile electrical system must be disconnected to avoid the threat of the electric welding system to the automobile electrical system. In fact, experienced maintenance masters, when welding in cars, basically connect the negative wire of the welding machine to the place where the welding point is located, and generally do not connect the welding point and the negative grounding point of the welding machine to different parts.

It should also be noted that when welding a hydraulic cylinder in an automobile, the welding point and the negative grounding wire of the welding machine must be connected to the cylinder and the piston rod respectively. If connected in this way, the current will pass through the contact part between the cylinder and the piston rod during welding, and an electric spark will be generated at this position to form metal welding attachment. When the hydraulic cylinder is working, the adhesion point is disconnected, and the scar formed will scratch the oil seal, resulting in oil leakage of the hydraulic cylinder. This is a profound lesson we learned after repairing multi-section lifting cylinders!