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Principle analysis of common lithium battery spot welder

September 2, 2021

After years of development, the lithium battery industry has continuously upgraded the spot welding equipment and improved the quality of spot welding from the initial AC pulse spot welding machine to energy storage spot welding machine, medium frequency spot welding machine, transistor spot welding machine, and then to laser spot welding machine. However, we often see that all kinds of spot welding machines gather in the same production workshop and play their respective roles. The spot welding machines with poor performance have not been eliminated. Why? We will have an in-depth understanding of their performance from the principles of various spot welders.

The AC pulse spot welding machine controls the thyristor through the single chip microcomputer to intercept the AC pulse voltage to the primary coil of the welding transformer, and then the transformer converts the high-voltage pulse into low-voltage high current and outputs it to the spot welding needle for discharge spot welding.

The advantages of AC pulse spot welding machine are low cost and low failure rate. The disadvantage is that once the thyristor is triggered to turn on, it must wait until the AC zero crossing to turn off, which leads to the extension of the welding time. With the extension of time, the welding power becomes lower and lower. Therefore, the extended welding time is not beneficial to the welding effect. It will only increase the solder joint temperature, which is easy to reduce the success rate, reduce the false welding, increase the explosion welding, tin melting, etc. This phenomenon is very common in spot welding of nickel sheet of protective plate, because the tin under the pad of protective plate is very sensitive, which reflects the indisputable fact that the temperature of AC pulse spot welding is high. Therefore, when spot welding the negative electrode of aluminum shell or the positive electrode of steel shell, due to its small rivet area, it is not easy to dissipate heat, the sealing ring is often burned, and the spot welding of nickel sheet of protective plate is prone to false welding or pad melting.

The energy storage spot welding machine controls the charging voltage of the energy storage capacitor through the single chip microcomputer, and then controls the discharge time of the capacitor to the welding transformer, so that the secondary of the welding transformer outputs the same amount of work to the spot welding needle every time. In particular, the so-called microcomputer high-frequency inverter spot welding machines on the market are energy storage spot welding machines.

The energy storage spot welding machine has the advantages of concentrated discharge energy, short welding time and low cost. It is very suitable for battery spot welding. However, it has large welding spark spatter and high failure rate. The capacity attenuation of energy storage capacitor will also lead to the decline of welding performance. Especially in recent years, the rise of welding automation is limited by the charging time of energy storage capacitor and can not output welding energy quickly and stably. Therefore, it is only suitable for manual spot welding.

The AC input current of the medium frequency welding power supply is rectified into DC, and the DC is converted into high-frequency pulse through the inverter to the primary of the medium frequency transformer. The secondary output waveform of the transformer outputs stable low-voltage high current to the spot welding needle for discharge spot welding after full wave rectification.

The frequency and control mode of inverter welding machine are important indexes to judge its performance. Generally, 1-10khz inverter frequency is collectively referred to as intermediate frequency, and more than 10kHz is referred to as high frequency. In the control mode, the primary constant current is an ideal control mode. The primary constant current adopts closed-loop control, which can adjust the high-frequency pulse width according to the actual output current. If it is worse, it adopts fixed pulse width modulation, which belongs to open-loop control. Because the spot welding needle and the weldment itself will have a great impact on the welding during spot welding, the stability is relatively poor. The frequency of medium frequency welding power supply is generally better at 4-5khz. On the one hand, stable welding waveform can be obtained at the output end. On the other hand, large discharge current can be fed back and controlled in this frequency band. The transformer needs a certain response time. The frequency is too high and the current is not large.

Transistor welding power supply is the most ideal resistance welding power supply. It does not need welding transformer. The current rises quickly and directly outputs the current waveform at high frequency. You can choose constant current, constant voltage or constant current and constant voltage control mode, but it also has the disadvantage of high price.

The laser spot welding machine is composed of crystal, xenon lamp, condensing cavity, optical resonator, cooling filter and laser power supply. In the battery industry, the welding of steel shell and aluminum shell cover plate has long been widely used, and spot welding of polymer pack protective plate has also been used in recent years. Compared with resistance welding, laser welding machine has the advantages of no need to grind the spot welding needle, firm welding, uniform welding spot and not easy to produce false welding, but it also has the disadvantages of high price and high maintenance cost.

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