How to solve the problem of inconsistent solder joint size and inconsistent soldering pin consumption?
In today's rapidly changing new energy technology, cylindrical cells are an important part of lithium-ion batteries, and their performance and manufacturing process are directly related to the overall efficiency and safety of the battery pack.
In the assembly of lithium battery PACK, multiple cylindrical cells need to be connected in series or in parallel to meet the required voltage and capacity requirements. The lithium ion battery spot welding machine is a device specifically used for the welding process in the assembly process of cylindrical batteries. Its working principle is to use a momentary large current to pass through the battery tabs and busbar of the cylindrical battery, so that the metal material at the contact point reaches the melting point in a short time, and then the metal is fused together under the action of pressure to form a firm solder joint. This welding method is based on the resistance thermal effect. When the current passes through two contacting metal parts, due to the resistance of the metal itself, according to Joule's law (Q=I²Rt, where Q is heat, I is current, R is resistance, and t is time), heat will be generated at the contact point, thereby achieving welding.
Automatic Double Sided Spot Welding Machine
However, in the welding process of cylindrical cell PACK, a phenomenon that cannot be ignored, the Peltier effect, often brings challenges to production.
Challenges of the Peltier effect in cylindrical battery PACK welding
The Peltier effect, first discovered by French scientist J.A.C. Peltier in 1834, is a thermoelectric phenomenon. When current passes through a circuit composed of two different conductors (or semiconductors), heat absorption or heat release occurs at the joint. This phenomenon arises from the energy level difference of charge carriers in different materials. When the charge moves from a high energy level to a low energy level, it releases energy, which is manifested as heat release; conversely, it absorbs energy, which is manifested as heat absorption. The Peltier effect is not only reversible, but the intensity of heat absorption or heat release is closely related to the current size, conductor properties and hot end temperature.
The resistance welding of cylindrical battery packs generally adopts parallel welding. Under this welding method, even if the left and right electrodes are under the same working conditions (welding pressure, electrode material, contact resistance, welding current, power-on time, etc.), the welding spots are often uneven on the left and right.
So, why are there problems such as "uneven size of left and right welding spots" and "inconsistent length of left and right electrodes" when the working conditions on the left and right sides are the same?
This is because when it comes to the connection of different metals or semiconductor materials, the current passing through the welding point may cause local heat absorption or heat release. If the heat is uneven, it will affect the welding quality and even the performance of the battery.
The uneven left and right welding spots in parallel welding are prone to problems such as cold welding, sticking needles, and over-welding. This not only has a great impact on the welding quality of the PACK, but also the consumption speed of the electrodes on both sides is quite different. The electrode with sticking needles on the left is shorter, and the electrode on the right is longer; and the difference in welding needle length will cause a difference in welding pressure between the left and right needles, affecting the stability of welding quality.
For the Peltier effect in the welding of cylindrical battery packs, effective solutions can be sought from the following aspects:
1. Adjust welding pressure: If it is a unipolar power supply, when the left side has sticky needles and cold solder joints, the welding pressure on the left side can be appropriately reduced to make the welding pressure on the left side lower than that on the right side. In this way, the contact resistance R between the left battery and the pure nickel connecting piece will increase. According to the energy formula Q=0.24I²Rt, after the welding is powered on, the resistance of the left solder joint increases, and the welding energy on the left side also increases accordingly, which will balance the welding energy on the left and right sides.
2. Use a bipolar power supply: After the power supply is powered on, first release a positive waveform current, and then release a negative waveform current. Using the reversible characteristics of the Peltier effect, the heat-absorbing point under the positive waveform becomes a heat-releasing point under the negative waveform, which can make up for the welding difference between the left and right sides of the positive waveform. Through debugging, after the two power-ons are superimposed, two weld cores of basically the same size are formed, and a relatively uniform welding residue is obtained.
3. Material selection and pretreatment: Select materials with small thermoelectric effect and good welding performance, such as nickel-plated connectors. The nickel plating layer not only enhances the weldability of the connector, but also weakens the Peltier effect to a certain extent. In addition, the surface of the battery cell should be thoroughly cleaned before welding to remove impurities such as rust-proof oil that may affect the welding quality.
4. Use high-quality welding pins: Optimize the design of welding pins, select materials with stable quality, improve the high temperature resistance and wear resistance of welding pins, further extend the service life of welding pins, and reduce the inconsistency of solder joint size and welding pin consumption caused by welding pin quality problems.
Welding Needles of Battery Spot Welding Machine
5. Strengthen heat dissipation measures: Set up effective heat dissipation devices in the welding area, such as heat dissipation copper column bumps or heat sinks, to quickly export the heat generated during the welding process and reduce the impact of welding points on the inside of the battery cell. Heat dissipation copper column bumps are particularly suitable for hot spot cooling due to their small size and high density, and can effectively reduce the local temperature rise caused by the Peltier effect.
6. Monitoring and feedback mechanism: Establish a real-time monitoring and feedback mechanism for the welding process, use temperature sensors and other equipment to monitor the temperature changes in the welding area, and immediately adjust the welding parameters or take other remedial measures once an abnormality is found to ensure welding quality and safety.
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