Electric Vehicles (EVs) are crucial in the fight against Global Warming & Climate Change because transport is a sector tough to decarbonize. Welding becomes important as it has a direct bearing on the safety, performance, and throughput of EV batteries.

Electric Vehicles (EVs) & the Decarbonisation of Transport

Welding technology used for EV battery assembly must minimize the cell-to-tab electric resistance for top battery performance and safety [1]. Thermal runaway is always a hazard given the hyper energy density of EV batteries [2]. Improper connection escalates this risk by increasing the cell-to-tab resistance [3].

Engineers choose the weld technology based on the production scale, battery cell geometry, and cost [1]. Weld joints have strength comparable to the parent material’s, making welding a preferred joining method [4].

EVs help decarbonize road transport, a tough task [6]. The sector emits 16% of the world’s greenhouse gases (GHGs) [5]. The Paris Agreement mandates limiting temperature rise to 1.50C [7] above the pre-industrial levels to avoid runaway Climate Change. This requires Net Zero Emissions by 2050 [7].

Net Zero requires 300 million electric cars in the world by 2030 [5]. As of end-2022, there are around 26 million EVs (including hybrids) [8]. Although improvements in EV range, policy support, and public spending have fuelled the recent EV demand, making 2021’s worldwide weekly sales exceed 2012’s annual sales [9], this is not enough.

China hosts 46%, Europe 34%, and the United States 15% of the worldwide EVs. Eventually, these markets will saturate. India, Southeast Asia, Brazil, Japan, and Mexico will then drive the demand. More than 50% of new sales of India’s 3-wheelers are electric. Latin America is rapidly electrifying taxis. Vietnam is fast migrating to electric bikes [8].

Cybernetik - EV Battery Welding - 02

Welding Technologies for EV Battery Assembly

Assembling lithium ion batteries is complex. These batteries have multiple layers with countless joints, and house different metals of varying thicknesses [10]. Welding technology used for EV battery assembly must deliver:

  • Least contact resistance between the connection tab and the cell to cut energy loss via heat generation [10].
  • Least inter-cell electrical resistance to reduce electrical losses to ensure high torque via large peak current [11].
  • Establish strong inter-cell mechanical connections to withstand vibrations [11].
  • Use minimal weld energy to prevent excessive heating of the internal cell separator [11].

High quality welds are of uniform width and have no gaps. Thin welds are more suited for EV battery packs. Thicker ones can penetrate the casings. Proper welds address the following welding challenges [1] & [10]:

  • Thermal & Electrical: Minimize contact electrical resistance, thermal fatigue, and thermal input.  
  • Metallurgical & Material: Establish robust connection between different surface types and dissimilar metals.
  • Mechanical: Create joints with sufficient strength without vibrational energy or residual stress.

Technologies used for EV battery welding are:

  • Resistance Spot Welding (RSW)
  • Laser Beam Welding (LBW)
  • Ultrasonic Welding: Ultrasonic Metal Welding (UMW) & Ultrasonic Wire Bonding (UWB)

Weld quality depends on [12]:

  • Welding technology
  • Joint geometry
  • Material
  • Cycle time
  • Weld access
  • Budget

Following features of the EV battery welding set up determine weld quality [12]:

  • Overall process management
  • Loading and unloading
  • Process data management
  • Fixtures
  • Weld quality checks

Equipment or system designers prefer the following communication protocols to achieve the transmission speed, processing capability, storage space, and networking that Industry 4.0 solutions demand [12]:

  • EthernetIP
  • Modbus TCP/IP

Resistance Spot Welding utilizes [1]:

  • Pressure in combination with electric current for welding without shielding gases or filler material.
  • An inverter to convert input current from AC to square wave DC, which is amplified and filtered subsequently.
  • Dual pulse current with the second pulse joining the metals after the first has eliminated oxides and contaminants on the metal surface.
  • Optimized applied current levels.
ProsConsCompatibility with Battery Types
Automation friendlyLimitations in joining high conductivity materialsSmall Prismatic
Does not require shielding gas or fillerLimitations in dissimilar metal weldingHình trụ
Contact process enables excellent quality controlElectrode sticking to base metal
Contact process eliminates need for fixturesQuality variance between different welds
Low capital requirementsSlower
Compatible with thin sheets

Table 1: Resistance Spot Welding (RSW) Features [1], [10] & [12]

Three main process parameters for spot welding are [1]:

  • Welding time
  • Electrode force
  • Welding current

Weld current is critical to weld quality, and needs to be focused. Electrodes with dome-shaped ends are used to focus the current. Low applied current fails to counter stray current from battery sources while excessive levels cause electrode sticking. Weld current is affected by [11]:

  • Electrode resistance and contact area
  • Material interface between parts
  • Current control

AI integration in RSW maps the relationship between weld parameters and quality. Apart from decreasing quality variations, automation slashes RSW’s operational expenses related to [1]:

  • Năng lượng
  • Labour
  • Material
Cybernetik - EV Battery Welding - 01

Factors that improve RSW quality are:

  • Sensors for force and displacement measurement [12]
  • Rapid rise in squeeze times
  • Polarity switching [12]
  • Closed loop control over feedback [12]

Monitoring electrode wear is the chief quality-related concern for RSW after process optimization [12]. Welding guns and power sources form the main capital expenses, making it less costly than others. Inverters slash operational expenses by improving uptime, quality, and yield [1].

Throughput is usually around 1 weld per second, and depends on:

  • Material and its thickness [1]
  • Surface finish and material coating
  • What moves during welding – weld heads or cell modules [1]

RSW is not suited for welding [1]:

  • Metals with high conductivity because electrodes must have greater conductivity than metals.
  • Dissimilar Metals as these have different melting points and the control system maintains constant temperature.

Laser Beam Welding uses a highly focused, minimally diffracted beam with a tiny spot size. Hyper beam intensity causes surface metal atoms to evaporate and generate vapour pressure, which creates a keyhole i.e. a depression in the molten material. The keyhole and the beam reflections inside the cavity boost the energy efficiency of laser welding [1].

ProsConsCompatibility with Battery Types
Automation friendly, being a non-contact, simple-to-imitate processComplex execution requires dedicated experience and trainingLăng kính
Low electrical resistance jointsExcellent joint fit-up essentialHình trụ
FastTough to weld high reflectivity metalsPouch
AccurateCapital intensiveUltra Capacitor
Suitable for narrow areas with low accessibility
Limited heat affected zone
Suitable for mass production
Minimal/zero post weld processing
Easier control over depth and width

Table 2: Laser Beam Welding (LBW) Features [1], [10] & [12]

Two primary lasers used for EV battery pack welding are [1]:

  • Pulsed Nd:YAG i.e. Yttrium Aluminium Garnet doped with Neodymium
  • Fibre Laser

Pulse rate, welding speed, and laser power influence [1]:

  • Geometry of the weld bead
  • Weld quality
  • Weld dimensions that increase with rising laser power but fall with climbing speed

Other influences on LBW quality include:

  • Gap between the parts being welded – minimal gap maximizes quality
  • Material coating
  • Surface finish
Cybernetik - EV Battery Welding - 03

Chief concerns for laser weld quality after optimisation are the maintaining of proper [12]:

  • Interface between the part and the weld at the focus of the laser
  • Part fit up

ANSI 136.1 dictates the use of a non-light-transparent enclosure for laser welding [12]. Thin welds are more suited for EV battery packs. Thicker ones can penetrate casings. Close parameter and process control prevent the high power density of the laser from causing unintended damage [1].

LBW speeds depend on the materials being welded and their thicknesses [12]:

  • Maximum: 20 welds/second.
  • General Range: 100 milliseconds/weld for mass production to 1-2 seconds/weld for low quantity production.

Ultrasonic Welding can be Ultrasonic Metal Welding (UMW) or Ultrasonic Wire Bonding (UWB). The mechanism for UMW is not completely understood. Most likely, a combination of 20 kHz oscillation and pressure form the joint via [1]:

  • Micro melting
  • Metallic interlocking
  • Chemical bonding

Sheets and foils of diverse metals and varying thicknesses are seam and spot welded via ultrasonic welding. UMW is automation friendly, its parameters can be monitored via software for data analysis, and it fits into existing manufacturing lines [1]. 

UMW does not melt the metals, and provides joints of the required strength with minimal or no brittle intermetallic zone. Such features make it suitable for connecting metals with high reflectivity and conductivity [13]. The nullification of melting makes it a low energy [14] and, therefore, less expensive process [1].

ProsConsCompatibility with Battery Types
Particularly compatible with dissimilar, thin, and numerous layers of metals with high conductivityRequires access from both sides with anvil on one side and sonotrode on otherPouch
Suited for welding high reflectivity materialsError prone when welding materials with high surface roughness
No filler metal or shielding gas requiredRequires excellent fixtures as vibrations can damage battery cell structure
Joints free from typical metallurgical issues viz. hot cracking, porosity, and bulk intermetallic compoundsUnsuited for hard materials as these do not vibrate much
Solid state welding enables lower energy inputs and shorter weld timesCannot weld think joints as these do not vibrate much
Lower production costCan only weld lap joints as it requires access from both sides
Low energy usageSonotrode is prone to sticking as built up metal on its surface bonds with workpieces of same metal

Table 3: Ultrasonic Metal Welding (UMW) Features [1] & [13]

A variant of UMW, UWB is used widely in microelectronics where access is available from one side only. A sonotrode welds a continuously fed wire to successive substrates via pressure and vibration.

UWB provides the same positives as UMW. However, it requires tight fixtures as lateral movement lowers the amplitude of ultrasonic vibration, so essential for producing robust joints. And because it can weld only small gauge wires, UWB can deal with low currents only [1].

Determinants of UMW and UWB weld quality are [1]:

  • Machine Parameters: Oscillation amplitude, welding energy, welding force.
  • Material Quality: Top part geometry affects weld energy absorption. Bottom part roughness influences friction.

Weld Technology Selection

Manufacturers choose welding technology based on [1]:

  • Cell Geometry
  • Overall Cost
  • Material Properties

Manufacturing Readiness Level (MRL) of various EV battery related weld technologies are as follows [1]:

  • LBW has highest MRL for cylindrical and prismatic cell welding. Micro-TIG, UWB, and RSW follow in that order for these two geometries.
  • LBW and UMW are tied at first spot for pouch cell welding. Micro-TIG and RSW come later in that order.

Guidelines to select the most compatible method [1]:

  • Except for UMW, which is unsuited for cylindrical cells, other methods are broadly ok with all cell geometries.
  • LBW is best for high weld strengths.
  • RSW’s low overall cost and simplicity are suited for smaller scale productions.
  • UMW and UWB are most suited for dissimilar metals.
  • LBW and UMW are compatible with high conductivity (thermal and electric) materials, RSW is not.
  • RSW and UMW do a good job with materials of large surface reflectivity, LBW does not.
  • LBW transfers immense heat on the substrate.
  • Micro-TIG and RSW involve hyper heat transfers.
  • For cylindrical cell welding:
    • LBW and UMW are more automation friendly than RSW or Micro-TIG.
    • RSW and Micro-TIG are least expensive per battery connection.
    • LBW has the same cost for each battery connection as UWB.
    • LBW is the fastest.
Cybernetik - EV Battery Welding - 04

Cuối cùng

Production scale, battery cell geometry, and budget have the strongest influence on the selection of welding technology. Whichever technology is chosen, the need to minimize the cell-to-tab electric resistance remains constant.

Tài liệu tham khảo

  1. Harald Larsson et al. “Welding Methods for Electrical Connections in Battery Systems.” Uppsala Universitet. June 2019.
  2. Jaywant Mahajan. “Basics of Battery Welding.” LinkedIn. 22 July, 2022.
  3. Peyman Taheri et al. “Investigating electrical Contact Resistance Losses in Lithium Ion Battery Assemblies for Hybrid & Electric Vehicles.” Journal for Power Sources. 2011.
  4. George S. Baker. “Welding, Brazing, and Soldering.” Britannica Kids.
  5. Leonardo Paoli. “Electric Vehicles.” International Energy Agency (IEA). September 2022.
  6. Hannah Ritchie et al. Our World in Data. “Electricity Mix.” 2020.
  7. United Nations Climate Action. “For a Liveable Climate: Net-zero Commitments must be Backed by Credible Action.”
  8. Colin McKerracher. “The World’s Electric Vehicle Fleet will Soon Surpass 20 Million.” Bloomberg. 8 April, 2022.
  9. International Energy Agency (IEA). “Global EV Outlook 2022.”
  10. Light Mechanics. “Laser Welding Applications in EV Battery Pack Assembly Lines.”
  11. “Information on Battery Tab Pack Welding: Technical Details about Material & Design of Bus Bar.”
  12. Geoff Shannon. “Improve Tab to Terminal Connections in Battery Pack Manufacturing.” Amanda.
  13. Rafael Silva et al. “Development & Evaluation of the Ultrasonic Welding Process for Copper-Aluminium Dissimilar Welding.” Journal of Manufacturing and Material Processing. 2022.
  14. Alex Yeung. “Ultrasonic Metal Welding: Empowering Advances in Battery Tech.” Electronic Design. 12 April 2021.


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