A single battery cell cannot move the entire vehicle, hence a collection of thousands of battery cells connected in series and parallel should be designed in order to meet the power, capacity, and voltage requirement as per the application.

Parallel connection increases Ah capacity and the Series connection increases the Voltage.

Here are some of the popularly used welding and bonding techniques in battery manufacturing today:

  1. Spot welding/resistance welding
  2. Ultrasonic welding
  3. Laser welding
  4. Wire bonding
  5. Tab bonding

Spot welding:

This welding process is used primarily for welding two or more metal sheets, in case of battery it is generally a nickel strip and positive terminal/negative terminal of the battery together by applying pressure and heat from an electric current to the weld area.

Pic: Spot welding machine

Advantages:

  • Low initial costs.
  • Manual, Semi-automatic and Automatic all three types of steps possible.
  • Easy to train, no high skill set required.

Challenges faced by using Spot welding:

  • Low flexibility.
  • Multi-layer welding not possible
  • suitable for welding partners having a low conductivity
  • Generates heat, which may damage or compromise the cell terminals. Spot welding cannot be used to weld components internally of the cells like tabs and cap.
  • Height variation cells cannot be welded, because and nickel strips are resistant to bends.
  • Cannot be used for complex battery design or shape.

Ultrasonic welding:

Ultrasonic welding is a solid-state welding technique. In this type of welding workpieces are not melted but pressed and scrubbed together with high frequency vibrations hence no need of electrode, filler material. The solid-state weld is formed through the high-frequency motion between the parts causing continuous shearing and plastic deformation, while also removing any oxide layers or contaminants. Most metals can be ultrasonically welded and the method is excellent for welding together thin foils, as well as thicker sheets (<3mm) which is very promising in battery applications (Tab welding, Busbar, nickel strip welding).

Pic: Ultrasonic welded pouch cell to thin metal plate, Pic credit: XIAOWEI INTELLIGENT

Watch here a video of Ultrasonic welding of pouch cell

Advantages:

  • Good for thin sheets, wires, or multi-layer sheets
  • Easily applicable on pouch cells
  • Heat generation only on specific area of application, hence used in critical cell components like tab welding, foil welding
  • No filler material or gases released
  • Low energy consumption and joining of different materials is possible.
  • High speed and precision

Challenges faced by using ultrasonic welding:

  • Sometimes the sonotrode might stick to the work, which can damage critical cell components.
  • Clamping battery may be critical: clamping is necessary because of high frequency of vibrations which also is a drawback.
  • High cost of spare parts.

Busbar Thickness: 0.5 mm to 2.5 mm

Laser welding:

In this type of welding, weld is made by generation of heat.

Heat is generated by a concentrated, high energy laser beam directed at the joint to be welded.

Due to its tailorable spot size, laser welding is an excellent joining method for thin or delicate metal parts like tabs, and while cutting application in the electrode.

Pic: Laser welding of prismatic battery, pic credits: Laserax

Video of Laser welding of Prismatic cells.

Advantages:

  • High precision of the weld
  • High speed
  • Thick and thin materials can be welded easily with high flexibility as compared to ultrasonic and spot welding
  • Can be used to weld critical parts like battery tabs and foils.
  • Automation possible

Challenges faced by using laser welding:

  • Quality control is difficult
  • High initial cost
  • Process monitoring is challenging, Due to material reflectivity
  • Heat generation

Bonding:

Wire Bonding:

Wire bonding is well matured technology which was invented for the semiconductor industry and standard technology for semiconductor chips since 1970s, and also Tesla and ola electric batteries are wire bonded.

The wires shall be of aluminium and have a diameter of 0.28 – 0.41 (mm). The collector plate shall be of any electrically conducting material, preferably metal.

The process takes place in room temperature and does not require external heat to weld to take place.

The aluminium wire will also act as a fuse, it will disconnect itself from the cell whenever extreme current flow through it.

Pic: Wire bondin, Pic credits: EVWEST

 

Video of wire bonding process

Advantages:

  • High productivity and efficiency
  • Makes battery packs safer by the virtue of fuse link effect per cell
  • Battery packs can be lighter as busbars and nickel strips are eliminated/reduced
  • Flexible, cells on various heights and distance can be easily joined
  • Bonds can be easily removed in case of defective manufacturing
  • Fast and fully automated manufacturing
  • Low resistance and better battery performance

Challenges while using Wire bonding:

  • Massively applied to cylindrical cells as of today!
  • Strength of bond is low
  • Complex manufacturing, hence requires training
  • The upper limit of round wire’s cross section was used to be less than or equal to 500µm.

Tab Bonding:

Tab bonding is somewhat like wire bonding but here tabs/terminals are needed to be pre-position before doing the bonding.

In tab bonding both the positive and negative terminal (At the edge) are connected on one side so the cooling of battery pack is easier on the bottom side.

Pic: Tab bonding, pic credits: KnS

Advantages:

  • Tabs can be removed easily, compared to busbar/nickel strip welding
  • Cold process
  • High speed automation as flipping is avoided while welding

Challenges while using tab bonding:

  • Pre placing of tabs/terminals is mandatory, makes process slow
  • Comparatively less flexible than wire bonding

Trends now and future potential:

Format

Current Interconnect legacyFuture interconnect
Spot welding

Ultrasonic wire bonding

Laser

Laser welding

Ultrasonic wire bonding

Spot Welding

Heavy ultrasonic weld

Laser Welding

Mechanical Clinching

Laser Welding

Heavy Ultrasonic Welding

Mechanical Clinching

Laser Welding

Mechanical assembly

Laser Welding

Mechanical assembly

Source: Li-Ion Battery Pack for Automotive and stationary storage applications Yolep.50