Battery Pack Assembly Bill of Process

A generic battery pack assembly bill of process that lays out the high level steps and challenges. In this process we are going from incoming battery cells and all sub-systems to tested complete battery pack.

1. Inbound Cells

In high volume manufacturing the cell to cell variation will be specified and managed by the supplier and hence minimise the test requirements.

In smaller volumes the cells will as a minimum require OCV measurements to allow matching and parallel assembly. For demanding applications cells may need matching in capacity and internal resistance.

Challenges

  • Cell ageing in transit and storage
  • Matching cells in small production volume
  • Large cell to cell differences will require a longer time for pack first charge to allow the BMS to balance

2. Inbound Pack Sub-Systems

All parts and sub-assemblies need to be batch tested with respect to critical characteristics.

Challenges

  • Part tolerances
  • Part cleanliness

3. Cell Module / Group Sub-Assembly

Depending on the cell format the assembly of the cells into groups or modules has different bill of process.

Cylindrical Cells

Prismatic cells are mechanically stable and don’t require external support to ensure performance and lifetime. However, the cases are normally a nickel plated steel and quite thin, hence welding development and quality are key processes. By definition the expectation is that cylindrical cells are cylindrical and this isn’t always the case.

Pouch Cells

The pouch cell needs the module assembly to apply the surface pressure to the cells to maintain performance over lifetime. Welding the busbars to the cell tabs needs to ensure a quality electrical and mechanical weld. Any particulates created in the welding process need to be removed as they could pierce the cell casing.

Prismatic Cells

Although these have a hard aluminium case they do still need a pressure applied to the main face to ensure lifetime. The terminals are larger and the busbars will be larger, hence welding can require more energy and result in heat damage.

Challenges

  • Cell cleanliness
  • Welding busbars to cells without damaging the cells
  • Applying an even pressure to surface of pouch and prismatic cells
prismatic module assembly

Manufacturing, Assembly and Test Process Flow

A look at the 7 steps in a prismatic module assembly process.

4. Cell Module Testing

Electrical testing of the module at power terminals plus testing of sensors and or BMS Cell Measurement Unit.

Challenges

  • Determining busbar connection quality
  • Handling of modules if >60V

5. Assembly into Enclosure

At this stage the enclosure will have been checked (maybe on a batch process) for sub-assembly quality.

Connectors, vents and breathers will be fitted to the main enclosure.

The cooling system will be assembled and is likely to require the application of Thermal Interface Material.

The Modules / Groups of cells will then be loaded into the pack and onto the cooling system.

Challenges

  • Ensuring no electrical shorst during assembly process.
  • Ensuring part cleanliess
  • Sealing of connectors, vents and breathers to enclosure
shape of thermal interface material application

TIM Application Patterns

There are some well defined application patterns for Thermal Interface Material. Thus optimising the spread and avoiding air pockets.

6. Cooling System Pressure Test

A leaking cooling system inside a battery pack will result in the pack shutting down and in the worst case catastrophic failure. Therefore, it is important to test the cooling system integrity prior to shipping the battery pack.

Challenges

  • Test time versus reliability
  • Fixing leaks in an embedded cooling system can be difficult and expensive and hence it is important to test components and sub-assemblies to minimise waste

7. HV BDU Assembly

In high volume manufacturing the cell to cell variation will be specified and managed by the supplier and hence minimise the test requirements.

Challenges

  • Busbar mating surface cleanliness and joint torque application

8. HV Busbar Assembly

The busbars between modules are normally assembled in stages to keep the system low voltage (<60V DC) for as long in the assembly process as possible.

The BMS Assembly is likely to be done before the final busbars are put into place as that then will make the battery pack high voltage.

Challenges

  • Cell ageing in transit
  • Matching cells in small production volume

9. BMS Assembly

The Battery Management System (BMS) normally consists of several control boards that need to communicate and to be connected to embedded sensors.

Challenges

  • Software updates
  • Validating sensors are working before pack completion

10. Closing the Enclosure

Applying sealant, placing lid onto correct position and fixing.

Challenges

  • Ensuring sealing system applied correctly
  • Correct ambient conditions for curing if required
  • Moving assembly prior to being fully cured
Battery pack assembly

11. Enclosure Sealing Test

The sealing of a battery enclosure is important and likely to require testing. Due to the size of some battery packs the sealing length can be very long and hence difficult to apply consistently.

Challenges

  • Closing off breather
  • Rework procedures

12. Labelling

Battery packs need to have clear labels and serial numbers. All of the data for the pack needs to be stored (dependent on country and application)

Challenges

  • Labelling to legal requirements by country

13. End of Line Testing

In high volume manufacturing the cell to cell variation will be specified and managed by the supplier and hence minimise the test requirements.

Challenges

  • Test quality and scope versus time
  • Rework limits and process

This post has been built based on the support and sponsorship from: MAHLE Powertrain Ltd, Thermo Fisher ScientificEatron TechnologiesAbout:Energy and Quarto Technical Services.

14. Storage and Logistics

Depending on build rates it may be necessary to buffer and hence store assembled battery packs. This could mean that the battery needs to be stored in a temperature controlled warehouse to minimise ageing of the cells.

Challenges

  • Ensuring no damage
  • Storing large packs safely in case of fire

References

  1. Ryan D’Souzaa, John Patsavellasa, Konstantinos Salonitis, Automated assembly of Li-ion vehicle batteries: A feasibility study, Procedia CIRP 93 (2020) 131–136
  2. Electric & Lightweight The future for electric vehicle battery assembly solutions, Atlas Copco

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