Preventing EV fires: Top factors to consider for battery pack design in India

This article is authored by Mehul Shah, Vice President Transport BU, Nexcharge

Lithium-ion batteries in electric vehicles will be most preferred in the future and will be suitable for all types of applications. Although all lithium-ion batteries have many aspects in common, there is a variety of different types of lithium-ion battery technologies that are available in the market. Each lithium-ion battery type has its own characteristics, and this means that different types of Li-ion batteries will be used in different areas.

Factors to be considered while designing a battery pack:

Cell Chemistry:

It is important to choose the right type of lithium-ion battery for each application. Understanding the different types and their characteristics enables the right type to be chosen for any application.
Few types of most preferred Lithium-ion batteries in an electric vehicle are
“Lithium Iron Phosphate Battery, LFP”
“Lithium Nickel Manganese Cobalt Oxide, NMC”
“Lithium Manganese Oxide”
“Lithium Titanate (LTO)”

Component Selection:

Child Parts used in battery packs influence the reliability of the pack.
Cells – Based on different form factors and usage of good quality cells and chemistry
BMS – Use of quality PCM or smart BMS to monitor and ensure the safety of battery packs
Connection System – Proper selection and sizing of all electrical components

Thermal Management:

Thermal management is of major importance in high power design, particularly for automotive applications in a tropical climate. As a feature of the battery framework it is important to give air or fluid cooling ducts, pumps or fans and heat exchangers for high temperature working or heaters for operating in low-temperature environments. The design of the cells ought to be helpful for overseeing heat flow inside the pack.

Robust design:

Cell Spacing: From empirical studies we know sufficient insulation, or an airgap is recommended to avoid adjacent cells from getting heated up and sustain the thermal runway and thermal propagation.
Adequate definition or verification of weld quality
Dust and Water Ingress Protection
Crush/crash resilient design

A battery pack is a hierarchical and repetitive assembly of individual cells. The challenges in battery pack assembly process are

Different Battery Cell Types: Due to different cell size, shape, form factor, and capacity the assembly process needs to be setup for each type of battery cell type. This adds to the investment cost if the decision is to make packs with different battery cell types.

Varying Pack Configurations: The pack design changes with the application requirements which in turn changes the number of cells in the pack. Thus, varying pack configuration makes it difficult to fully automate the assembly process if the target is to cater to several different markets. The assembly lines need to be flexible to accommodate different battery pack designs and their varying rates and capacity.

Shock and Fire Hazards: The packs are assembled with cells at 40-50% state of charge. As the pack size increases the voltage and current capability of the pack increases. This increases the shock and fire hazard risk during the assembly, shipment, storage & integration process.

Auxiliary Components: Modules and packs need to have a battery management system installed for better operational control. Also, the pack may need a cooling system component. All such and other auxiliary structural, mechanical and electrical components add to the complexity of the assembly process.


Assembly of the battery pack is divided into below steps

Cell Sorting: This is most crucial step to assure the safety of the battery pack, it is a process of identifying and grouping the cells with similar parameters and characteristics, so they have similar performance during operation cycles. The common sorting method is based on voltage, capacity, and AC internal impedance (ACIR). The other sorting methods are based on electrochemical impedance spectroscopy and its equivalent model parameters, charge and discharge voltage curve, dynamic parameters and thermal behaviour. The cells with same operating voltage, capacity and internal resistance are grouped together in the battery pack, so that consistency and stability of the whole pack is assured during its life.

Module Assembly: The cells from a group are welded in a series and parallel combination to form a module. The cells can be first joined in parallel then series or first series then parallel or mixed series and parallel connections. The cells are welded either tab-to-tab or tab-to-bus bar connections. The modules are tested as per the specifications for quality assurance.

Pack Assembly: The modules are assembled in series and parallel configuration to achieve
the final pack level voltage and capacity. The pack is the housing and is sealed to avoid any internal damage. Other auxiliary components such as cooling systems and mechanical connections are also made. Finally, the master BMS is installed into the pack.

Final Testing and Storage: The pack is tested for uniform charging of all cells and other pack
parameters such as voltage, and impedance. The pack is then stored at <50% SOC in the warehouse as per first-in-first-out method (FIFO). The warehouse temperatures are maintained to approximately 25°C for safety and performance. Due to high voltages of the battery pack, there are electrical shock hazards and appropriate safety measures need to be taken during storage, handling and shipping of the packs.

It is necessary to follow these processes in the defined order so that the quality and performance of the product are maintained.

Disclaimer: Views and opinions expressed in this article are solely those of the original author and do not represent any of The Times Group or its employees.