Automotive EV Industry uses high energy density Lithium-ion cells to meet the range requirement. Popular cells are made of NMC (Lithium-Nickel-Manganese-Cobalt-Oxide) or LFP (Lithium iron phosphate) which are used on the positive electrode, graphite as the negative electrode and a polymer-based film as separator between the two.
During charging and discharging cells are subjected to Joule heating because of resistance of ion movement in the electrolyte and the electrodes. This will vary depending on the drive cycle and Indian ambient conditions. In the absence of effective thermal management, the battery packs can get warm and through thermal runway at approx. 120°C to 140°C (Rapid cathode decomposition and electrolyte oxidation in the presence of air) even catch fire.
Through a key focus on thermal management in battery pack design, thermal events can be avoided.
Apart from thermal runaway other parameters related to design also needs attention.
A few references from the design observed in EV battery packs
- Direct contact of cell to cell, no cell holders – this leads to thermal propagation
- Cell Interconnection done by spot welding in a crude way
- Plastic insulation around the cells, which traps heat
- Bad Wire harness routing and Module packing. No fuse in voltage sensing wires.
- Bad spot welding, leads to local resistance, irregular cell aging -> local heating-> venting or thermal runaway initiation at cell level
For battery packs to be safer and reliable, following points are recommended
- Component Selection – Child Parts used in battery pack influence the reliability of the pack.
- Cells: Usage of good quality cells and testing prior before use is mandatory
- Cell Holders: Non-Flammable grade selection instead of generic plastic
- Bus Bars: Proper thickness, size and coating
- BMS: Use of quality PCM or smart BMS to monitor and ensure the safety of battery 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 (thermal propagation).
- Adequate definition or verification of weld quality
- Dust and Water Ingress Protection
- Preference to be given to cell connection such as wire bonding over spot welding that help to reduce the cell temperature gradient.
- Enable automated round the clock battery monitoring, use data analytics to increase awareness & abuse feedback
- Automation of key manufacturing processes will increase the product quality and durability
- Proper definition of welding/joining techniques
- Proper Wire Harness Assembly
- Optimum creepage clearance during pack assembly
- Handling & storage
- Design Validation to be done for shipping and handling
- Proper handling during transportation and adopt UN38.3
- Safe handling before and after installation of battery into vehicle
- Usage of battery pack
- User education for correct use of the battery pack
- Use of authorised chargers
- Avoid charging near heat source
Researchers are changing key features of the lithium-ion battery to make an all-solid, or “solid-state,” version. Hereby the liquid electrolyte is replaced with a thin solid electrolyte that’s stable at a wide range of voltages and temperatures. With that solid electrolyte, a high-capacity positive electrode and a high-capacity, lithium metal negative electrode can be used that’s far thinner than the usual layer of porous carbon. Those changes make it possible to shrink the overall battery considerably while maintaining its energy-storage capacity, thereby achieving a higher energy density. These features — enhanced safety and greater energy density — are probably the two most-often-touted advantages of a potential solid-state battery. It will probably take the better part of the decade before this technology is widely adopted and in mass production around the world.
EV batteries undergo tremendous stress during their long life. The two leading stressors are fast charging and extreme temperature. The battery’s capacity to accept charge varies significantly with temperature. Traditional battery management systems do not adjust the degree and rate of charge based on health. Temperature gradients across the pack cause uneven degradation, especially during fast charging. Therefore, any one of the hundreds of cells in the battery pack can develop an internal short and ignite the whole pack.
A more intelligent approach is to control the degree and rate of charge in real-time based on detailed diagnostics of the cells and the pack.
There are two other complementary ways to improve battery safety for safer EVs:
- Develop safer cell chemistries. Considerable research and investments are pursuing new technologies. An intelligent approach would be required to control the degree and rate of charge in real-time based on detailed diagnostics of the cells and the pack
- Deploy software intelligence to predict the rare presence of defects and ensure the batteries operate safely, the predictive safety software stack is essential to identify the rare battery defect before they lead to fire comes standard.
We believe that intelligent software with predictive capabilities is essential to provide additional safety safeguards. Moreover, the technology is available today and can deliver the required safety at a fraction of the cost.
Failures can appear suddenly and violently, with the potential of severe injuries, product recalls, and loss of reputation. It is crucial to invest in intelligent software to monitor battery diagnostics, predict the presence of defects, and be prepared to remove hazardous batteries from circulation before they become catastrophic incidents.
Views expressed above are the author’s own.
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Tags: #batteries #safer #reliable #Indian #market