After the recent fire incidents with Ola Electric, Okinawa and Pure EV electric 2Ws, everyone is wondering why are Lithium-ion batteries catching fire?
Lithium-ion batteries are classified under the dangerous goods category. In this article, Rahul Bollini explains the multiple reasons an EV battery pack can catch fire and a few tips for the nascent Indian EV industry to get the battery safety issues under control.
A Lithium-ion battery pack comprises of multiple Lithium-ion cells in series and parallel arrangement to achieve the required voltage and capacity. A Lithium-ion cell catching fire is termed as a thermal runaway in technical terms. Thermal runaway is caused by a chemical reaction that has a ripple effect inside the cell and results in fire. It is the nature of Lithium-ion cell chemistries such as NMC (Lithium Nickel Manganese Cobalt Oxide) and NCA (Lithium Nickel Cobalt Aluminium Oxide). This reaction is very difficult to stop once it has begun. It begins when the cell reaches high temperatures leading to an internal chemical reaction, which further increases the heat generation and the next thing we know there is smoke and fire coming from the cell. The chemical reaction leads all of the energy of the cell being released at the same time and all this happens in a very short period of time.
Let us discuss the reasons that lead to thermal runaway:
Internal short circuit of a Lithium-ion cell due to physical damage
Physical damage can lead to an alteration in the internal structure of the cell. When the structure changes internally, the cell can no longer be expected to perform normally, and it is a matter of time before the cell will short circuit. External mechanical vibrations could also cause physical damage.
Improper heat management: No batteries are perfect when it comes to charge-discharge efficiency. Efficiency losses are more during fast charging and high-power discharge. The energy loss is in the form of heat. This heat needs to be carefully managed and the cells should not be exposed to heat for too long. Long term effects of exposing the cells regularly to high temperatures include faster degradation of the cell’s capacity. If exposed to higher temperatures, the cells should not charge/discharge anymore and it is the duty of the BMS to cut off the battery operation. BMS failing to cut off the battery at this instance can quickly accelerate thermal runaway of the cells. BTMS (battery thermal management system) is a good way to tackle this. However, most electric 2Ws on roads have no thermal management and hence electric 2W are seeing a number of fire incidents compared to other vehicle segments. The cells in these battery packs are very tightly packed leaving no breathing space. BTMS is an emerging field and needs more R&D focus.
Overcharging: Charging the battery beyond its limit can rapidly elevate a chain reaction inside the cell. It is the duty of the BMS to perform functions such as overcharge protection, under-voltage protection, continuous current, over-current detection and over-temperature protection. The battery companies must use reliable BMS that doesn’t fail on the field.
Most low-cost BMS come with one year warranty, and they fail after the warranty period. Some BMS also fail due to their inability to handle regenerative energy coming back from the electric motor. This is very common in electric 2Ws using universal controllers that have the ability to handle 48V and 60V. The reverse energy is unregulated and slowly damages the BMS over time.
Overusing the battery after its EoL (end-of-Life):
Lithium-ion cells come in two grades – EV (electric vehicle) grade and ESS (energy storage
system) grade. EV grade cells are called power cells; ESS grade cells are called energy cells.
EV grade cells have lower internal resistance, allow for fast charging and have higher maximum continuous discharge capability. These cells have a shorter life and a lower cycle life. On the other hand, ESS grade cells have higher internal resistance, do not allow for fast charging and have lower maximum continuous discharge capability. These have excellent cycling performance with high cycle life.
EV grade cells are expensive, and the way they are designed internally is different from ESS grade cells. An EV grade cell loses the capability to behave like one after it has cycled for a given number of cycles. At this point, the cells have developed higher internal resistance, and they become unfit to be used for an EV application. The battery should no longer be deployed for an EV application. For example, Tesla says its battery needs to be replaced when degraded to 70% of its original capacity. Some companies set this limit to 80%. This terminology is called as end-of-life of the battery in the EV application. The vehicles must have inbuilt systems tracking the battery’s health and notify the user when the battery has become unfit to work in an EV, e.g. how Apple sends out a notification to iPhone users to replace the battery when the battery health has hit 80%.
If a battery is used in the EV application beyond its end-of-life, the higher internal resistance will generate a high amount of energy losses in the form of heat during charging and high- power discharge. In this case, the battery constantly experiences high amounts of heat, and it can lead to constant battery cut-offs from the BMS due to the over-temperature cut-off function. Constant cut-offs can lead to longer charging times and the vehicle turning off constantly during driving. In a rare scenario, it can also lead to thermal runaway.
Using lower grade cells in EV application:
EV applications need A grade EV cells. Many battery packers are using B grade cells for EV applications, which can at the most be used in small ESS applications. I wouldn’t suggest them even for higher capacity ESS applications. B grade cells seem to perform similar to A grade cells during initial charge-discharge cycling; however, they are not going to perform uniformly after ageing a few hundred cycles – this is when they create balancing issues. B grade cells also tend to have a sharper rise in internal resistance with ageing. Balancing issues and the sharp rise in the internal resistance can lead to lower output overall and generate heat due to some cells heating up.
A common scenario with B grade cells is when one NMC cell in the pack hits the upper cut-off voltage of 4.2V and the rest of the cells are not fully charged, and the battery pack stops charging. A similar scenario during discharging is when one cell hits the lower cut-off voltage faster than the others, and it leads to the cutting off of the battery pack energy supply. B grade cells have a lot of variations in their performance since there is no fixed way of determining why a cell has been classified as B grade.
As a professional working on Lithium-ion cells at a mass manufacturing level, I can share that the grading of Lithium-ion cells is done on the following basis:
- Self-discharge in terms of millivolt/day.
- Internal resistance at a particular SoC (state of charge).
- Nominal voltage and voltage range.
- Capacity of the cell.
- Cosmetic defects.
Some mass manufacturing companies have up to 9 varieties of B grade cells starting from B1 up to B9, while some simply grade them as A, A-minus and B. Just think of the combinations of cells that fit in these categories and imagine them working with each other. It cannot get any more non-uniform than this.
Another kind of B grade cells are refurbished cells. These cells previously had higher capacity in terms of mAh and now they have reached their end of life. These are dismantled from the battery pack and sold as a new lower capacity cell with a new PVC cover. These cells are definitely not fit for EV applications, and it is hard to figure out where the received cells are refurbished since they are imported from outside.
In a nutshell, here are some tips to avoid thermal runaway from Lithium-ion cells:
1. Avoid physical damages to the battery.
2. Explore basic BTMS options and not to forget that India’s climate is hotter than other countries that are seeing high rates of EV adoption.
3. Use a good and reliable BMS that has branded MOSFETs and ICs.
4. Validate the BMS well before deploying it in a large number of battery packs.
4. Recall the EV battery packs at the end-of-life period.
5. Use A grade cells for EV applications.
About the author:
Rahul Bollini is an independent R&D consultant in the field of Lithium- ion cells and batteries with 7 years of industry experience. The author can be reached at firstname.lastname@example.org and +91 72049 57389.
Subscribe today for free and stay on top of latest developments in EV domain.