Over the course of the last three decades, lithium-ion (Li-ion) batteries have become the dominant rechargeable battery technology, offering long service lives and excellent power density.
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This can be seen in almost every major auto manufacturer – in just the last two years, most have made a major shift toward electric vehicles (EVs) that are powered by Li-ion battery technology and are phasing out cars and trucks with internal combustion engines.
This shift is certainly a revolution in modern transportation, but there are high levels of concern surrounding the adoption of this new type of vehicle, namely regarding practicality and safety – stemming largely from occurrences of thermal runaway, where the energy in a cell is uncontrollable and is transformed into heat, often violently.
To meet the demands of use by electric vehicles, the adaptation of existing sensor technology in the introduction of new system monitoring devices is required.
In the event of thermal runaway – which matters for all vehicles powered by Li-ion batteries – sensor technology that is robust, accurate, and fast-responding is a must for vehicle design.
The Case for Li-ion Technology
When the first lithium-ion batteries were brought to market in 1991, they represented a significant improvement on other commercially available batteries.
Rechargeable Li-ion batteries rely on the transport of lithium ions through a non-aqueous electrolyte, and hence they offer extremely high energy densities and long service lives – a notable step away from other nascent rechargeable battery technologies that were commonplace at the time.
With the threat of climate change becoming ever more pressing, the years following saw a boom in demand for an energy storage solution that could replace fossil fuels in vehicles and give longer-term energy storage for a distributed renewables-based grid.
The obvious candidates here are lithium-ion batteries, and over the last 30 years, there have been enormous research and development efforts to reduce their cost and improve their performance.
The Li-ion batteries currently used are – with very little exception – the favored energy storage medium for electric vehicles and portable electronics. They are serving other applications as well, such as large-scale stationary energy storage systems and mobile power generation units.
EV Li-ion Battery Safety Concerns and Thermal Runaway
As well as having excellent performance characteristics, lithium-ion batteries were – and still are – widely regarded as safe. However, the history of Li-ion batteries - which is otherwise successful - has been marred by instances of catastrophic EV battery pack failures known as thermal runaway.
With the seemingly unknown territory of adopting new technology, these battery thermal events are raising concerns for future EV owners.
Thermal runaway is an example of what is considered by chemists as an uncontrolled exothermic chain reaction.
In mere milliseconds, a rise in temperature of the battery cell results in the exothermic decomposition of materials. This leads to other cells in the battery pack being heated, and they may begin to decompose as well.
Eventually, the battery begins to heat itself at a rate exceeding that at which it can dissipate heat to its surrounding environment. Stability is lost as the temperature of the battery rises exponentially, resulting in all remaining chemical, thermal, and electrochemical energy being released to the surroundings.
Testing shows that thermal runaway is triggered by multiple different mechanisms, including:
- High temperatures
- Overcharging
- Mechanical failure
- Leaks
- Internal short-circuits
Thermal runaway results ultimately in a vehicle fire, which causes serious concerns regarding the safety of not only drivers and passengers but - given the difficulty of extinguishing battery fires – of first responders too.
Thermal runaway events are not common – but, given their severity, they are acknowledged as a serious problem. The lack of an apparent cause in many thermal runaway incidents demonstrates the importance of robust monitoring and detection technologies integrated into the design of EVs.
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Mitigating EV Thermal Runaway Events
Research has frequently revealed that thermal runaway events are caused by “abuse” of the battery. Lab tests commonly rely on a handful of standard procedures for purposely inducing thermal runaway - by overheating or puncturing a cell, for example.
These testing methods are indeed effective at inducing thermal runaway; however, they are not representative of the cause of real-world battery failures. In fact, in a large number of real thermal runaway events, no apparent connection with any kind of system “abuse” or obvious stress factors have been found.
As with mechanic or system failure in vehicles of all types, the most effective way to hinder the effects of thermal runaway is through early intervention. Robust early detection of thermal runaway via gas sensors is essential in EV design.
Thermal Runaway Gas Detection Sensor Technology for Li-ion Batteries
It is not common for thermal runaway to occur in Li-ion batteries. However, the effects can be severe, including explosions in stationary energy storage systems and the spontaneous combustion of electric vehicles.
Alarmingly, research suggests that many instances of thermal runaway in Li-ion systems take place despite there being no known damage to the battery. Such events can therefore only be effectively mitigated through the use of robust early detection systems, with gas sensor technology playing a key part.
As thermal runaway is an uncontrolled chain reaction, detecting it as rapidly as possible is crucial.
At the beginning of this multi-phase process, evaporated electrolyte is typically vented at high temperatures by the battery cell. Voltage and temperature often react slowly to this initial venting, but gases, smoke, and pressure serve as reliable rapid indicators of this type of failure.
They may provide quick detection of cell venting, but pressure sensors have limitations to their application in EV battery pack systems.
Pressure sensors are inexpensive, small, and durable, but they usually exhibit a poor signal/noise ratio and are too sensitive to pack volume/venting effects for use in thermal runaway detection systems.
On the other hand, while different battery chemistries result in different chemical signatures during initial venting before thermal runaway, carbon dioxide and hydrogen gas have been shown to be reliable indicators of such failures.
When integrated, CO2 and H2 sensors offer:
- Stability in long-term applications
- Fast response times (5 – 8 seconds for CO2 and <1 – 3 seconds for H2)
- Low risk of type 1 and type 2 faults
- High specificity (CO2 sensors have no known cross-sensitivity, and H2 sensors are only cross-sensitive to helium, which is not present in battery packs)
- Excellent signal-to-noise ratios
The integration of gas sensors into EV battery thermal management systems enables the detection of cell failure within seconds – giving sufficient time to deploy countermeasures and allowing occupants to exit the vehicle safely.
In parked vehicles, a system like this would enable people in close range to reach a safe distance away.
Enhanced EV Performance and Safety
Learn about our sensor technology & its applications in electric vehicle design:
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New Technology = New Regulations
As is the case for all new technology used by the masses, new regulations typically follow. In the case of Li-ion batteries for vehicles, new laws and standards have emerged.
The Chinese government’s regulations for eclectic vehicles are strict guidelines on the inclusion and function of thermal runaway detection systems inside electric vehicles.
These mandate that any electric vehicle system has to be able to give a warning of a thermal runaway event five minutes before the occupancy of a hazard in the passenger compartment.
The UN has issued guidelines for similar safety measures in electric vehicles – yet these remain just guidelines. Whilst there are regulations regarding electric vehicles in the U.S., there are none around the detection of thermal runaway.
Together with laws and regulations, the implementation of new industry standards for thermal runaway detection is underway as EV technology takes hold in transportation.
Performance requirements for thermal runaway detection systems are extremely high. For example:
- A thermal runaway detection system must also provide stable operation throughout the entire service life of a battery pack (which can run as high as 10-15 years in automobile applications.)
- Any thermal runaway event must be detected well before any hazard is posed to the vehicle’s inhabitants.
- Electric vehicles and energy storage systems commonly use one of several different battery electrochemistries, as well as using different cell geometries and configurations. Any thermal runaway detection solution must be agnostic to all these and provide reliable detection for any battery system.
- Electric vehicles typically use a 12 V secondary battery system to power auxiliary systems such as keyless entry and alarms, which severely limits the power draw of a thermal runaway detection system. A robust thermal runaway detection system ideally needs to have a current draw in the <1 mA range to avoid depleting this secondary battery system.
Thermal Runaway Detection Beyond Electric Vehicles
Thermal runaway is not unique to electric vehicles – there can also be occurrences in stationary energy storage systems, which typically have much higher capacity.
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In energy storage applications, gas detectors can be deployed as standalone devices and/or integrated into battery systems within facilities to protect against thermal runaway on a large-scale.
As in electric vehicles, these systems enable the implementation of countermeasures by providing nearby personnel with an early warning, thereby protecting infrastructure and minimizing the effects of any thermal runaway in development.
Developing a Robust Thermal Runaway Detection System Using Gas Sensor Technology
Having started with two engineering test platforms consisting of a variety of sensors with a digital output, Amphenol Advanced Sensors have spent the last few years developing industrial H2 and CO2 sensor technologies to provide detection systems for thermal runaway in Li-ion battery systems which are both versatile and robust.
Working closely with OEMs, Amphenol found a way to characterize the plumes of material ejected from failing cells such that it detects when cell failure has occurred and determines whether the problem is localized or spreading to other cells in the battery pack.
With these results, Amphenol has been able to create the Robust Early Detection of Thermal Runaway (REDTR) system. REDTR is reliant on H2 and CO2 sensors, in addition to monitoring temperature, pressure and relative humidity to accurately detect the early indicators of thermal runaway in Li-ion battery systems.
By monitoring CO2, H2, pack pressure, relative humidity and temperature, REDTR is completely system-agnostic and will function effectively irrespective of the specifics of a given battery system.
Low rates of power consumption mean that REDTR sensors offer a service life of up to 20 years, ideal for any EV or ESS applications. A compact configuration enables the sensor system to be used as a standalone device or integrated directly into the battery management system.
Gas Sensor Technology for Continous EV Battery Improvement
Although gas detectors are fairly robust, they are not indestructible. In a failing battery pack, H2 and CO2 detectors stop working when temperatures start to surpass 150 °C.
Before this happens, however, gas sensors collect invaluable data that provides OEMs with a powerful diagnostic tool and a window into what is going on at the cell level to cause battery thermal events.
As well as enabling danger mitigation and rapid response and in the event of thermal runaway, sensors will help eliminate the root causes of cell failure and make thermal runaway a thing of the past.
This information has been sourced, reviewed and adapted from materials provided by Amphenol Advanced Sensors.
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