This article discusses the reasons and various ways to control thermal runaway in batteries. Maintenance-free batteries do not mean that these will operate perfectly forever!
Thermal runaway is a condition caused by a battery charging current or other processes that produce more internal heat than a battery can dissipate. It increases the temperature in a way that causes a further increase in heat and often leads to a destructive result.
It is an increasing threat to electronic devices where more and more power is being packed in smaller spaces. Causes of thermal runaway are varied and often random in nature. Its consequences depend on cell chemistry or battery components. Samsung Galaxy Note 7, Boeing Dreamliner, Panasonic e-bikes and Tesla EV cars have shown how real the dangers can be.
Although, thermal runaway clearly has the capacity to evolve into a true disaster, the good news is, it is unlikely to occur if just a few simple rules are followed. For instance, routine service calls, regular battery replacement and a monitoring system can go a long way towards avoiding thermal runaway. Also, adhering to simple guidelines, proper application and system design, proper installation and operation can reduce the likelihood of thermal runaway.
Potential causes and prevention of thermal runaway
Initially, batteries show signs of heat and may begin to bulge or ooze as well, which increases continuously, leading to fire. Because heat is one of the primary factors that lead to thermal runaway, it is critical to maintain the temperature of the battery’s environment at 25°C (77°F) or below.
The second factor related to thermal runaway is charge current.
Several factors can lead to thermal runaway of a lithium-ion battery, such as internal short-circuit, external short-circuit, overcharging of battery, excessive current when charging or discharging battery, high AC ripple in DC circuit and charger malfunction. Once thermal runaway begins, it usually becomes unstopable, and the battery eventually has to be replaced.
The thermal management system must be able to absorb heat when thermal runaway occurs in a single cell, and prevent adjacent cells from entering thermal runaway as well. Installing a battery in a cool area or ventilated space, using temperature-compensated charging voltage, and installing an active or passive cooling system can avoid thermal runaway. Also, observing battery containers for damage during installation helps.
The battery case should prevent damage from shock and vibration during transport and use. It should also minimise damage during vehicle collisions. Multiple insulation barriers should be considered where high voltages are present. A flame-retardant compound should be used to suppress or delay the production of flames, or prevent the spread of fire.
The simplest form of protection in an electric vehicle (EV) against thermal runaway is to place protective material between the battery pack and chassis. The next form of protection against thermal runaway is to place protective material between battery modules. A thermal conducting film, thermal conducting plates, fans and liquid cooling systems are used to cool down the battery or to dissipate heat from a hot spot.
Battery system design can reduce thermal runaway
Cell manufacturers have lately modified lithium-ion cell chemistries of the electrolyte, anode, cathode, separator and case to reduce the incidence and intensity of thermal runaway. They all are now designed to maximise safety and temperature range while attempting to maintain or increase cell watt-hour capacity. However, these changes come with some trade-offs such as lower cell capacity or higher cost.
Dr Daniele Suzzi, lead engineer – HV battery and EE thermal, AVL List GmbH, says, “Safety concern is one of the major issues during development and homologation of EVs with lithium-ion battery systems. Therefore thermal propagation must be effectively considered in battery safety design, with both support of numerical simulation and real-life tests.”
Thermal fuses are good solutions that can be reflow-soldered at 260°C and still open at 210°C. These include high operating current (up to 100A), high rated voltage (60V DC), low resistance (120µ) and very high breaking capacity.
AllCell’s PCC material is designed to prevent thermal runaway from propagating throughout the battery pack. When thermal runaway occurs in a single cell, wax in the PCC rapidly absorbs a large amount of heat, while the graphite spreads the remaining heat evenly throughout the entire pack.
Key to an efficient lithium-ion battery is to conduct cell selection and battery system design according to the target application to facilitate safety, performance and cost-efficiency. Mechanical and thermal stability of the battery is ensured by appropriate monitoring mechanisms of battery cells and the battery pack.
Temperature sensors on every block connected to a battery monitoring system allow for early detection of thermal runaway. If a permanently-installed monitor takes daily temperature differential readings and floats current readings, the battery can never go to thermal runaway without the monitor first detecting and alarming the condition.
Fans are available in various sizes, some with integrated pulse width modulation (PWM), speed measurement, speedometer signal or automatic restart for better results.
Battery management system design for safety
A battery management system manages the battery within acceptable performance and safe operating conditions. It requires precise measurement of cell voltage, charge and discharge currents, and temperature. As all of these could cause a rise in cell temperature, cells are also equipped with a polyswitch that prevents excessively high currents out of the cell. Cells are equipped with a burst valve that opens when the pressure gets too high, or the battery is exposed to heat externally, to reduce the temperature and prevent thermal runaway.
It is important to consider the cell’s shape when specifying cell protection, as different cells have different insulation needs. There are three main shapes for cells, namely, cylindrical, prismatic and pouch. There are two options for thermal protection in automotive, namely, active or passive management.
Active thermal management relies on cooling technologies using a substance such as air, liquid and refrigerant cooling. It involves an external device that helps dissipate heat. Active methods are generally more expensive and complex than passive techniques.
Passive methods rely upon the thermodynamics of heat transfer conduction, convection and radiation. Passive battery cooling includes metal heat-sinks, PCMs and specialised heat-shields. These are typically less expensive than active technologies and are easier to put in place.
Development in battery manufacturing
Lithium batteries that use cobalt are prone to thermal runaway and fire and, hence, require additional equipment for thermal monitoring and cooling. Lithium battery chemistries that do not contain cobalt, such as lithium iron phosphate, are not prone to thermal runaway or fire and, are, hence, quite safe.
Scientists at Massachusetts Institute of Technology (MIT), Stanford University and Toyota Research Institute (TRI) discovered that combining comprehensive experimental data and artificial intelligence (AI) can predict the useful life of lithium-ion batteries before their capacities start to wane. Their machine learning model enables the algorithm to predict how many more cycles each battery would last, based on voltage declines and a few other factors among early cycles.
Lithium-sulphur batteries offer a theoretical energy density more than five times that of lithium-ion batteries. Researchers at Chalmers University of Technology, Sweden, have developed a porous, sponge-like aerogel, made of reduced graphene oxide, which acts as a free-standing electrode in the battery cell and allows for better and higher utilisation of sulphur.
Dr Suzzi adds, “During battery pack development at AVL, thermal runaway is modelled applying CAE methods, allowing time-effective and preventive evaluation of design countermeasures. Different design scenarios are virtually evaluated, for example, specific cell and module geometry, electrical connections, thermally-resistant layers serving as heat barriers, different cooling concepts, different triggering methods, position of venting plugs/burst disks on the battery housing and so on. These investigations are of significant relevance for developing strategies to prevent or postpone thermal propagation, as well as to meet safety requirements for batteries in EVs.”
Why venting is needed
Jake Sanders, product development manager, Donaldson Venting Solutions, says, “Unlike smaller lithium-ion batteries used in home electronics, automotive lithium-ion batteries need robust protection from harsh external conditions, along with adequate venting for temperature and pressure fluctuations.”
A battery pack for an EV may have hundreds of cells contained within several modules. In addition to keeping out contaminants, battery enclosures need to handle pressure differentials between the enclosure interior and the surrounding atmosphere. Pressure differential can fluctuate widely during normal vehicle operation due to ambient temperature changes, heat generation in cells and atmospheric pressure changes. To prevent the collapse or compromise of the protective shell, enclosures must be vented to equalise internal and external pressures, while releasing gases produced in a rapid pressure. Whether gradual or sudden, pressure buildup can stress seals, leading to leaks and potential explosions.
To sum up
Lithium-ion batteries can be a safe and reliable means of energy for EVs by means of proper cell selection and design and manufacturing, including an advanced battery management system to prevent thermal runaway. Maintenance-free batteries do not mean that these will operate perfectly forever! Hence, it is important to have the batteries checked at least once a year. Without proper maintenance, monitoring and proper installation, the batteries will end up costing a lot in the end.