In the following article, which was published in Engineering magazine, Unitemp Sales Director Jack Brown discusses testing for propagation caused by secondary battery thermal runaway.
A thermal runaway event in a lithium-ion battery can cause the propagation of fire to battery cells sitting in close proximity to one another. This occurrence has been observed in many industries, including transportation, aerospace, and military. Battery test chambers give managers and engineers the opportunity to create a field-like environment for triggering propagation and runaway. For example, a hot Arizona summer or a cold night in Iceland can be recreated in a test chamber. The results of these tests are used to validate the battery assembly’s integrity and assess the need for modification to the structure.
What is thermal propagation?
Thermal propagation is the spread of heat from a localised area of an object to other parts of the object. The increase in temperature can cause the object to break down and can lead to a fire. The term “thermal” refers to the heat energy that is transferred, and “propagation” describes the movement of that energy.
What is thermal runaway?
Thermal runaway occurs when this uncontrolled increase in temperature leads to a catastrophic failure such as a fire. When a fire starts in one area of a battery, the heat can spread to other batteries and cells in close proximity. This can cause the fire to rapidly spread, leading to extensive damage. In some cases, the fire can even reach the main battery pack, causing an explosion.
What is a secondary battery?
A secondary battery, also known as a backup battery or standby cell, is an additional battery that can take over power supply when the primary or main battery runs low on current. The secondary battery ensures continuous operation of the equipment until the primary battery recharges. Secondary battery testing is important to ensure that the battery will not cause a thermal runaway event and propagate a fire. Some common causes of thermal runaway are overcharging, short-circuiting, physical damage, and manufacturing defects.
Thermal propagation caused by secondary battery thermal runaway
When the secondary battery is used, an internal short circuit related to mechanical or electrical disruption, or perhaps weather conditions, can cause rapid overheating and thermal runaway. In this situation, a domino-like reaction can propagate to nearby cells. Fire, explosion, or another disaster affecting the entire system is a likely result. With the capacity of battery cells increasing in all markets and industries comes an increased risk for lower thermal stability combined with too much heat from large currents. This is especially concerning when cells are close together and the heat is further intensified.
Thermal propagation and runaway in secondary batteries is a serious safety issue that’s claiming attention by many different companies around the world. All markets and industries are testing for thermal runaway, most commonly with overcharge and short-circuit tests. Testing can help identify potential problems with the battery and prevent a fire from spreading, but each test must be precisely calibrated to the battery in question. There is no one-size-fits-all answer for thermal runaway testing, and each market has specific requirements. However, some general considerations for thermal runaway testing include;
- Simulating real-world conditions as closely as possible
- Repeating tests to ensure reproducibility
- Tailoring to the specific battery and its intended use
- Performing tests on both new and used cells and batteries.
Thermal runaway in battery testing
The most common tests for thermal runaway are the overcharge and shortcircuit tests. In the overcharge test, the battery is charged beyond its rated capacity. This can cause the battery to overheat and experience a thermal runaway event. In the short-circuit test, the battery is subjected to a short circuit, causing rapid increase in temperature and a thermal runaway event. In order to test for thermal runaway, it is important to understand the events that can trigger it. Overcharging, short-circuiting, physical damage, and manufacturing defects are some of the most common causes. By understanding what can cause the battery to overheat and experience a thermal runaway event, you can design tests to simulate these conditions. Testing chambers, manufactured by companies such as Espec, who are represented by Unitemp ltd. In the UK, can help to create these conditions for thermal runaway testing. In some cases, it is not possible to recreate the exact conditions that can cause a thermal runaway event. In these cases, you can substitute different trigger conditions that will cause the battery to overheat and experience a thermal runaway event. For example, you can use a higher current in the overcharge test or increase the resistance in the short-circuit test.
Technical requirements for testing
In secondary battery testing, managers and technicians usually mimic the conditions that cause thermal runaway in the field and often select a trigger cell as a target for the event. This trigger cell is usually the cell where thermal propagation and runaway would be most destructive to the system, with a position in the centre of a group of adjacent cells being ideal. However, in cases where the battery assembly must be modified, such as with a heating method (heater, burner, laser) and a nail inserted from the outside, a cell on the outside might be the only choice. Some of the factors included in tests are usually, battery model number, trigger condition (such as overcharging), cell position, duration of test, temperature, the state of charge and the resulting failure such as cell fire or case rupture. A simulation of the worst possible conditions will strengthen the test’s ability to deliver a range of thermal propagation and runaway attributes. Because of the number of variables at play, it’s a good idea to trigger conditions with a single cell before testing the entire system.
Environmental test chambers manufactured especially for battery evaluation provide managers and engineers with the ability to create the temperature and humidity levels that the battery will experience in use. Besides extreme heat and cold, conditions such as smog, smokiness, rain, and extreme humidity are also possible.
Find out more about Espec’s Specialised Battery Test Chambers from Unitemp Ltd.