Although very rare, cell internal short circuits are a leading cause of battery thermal runaway. They are a major safety issue for any application of a battery pack. Hence there is a requirement to prevent them and to detect them.
Common hazards of battery thermal runaway include toxic off-gassing, smoke, fire, and even an explosion.
Preventing Cell internal Short Circuits
There are a number of things that can cause an internal short circuit within a battery cell. The primary focus has to be on manufacturing and the processes deployed to mitigate or reduce these risks.
- Metallic foreign body in the raw materials
- Introduction of a metallic particle during cell production
- Damage to electrodes
- Particles or damage during tab welding
- Particles from cell case welding
- Dendrite growth during cell use
Finally, in cell formation and ageing, methods can be deployed to pick up some of these issues. However, there is always a chance that one of these will make it all the way through to the product.
Type of Internal Short Circuit
Steve Grodt’s white paper from Chroma Systems Solutions [4] shows that the temperature versus time graph is very dependent on the type of short-circuit within the cell.
The worst case is shown to be for the aluminium current collector to the graphite anode. This could be caused by a foreign particle in the cathode layer or by a burr on the edge of the aluminium current collector.
Detecting Cell Internal Short Circuits
Once the battery pack has been assembled from multiple cells in series and parallel the detection of an internal short circuit in one of the cells will be very difficult. The challenge is detecting it, shutting the pack down and ensuring anybody in the vicinity can be warned and get away. Some of the measurement techniques being investigated and deployed are:
- Voltage measurement
- Gas pressure
- Cell swelling force
- Gas composition
- Temperature measurement
A further challenge is the reliable detection of an internal short circuit, avoiding false positives that are likely to reduce confidence in the use of the battery or even neglect of any messaging.
Managing Cell Internal Short Circuits
Finally we need to deploy methods to manage an internal short circuit. These methods can be deployed at cell level with the use of:
- Ceramic coated separators
- Shutdown separators
- Current interrupt devices
These devices can reduce the impact of an internal short circuit. However, we need to test any battery pack to approved standards to ensure that the effect of these electrical shorts results in an overall safe battery system.
References
- Ting Cai, Peyman Mohtat, Anna G. Stefanopoulou, Jason B. Siegel, “Li-ion Battery Fault Detection in Large Packs Using Force and Gas Sensors“, University of Michigan, Ann Arbor, MI 48105, USA
- Shan Huang, Xiaoniu Du, Mark Richter, Jared Ford, Gabriel M. Cavalheiro, Zhijia Du, Robin T. White, Guangsheng Zhang, “Understanding Li-Ion Cell Internal Short Circuit and Thermal Runaway through Small, Slow and In Situ Sensing Nail Penetration“, Journal of The Electrochemical Society, Volume 167, Number 9
- Naha, A., Khandelwal, A., Agarwal, S. et al. Internal short circuit detection in Li-ion batteries using supervised machine learning. Sci Rep 10, 1301 (2020).
- Decreasing Risk of Electrical Shorts in Lithium Ion Battery Cells, Steve Grodt, Chroma Systems Solutions
This post just gives an introduction to this field. If you would like to contribute to these pages then please do drop us a line: editor@glue-it.com
What happens when batteries are abused?
Lithium-ion batteries are an essential component in electric vehicles, however their safety remains a key challenge. This video explores the science behind what happens when batteries are abused and when they fail.
A great introductory presentation by Billy Wu, Dyson School of Engineering, Imperial College.