Isolation Tests on Assembled Cells come in a number of forms, but the intention is to check for:
- short circuits inside the cell
- issues with the separator
- burred edges on the electrodes
- foreign metallic particles
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.
Steve Grodt [4] Looks at the different types of short circuits and finding these with HiPot Testing:
Many conditions will cause temperature to rise, but when the aluminum (AI) shorts with the anode material, the rise is significant [Figure 2]. This combination of high energy and rapidly rising temperature on a concentrated location could easily ignite the electrolyte. The risk of a fire is more dependent on the heat generated in the localized spot rather than the current magnitude of the short circuit.
This post has been built based on the support and sponsorship of: AVANT Future Mobility, Quarto Technical Services, TAE Power Solutions, h.e.l group and The Limiting Factor.
The tests are conducted on the “dry” (pre-electrolyte filling) stage of cell assembly. Hence the cells are not active batteries at this stage in their assembly. Thus allowing a number of electrical tests/analysis to be conducted:
- High Potential (HiPot) electrical isolation test
- Capacitance measurement
Hi-pot test is conducted multiple times before insulation resistance test. Hi-pot informs you if there are any mechanical issues with your separator (dents, scratches, separator folding etc.). Hi-pot is your first defence from large metallic particles as well -> of course you can detect them by K-value (self discharge rate), but that is after cell formation.
Note: Repeating a HiPot test on a cell will give a different result. Discharging after the first test removes the charge on electrode as it is conductive, but not on the separator as it is not conductive. This means, the separator is still charged after the test and hence the conditions are not exactly the same as the first test.
Hoffmann et al [3] show that the HiPot test on a cell could be used to identify the defect with the cell.
Voltage curves of clean cell stacks (a–c) and cell stacks with defect structures (d–f). Clean stacks at (a) 350 V, (b) 450 V, and (c) 500 V, the latter with a hard-discharge (HD) and no recovery. Cell stacks with defect structures charged up to 450 V: (d) mass of small particles with HD and full recovery; (e) one large particle with several HD events and partial recovery; (f) hole with HD and no recovery. The mechanical pressures are given in the insets.
Test Pressure
For pouch cells it is important to apply a pressure to the cell stack, Hoffmann et al [3] show that the likelihood of detecting defects improves with a pressure of 0.3N/mm2 (300kPa). This pressure is much higher than would be applied to a cell at the beginning of life.
Modelling
Using an ECM to Model HiPot Tests by Siddharth Kurwa
With the RC model, we can predict the expected dynamic response (leakage current) through the system for a given capacitance. The nominal capacitance and insulation resistance of the cell can be estimated by fitting this model to empirical current response.
Isolation Tests on Assembled Cells is an initial draft where we need the power of the Battery Community to help expand this topic and the knowledge base. Please drop me a line at nigel@batterydesign.net if you can contribute.
References
- IEC 60950 – Information technology equipment – Safety
- Replaced by IEC 62368-1
- What is HiPot Testing?, Electrical Engineering Portal
- Hoffmann, L.; Kasper, M.; Kahn, M.; Gramse, G.; Ventura Silva, G.; Herrmann, C.; Kurrat, M.; Kienberger, F. High-Potential Test for Quality Control of Separator Defects in Battery Cell Production. Batteries 2021,
- Decreasing Risk of Electrical Shorts in Lithium Ion Battery Cells, Steve Grodt, Chroma Systems Solutions