Author: Siddharth Kurwa
A DC high-potential test (HiPot) is used to detect manufacturing defects in the electrode stack/jelly roll. During the test, a high voltage (orders of magnitude higher than the cell operating voltage) is applied to the cell to screen out early-in-life and latent risks from the production cell population. It is preferred to do this test after jelly roll winding and tab weld processes but before electrolyte fill to cast the widest net for potential defects (examples of defects will be discussed later). In this condition, the idealized cell can be thought of as a parallel RC circuit, the resistance is driven by the insulation resistance of the separator and the capacitance is the charge that can be stored between the parallel electrodes.
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.
The expected response is shown below for a set capacitance (1 µF), a set insulation resistance (100 MOhms), varying the set voltage from 250 – 750V. In this example, the applied DC voltage is ramped from 0 V to the set voltage over 5 seconds and then held constant for 5 seconds.
Empirical data has been found to match this model quite well, but this data is proprietary and cannot be shared. This model can be used to predict how to set leakage current limits for the cells.
If the leakage current is substantially higher than what is predicted by the ideal cell, it indicates the presence of a defect. Typical defects include:
- Pinholes/tears in the separator
- Burrs in the electrode that impinge the separator
- Electrode – separator misalignment
- Metallic contamination that impinge the separator (loose active material particles, welding slag, stamping burrs, etc.)
That said, HiPot testing is not a perfect detection method and the parameters need to be carefully selected. There are 2 competing constraints:
- the set voltage and test duration must be well below the separator breakdown voltage to avoid damaging the cell during the test
- the set voltage should be maximized to improve detection capability for more subtle defects
Experiments should be run to find the appropriate voltage setting based on process and product design constraints and requirements.
Cell Manufacturing Process
In order to engineer a battery pack it is important to understand the fundamental building blocks, including the battery cell manufacturing process. This will allow you to understand some of the limitations of the cells and differences between batches of cells. Or at least understand where these may arise.
Battery Cell Modelling
By conducting experiments to measure the battery voltage at various SoCs and temperatures it is possible to develop phenomenological models that relate the applied current and the voltage. An equivalent circuit model (ECM) is one such phenomenological model most widely used in industry to simulate the voltage response for subsequent Battery Management System control and state estimation.