In this Article, we are discussing different types of Battery safety standards to minimize the user’s risk of injury.
INTRODUCTION
Battery safety standards are developed to evaluate the design and manufacturing of a cell, battery, battery system or product device as a single entity or a combination for regulatory compliance and certification. During the evaluation process, various components are tested to ensure the product’s fail-safe operation and minimize the user’s risk of injuring. If a product is not evaluated and certified, it cannot be sold to consumers. As the first safety standard for power lithium batteries in The International Electrotechnical Commission (IEC), the latest version IEC 62660-3:2022 was officially released by IEC Battery Technical Committee TC21 on March 1, 2022.
1. Test Preparation
Thinning or removal for prismatic cells:
The casing can be thinned or removed using appropriate methods and indenter presses recommended by the cell manufacturer to avoid the effect of casing deformation on internal short circuit events. The outer casing of the pouch cell does not need to be thinned or removed.
The following are the recommended methods for removing hard shells using ahead press:
i) The surface of the cell-shell is partially thinned to a thickness of 90% of the shell thickness. The thinning tool should be designed so that the edge of the cutting head enters 90% of the shell thickness.
ii) Apply glue to the attachment surface of the removal pin and attach it to the thinned part of the case. The attachment surface of the removal pin should be the same size as the thinned housing hole. It is recommended to use glue with a tensile strength above 20MPa (such as glue gun or epoxy glue). After the glue has hardened, pull out the removal pin. The holes of the dismantled part shall be sealed with insulating tape before the test.
Figure 1 shows an example of the shell removal process.
Figure 1: Demonstration of removing the case
The following is an example of removing the hard case during cell manufacturing, as shown below;
• Prepare weak areas, such as soldering, an additional vent hole during cell manufacturing;
• Glue the pull tab to the vent sheet and wait 10 minutes to make sure the glue is cured. Use the pull tab to remove the vent tab.
2. Experimental Setup
i) Fix the cell, use Bakelite and rubber sheet to fix the cell;
ii) Voltage measurement, according to the design of the cell, measures the voltage between the negative or positive electrode of the cell and the indenter to detect internal short circuits. The voltage line cannot be connected to the terminal of the outermost electrode.
iii) Indenter device, the indenter adopts steel nails such as SKD-11, SUS-440C, SUS-300 series with a diameter of 1-3mm.The angle of the nail head should be between 20-45 degrees.
3. Preliminary test
The situation, where the pressure head stops, (that is, the voltage drop value corresponding to the short-circuited of one or two layers of the cell under test) should be determined before the test. The steps are as follows:
i) Prepare the battery according to point 2,
ii) Adjust the cell voltage to the upper limit of the charging voltage,
iii) Fix the cell on the test device according to point 3. The indenter should be placed perpendicular to the electrode layer of the cell. The cell or indenter should move along the vertical axis,
iv) The preliminary test shall be carried out at the test temperature of 25℃ ±5. During the test, the voltage shall be recorded at a sampling rate of 1000 Hz or above,
v) Press the indenter onto the cell at a constant speed of 0.01mm/s or press the cell up to the indenter. If one or two layers of internal short-circuiting can be achieved, the press speed can be faster than 0.01mm/s. When the voltage between the indenter and the terminal reaches the voltage between the negative electrode and the positive electrode and a sharp drop in voltage is detected, the pressing should be stopped. After pressing stops, release the indenter from the cell within 10 seconds,
vi) Discharge the cells to a safe voltage, then remove the cells to check the number of short-circuit layers. Repeat steps 1 to 6 until one or two layers are shorted inside and determine the head stop voltage drop parameters. The number of short-circuit layers is equal to the number of short-circuit points between the positive and negative electrodes. Short points which are not touched by the indenter are not counted in the number of short-circuit layers.
4. Test procedure
The test shall be carried out as follows:
i) Prepare the cells according to point 2,
ii) Adjust the cell voltage to the upper limit of the charging voltage,
iii) Fix the cell on the test device according to point 3. The indenter should be placed perpendicular to the electrode layer of the cell. The cell or indenter should move along the vertical axis,
iv) The test shall be carried out at the temperature 25℃±5. During the test, the voltage shall be recorded at a sampling rate of 1000 Hz or above. Cell temperature can be recorded at a sampling rate of 1 Hz or higher. The temperature measuring device should be set on the surface of the cell, less than 25mm away from the center of the indentation,
v) Press the indenter onto the cell at a constant speed of 0.01mm/s or press the cell up to the indenter. The press speed can be faster than 0.01mm/s if one or two layers of internal short circuit can be achieved. When the voltage drop determined in point 4 is detected, the pressure shall stop immediately,
vi) After stopping the pressure, observe the cells for 1 hour. When stopped, the indenter shall remain in place and remain stable within ±0.02mm in the three directions of X, Y and Z until the end of the test including the observation period. After the observation period is over, remove the cell and check the number of short-circuit layers.
CONCLUSION
Inclusion of faulting of battery pack safety circuit components was endorsed by some developers of the standard, but in the end was implemented only as recommendations versus mandatory requirements. Specifically, the new standard states that a safety analysis “should” be provided identifying critical components and that the components “should” be considered for single faulting during short-circuit testing of the battery pack. Whether or not such faults will become a mandatory part of country-specific deviations to the IEC standard remains a distinct possibility.
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