It was confirmed that the safety margin of Hyundai’s’Kona Electric’ battery, which had 13 fire accidents from last year to recent years, was less than half of that of other competitors. A design with a reduced safety margin to improve driving performance is pointed to as the key cause of this accident.
The safety margin is a kind of’safety securing section’ in which some sections of charge and discharge are deliberately left unused in order to maintain the safe charging and discharging performance of the battery and secure a long life. The more these sections are secured, the higher the safety of the battery, but the possible driving distance will inevitably decrease. This magazine analyzed the report of the National Institute for Scientific Investigation with Professor Park Cheol-wan, Seo Jeong University.
◇ 3.2% Kona safety margin… 30-40% compared to competitors
As a result of an analysis of the legal safety appraisal for the’Kona Electric’ fire accident (Gangneung on July 28, 2019) recently submitted by the National Forensic Research Institute to the National Assembly, it was confirmed that the vehicle’s battery system safety margin is up to 3%. Compared to other competing vehicles, this is less than half of the battery design requirements.
For products that connect batteries in series and parallel, such as electric vehicles, for safety, 8 to 12% of batteries are not actually operated, but are generally left. For this reason, Tesla Model 3 (based on LR trim) is actually equipped with a battery of 80 ㎾h, and the capacity is released at 75 ㎾h outside. In addition, it is a common practice in the industry to secure a safety margin once more under set conditions, such as limiting the battery charge rate (SOC) through a battery management system (BMS).
However, Hyundai Kona’s battery safety margin was only up to 3%. The actual battery settlement capacity of Kona Electric Vehicles is 64㎾h, and in this survey, the operating range of the battery is found to be 97~98%. According to the National Fruit Tree report, the nominal voltage range, which is the charging/discharging range of the battery pack, is designed to be 240 to 412.8V, and the battery pack voltage measured by the National Fruit Tree after charging of the accident vehicle is completed was 409.3V. This is only a difference of 3.5V from the rated maximum charging section (412.8V), which is the allowable standard of the product, and the deviation rate was only 0.86%. Applying this ratio, the safety margin in the charging section is about 1%.
Regarding this, an official from Hyundai Motors explained, “Without separate physical (nominal) capacity and rated capacity, the battery charging rate (SOC) range was designed to be 97%, so we set a safety margin of 3%.”
Even if the 3% safety margin disclosed by Hyundai is applied, the actual battery usable capacity is 62㎾h, and the proportion of the safety section is less than half compared to other competing models. Tesla Model 3 is equipped with 80㎾h, so the actual usable capacity is 72~75㎾h, and GM’Bolt’, which uses the battery most similar to Kona, also has a 60㎾h battery, but the usable capacity is 55㎾. It was only h~57㎾h.
◇ Cause’Battery bad’, not’progressive bad’
It is analyzed that the cause of many Kona electric vehicle fire accidents is due to progressive battery failure. As severe operating conditions were added to the normal battery, the battery was subjected to extreme stress, and there is a high possibility that a poor progression occurred due to damage to a single cell of the battery, followed by a short-circuit process and then ignition.
If the safety margin section is enlarged, the problem of battery cell defects can be suspected, but the safety margin is very small, and the poor progression made during vehicle operation is dominant. In the end, severe battery system design such as battery operating conditions can be interpreted as the main cause. The reason is an attempt to increase the mileage by increasing the available capacity of the battery as much as possible. The industry agrees that the Kona electric vehicle fire accident and the energy storage system (ESS) fire mainly occurred in Korea have a lot in common.
In the case of ESS, the maximum charging voltage section is usually 4.1V to 4.15V, but in this investigation, the average single cell voltage of the accident vehicle Kona was 4.17V. About 20 ESS fire accidents were similar to the Kona case in that the cause of the accident was an unreasonable increase in the charge/discharge section.
Professor Park Cheol-wan said, “For battery packs that are connected in series or parallel with hundreds of cells, it is necessary to design a particularly well-designed safety margin to ensure long life and safety in charge/discharge cycles. Batteries that are filtered and defective in manufacturing are damaged or cause fire regardless of whether the use of SOC is restricted.” In the case of battery packs made in series and parallel, if the safety margin is not sufficiently secured, there is a high possibility of exposure to poor progression.
◇Premature announcement by the Ministry of Land, Infrastructure and Transport…Is’Niro EV’ okay?
On the 8th, the Ministry of Land, Infrastructure and Transport pointed out a defect in the battery cell as the cause of the Kona electric vehicle fire accident, and said that damage to the battery separator is likely to be the cause of the ignition. However, this announcement from the Ministry of Land, Infrastructure and Transport needs to be reviewed once again. The reliability of the investigation is poor because all vehicles investigated by the Ministry of Land, Infrastructure and Transport (KATRI) are accident vehicles and are batteries that have already been stressed under severe operating conditions without a safety margin.
Kona Electric, which has already been sold in Korea, has experienced such operating conditions. For this reason, accurate investigation is possible only by examining new battery cells and packs installed in Kona.
Accordingly, it is necessary to upgrade the battery management system (BMS) to alleviate battery operating conditions through a recall measure by the Ministry of Land, Infrastructure and Transport, and to accurately filter out any damage to the batteries of existing vehicles sold or operated. It is pointed out that these two methods must be used in parallel to prevent future accidents.
Professor Park said, “The results of the investigation by the Ministry of Land, Infrastructure and Transport and KATRI did not understand LG Chem, which led to a bigger dispute, and the battery is the starting point of ignition, so it is not at all true.” It’s the result, not the cause of the ignition, so we have to find out what caused the short circuit.”
Kia Motors’ Niro EV’ cannot guarantee safety if it is operated under the same conditions as Kona Electric, but the battery installed in Niro is a core material of NCM811 unlike Kona’s NCM622 (nickel 60%, cobalt 20%, manganese 20%) The design technology is different according to the configuration.
Professor Park said, “Since the two companies’ single cell technology itself is very different and the safety margin of the cell may be different in design, it is difficult to say that the Niro electric car is exposed to the same risk. Even if the Kona fire is not a battery defect or defect, the battery Manufacturers will have to continue their efforts to develop technologies to improve safety.”
【Table】 Main contents of the “Kona Electric” fire accident legal safety appraisal (based on the National Fruit Tree Report)
【Table】Comparison of battery pack design voltage and operating voltage of accident vehicle
【Table】 Comparison of electric vehicle model 3, bolt, and Kona battery safety margins (each company and country fruit report)
※Nominal battery capacity is 98 cells × nominal single cell voltage (3.63V) × nominal capacity (180Ah), rated battery capacity is 98 cells × rated pack voltage (352V), and available battery capacity is nominal capacity (64kWh) × 97% ( Each calculation is based on the charging rate (SOC) setting value). (Wh is the energy amount unit, Ah is the battery capacity unit)
Reporter Park Tae-jun firstname.lastname@example.org
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