Abstract
Voltage measurement faults may cause overcharging of lithium-ion batteries, leading to the formation of internal gases and heat generation, resulting in uncontrolled heating. To reduce this risk, the cylindrical battery is equipped with a current interruption device (CID), which acts as a pressure relief valve. When the internal pressure rises, the CID can disconnect the internal circuit of the battery. However, this disconnection causes the battery to suddenly become high in resistance, causing serious problems in series connected batteries. In this configuration, some or even the entire system voltage may drop on the disconnected battery, greatly increasing the possibility of arcing. This type of arc may ignite any escaping flammable gas, leading to catastrophic failure.
In a series of tests conducted on three different battery chemists, NMC (nickel manganese cobalt), NCA (nickel cobalt aluminum), and LFP (lithium iron phosphate), it was found that the safe operation of CID cannot be guaranteed at system voltages exceeding 120V. Although comparative tests at twice the nominal battery voltage did not show the same behavior, these findings suggest that current safety standards that recommend testing at twice the rated voltage may not fully address the risks involved. Further testing has shown that the serial connection between the battery and CID is inherently dangerous, as in the worst-case scenario, the entire system voltage can be concentrated on a single battery, leading to potential system failures.
1. Introduction
With the advancement of electrical and electronic engineering, modern life heavily relies on devices such as smartphones, tablets, electric bicycles, electric vehicles, power tools, and home energy storage systems. According to the IEC 61140 standard, these devices can be divided into two voltage levels: devices below 60V AC and 120V DC, and devices with voltage ranges up to 1000V AC and 1500V DC.
The former includes electric tools, electric bicycles, laptops, and mobile phones, which are usually considered safe because of their extremely low voltage. The latter is also known as low voltage range equipment, such as electric vehicles with a nominal voltage of 400V DC to 800V DC. Electric vehicles and other applications obtain the required operating power from lithium-ion batteries, with a maximum voltage of 4.2V. Generally speaking, this voltage level is sufficient for smartphones, but for electric bicycles (36V DC) and electric vehicles (400V DC), approximately 10 and 96 batteries need to be connected in series, respectively.
Lithium ion batteries are particularly sensitive to overcharging reactions, which can lead to the formation of gas inside the battery. To ensure that each battery operates within the correct range, a Battery Management System (BMS) is used in the battery to monitor parameters and ranges. In addition, cylindrical batteries are equipped with passive safety systems such as current interruption devices (CIDs), which are used to disconnect the internal circuits of the battery when gas formation and pressure increase occur due to decomposition reactions inside the battery.
Due to the disconnection of CID, the potential risk of arcing increases, leading to a question of whether batteries with CID are dangerous when used in series. For example, an electric vehicle with a 400V system may encounter technical issues that result in a single battery voltage being very high, exceeding twice the nominal voltage. In this case, the testing conducted during the approval of the electric vehicle battery is meaningless because using CID in this situation may lead to dangerous situations.
In order to find the best answer to this question, this article conducted extensive testing at different voltage levels (120V DC to 800V DC) commonly used in electric and hybrid electric vehicle applications.
2. Theoretical background
The consequences of overcharging: Overcharging is one of the most critical situations in battery applications. Compared to deep discharge, the consequences of overcharging are more serious, which may lead to the decomposition of electrolytes and cathode materials, as well as adverse reactions between electrodes and other battery components, resulting in catastrophic battery failures such as fires or explosions.
Reasons for overcharging: including charging controller failure, BMS failure, or incorrect voltage measurement. For example, BMS balancing the battery based on incorrect voltage values may ultimately lead to overcharging and potential thermal runaway.
Internal reactions of batteries: Depending on the materials and chemicals used in the battery, oxygen is produced during cathode decomposition (depending on the charging state and cathode material). Oxygen reacts with carbon and electrolyte solvents, resulting in the release of flammable gases such as carbon monoxide, carbon dioxide, and hydrogen. In this case, lithium nickel manganese cobalt electrodes (NMC 622 and NMC 811) and lithium nickel cobalt aluminum electrodes (NCA) demonstrate criticality, while lithium iron phosphate electrodes are considered the safest materials due to their low release of toxic carbon monoxide gas. Electrolyte is the main responsible element for gas generation in batteries, and the formation of gas in each battery establishes high pressure. Due to the sealing of the environment by lithium-ion batteries, the generated gas escapes, and together with the stable metal shell, the gas pressure can reach up to 20 bar. In uncontrolled failure events, these gases may explode.
Safety devices: In order to reduce the potential hazards of energy storage equipment, various safety devices and control mechanisms are adopted. Internal safety measures such as positive temperature coefficient (PTC) devices and current interruption devices (CID) are used at the battery level, and BMS is used as an external safety measure to continuously monitor the battery at the system level. PTC increases resistance and reduces current flow during heating, while CID consists of a top disk and a bottom disk. When overcharge causes an increase in pressure, the top disk will bend and the welded joint will break, thereby disconnecting the current path with the active material. Triggering CID is similar to opening a switch under load, which may ignite an arc. For cylindrical batteries with CID, a voltage of 18V is sufficient to generate an arc. In a series connection, a single battery may not reach such a high voltage value, but it may occur in the system, which can cause voltage concentration on one battery, making it particularly dangerous.
Testing standards: The United Nations Recommendations on the Transport of Dangerous Goods are very important for battery testing, among which UN 38.3 T3 specifies multiple testing requirements, including overcharge testing. According to this standard, the overcharge test is to determine whether the battery is dangerous in case of abuse, and the battery should be charged to twice the maximum charging voltage during the test. The UN ECE Regulation No. 100 is the legal basis for the approval of electric vehicles by the European Union, which describes the overcharge test of electric vehicle batteries. The FreedomCAR Electrical Energy Storage System Abuse Test Manual is also one of the important standards. For overcharge testing, this standard uses a constant DC charging current and the voltage should be set to twice the normal voltage. These standards do not always meet the requirements of practical applications, as the batteries are installed in series in modules and the voltage may be higher, increasing the risk of arcing when the CID is disconnected.
3. Experimental section
Experimental design: In the overcharge test, three batteries with different chemical properties (LFP, NMC, and NCA) were used for comparative behavior analysis. The reason for choosing these batteries is that LFP has a mild overcharge reaction, NMC electrode has stronger reactivity as a cathode material, and NCA oxide releases oxygen and causes thermal runaway. The selection of batteries is based on the main criteria, which is that the batteries should have a CID. Before the experiment, samples of each battery type were opened and inspected.
Testing device: The testing device includes a power circuit and a measurement circuit. The measurement circuit includes a high-voltage measurement module, current clamp, temperature sensor, and data acquisition equipment. The power circuit consists of a voltage source, a load contactor, and a battery. The overcharge abuse test was conducted in outdoor testing facilities, and high-definition cameras and infrared cameras were used to record the events.
Testing process: The testing is conducted according to the FreedomCAR testing specification, but at the normal operating temperature of the battery. The testing equipment is charged to twice the rated voltage, and data collection stops after 30 minutes, regardless of the battery's reaction state. The reaction of the battery was evaluated using the EUCAR hazard level, dividing its behavior into eight hazard levels. Three color levels were defined to represent the safe behavior of the battery, and binary logistic regression analysis was conducted.
Test parameters: Conduct ten tests on each battery at voltage levels of 120V, 400V, and 800V, as most electric vehicles are within these voltage ranges. We compared the situation of double rated voltage in higher voltage levels and FreedomCar overcharge tests to check if the danger is proportional to the voltage. According to the manufacturer's battery data sheet, the current level of each battery was selected, with NCA and NMC batteries set to 4A and LFP batteries set to 1.5A. The battery is charged until the CID interrupts the charging flow or the test is terminated, with each test lasting for 30 minutes.
Data analysis: SPSS software is used for statistical evaluation of data, with a focus on the safety of batteries. Binary logistic regression is used for evaluation based on binary expressions of "safe" or "unsafe". The statistical evaluation of the test includes discrete (descriptive) and analytical (inferential) parts. The test can be described using three variables: chemical properties (discrete categorical variables), voltage (continuous ratio scaling variables), and test results (binary 0-1 variables, safe and unsafe).
4. Results
Classification of test results: In order to provide an overview of the raw data, three categories with hazard levels 3-5 have been defined for the test series.
The behavior of correctly triggering CID: The first test result category summarizes the data on the correct behavior of CID (hazard level 3). All tested batteries, after being overcharged for 10 minutes, had internal air pressure sufficient to open CID, causing battery exhaust (current drop, voltage increase). CID correctly interrupted current flow and prevented further overcharging of the battery, classified as a safety condition and marked as hazard level 3 (green safety behavior).
CID triggered incorrect behavior: The second category summarizes CID triggered incorrect behavior, in which the CID partially interrupts current flow, resulting in strong smoke and temperature rise, and is classified as unsafe condition hazard level 4 (yellow unsafe behavior).
Behavior triggered by CID errors: The last category includes data triggered by CID errors, where CID can only briefly or completely separate current and voltage, and therefore cannot prevent battery overcharging, ultimately leading to battery combustion or explosion, classified as an unsafe condition of hazard level 5 or higher (red unsafe behavior).
5. Discussion
Limitations of testing standards: According to FreedomCAR's battery testing standards, it is difficult to push the battery to the safe limit, that is, when overcharged at twice the rated voltage, the battery will not be pushed to extreme limits and will not exhibit dangerous behavior. Within this voltage range (2-5V), CID can correctly separate the positive and negative poles without igniting the battery. However, the testing standards do not reflect the actual use of lithium batteries. In the energy storage market, there are higher interconnected series switching systems with voltages up to 800V.
The performance of batteries with different chemical properties: Considering the results of the 120V test series, NMC and NCA chemical batteries exhibited the first critical battery behavior, while LFP chemical batteries were relatively safe and did not experience ignition or fire with a hazard level of 5 or higher. In the 400V test, the critical conditions of NMC and NCA chemistry batteries doubled compared to the 120V test, but LFP batteries can still be considered non critical. In the 800V test, the performance of NMC and NCA batteries was almost the same, in the ignition stage, while LFP batteries showed the first key behavior compared to the 120V and 400V test series.
Reasons for unsafe behavior: For all batteries classified as "unsafe", the energy supply cannot be stopped, that is, the charging current cannot be interrupted, which may be due to the arc generated when CID is triggered, causing the charging current to continue flowing, resulting in a small contact point between the anode and cathode, leading to high current density. In addition, the distance between the two contacts created when CID is triggered is very short, which also increases the breakdown voltage and may cause arcing.
6. Conclusion
Shortcomings of current standards: Based on the results of all test series, it can be concluded that the current standards for testing battery safety in battery systems are insufficient. In the battery system of cylindrical batteries connected in series, the disconnection of CID under high system voltage may lead to the formation of critical arcs, resulting in battery combustion or explosion. Therefore, if the batteries are connected in series in the battery system, battery testing at twice the rated voltage is not important for the safe behavior of the batteries, and the current standards must be revised. It is recommended that the testing conducted at the battery level should at least reach the maximum voltage level of the battery system planned for installation and operation.
Consideration for CID application: It has been found that overcharging the battery with very high voltage increases the potential for danger. Therefore, when a large number of batteries with CID are used in series in the battery system, their application should be reconsidered, as triggering CID may lead to catastrophic battery failure. The alternative solution to this problem is to design a CID battery that can withstand such high voltage.