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. 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 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.