Therefore, with the rapid increase in the use of lithium-ion batteries across various industries, battery safety performance has become increasingly important. Not only are excellent charge and discharge performance required, but also higher safety standards. So why do lithium-ion batteries catch fire or even explode, and what measures can be taken to prevent and avoid this?
Laptop battery explosions are related not only to the manufacturing process of the lithium-ion battery cells used, but also to the battery protection board encapsulated within the battery, the laptop's charge and discharge management circuitry, and the laptop's heat dissipation design. Inadequate heat dissipation design and charge/discharge management in laptops can cause the battery cells to overheat, significantly increasing cell activity and thus increasing the likelihood of explosion and combustion.
Analysis of the Composition and Performance of Lithium-ion Battery Materials
First, let's understand the material composition of lithium-ion batteries. The performance of a lithium-ion battery depends primarily on the structure and performance of its internal materials. These internal materials include the negative electrode material, electrolyte, separator, and positive electrode material. The selection and quality of the positive and negative electrode materials directly determine the performance and price of the lithium-ion battery. Therefore, research into inexpensive, high-performance positive and negative electrode materials has always been a key focus of the lithium-ion battery industry.
Carbon materials are generally used for anodes, and their development is relatively mature. However, the development of cathode materials has become a crucial factor restricting further improvements in lithium-ion battery performance and reductions in price. In currently commercially processed lithium-ion batteries, the cost of cathode materials accounts for approximately 40% of the total battery cost; therefore, a decrease in the price of cathode materials directly determines a decrease in the price of lithium-ion batteries. This is especially true for lithium-ion power batteries. For example, a small lithium-ion battery used in a mobile phone requires only about 5 grams of cathode material, while a lithium-ion power battery powering a bus may require as much as 500 kilograms of cathode material.
Although theoretically many types of materials can be used as positive electrodes in lithium-ion batteries, the most common and essential component is LiCoO2. During charging, the potential applied to the battery electrodes forces the compound at the positive electrode to release lithium ions, which then embed into the layered carbon structure at the negative electrode. During discharging, lithium ions are released from the layered carbon structure and recombine with the compound at the positive electrode. This movement of lithium ions generates an electric current. This is the working principle of a lithium-ion battery.
Lithium-ion battery charge and discharge management design
During charging, the potential applied to the battery's electrodes forces the compound at the positive electrode to release lithium ions, which then embed into the layered carbon structure at the negative electrode. During discharging, lithium ions are released from the layered carbon structure and recombine with the compound at the positive electrode. This movement of lithium ions generates an electric current. While the principle is simple, in actual industrial processing, many more practical issues need to be considered: the positive electrode material requires additives to maintain its activity after multiple charge-discharge cycles; the negative electrode material needs to be designed at the molecular level to accommodate more lithium ions; and the electrolyte filling the space between the positive and negative electrodes, besides maintaining stability, must also have good conductivity to reduce the battery's internal resistance.
While lithium-ion batteries offer the advantages mentioned above, they place high demands on protection circuits. Strict precautions must be taken against overcharging and over-discharging during use, and the discharge current should not be too high; generally, the discharge rate should not exceed 0.2C. The charging process of a lithium-ion battery is shown in the figure. Before charging begins within a charging cycle, the battery voltage and temperature must be tested to determine if it is ready for charging. If the battery voltage or temperature exceeds the manufacturer's permitted range, charging is prohibited. The permissible charging voltage range is 2.5V to 4.2V per cell.
When the battery is deeply discharged, a pre-charge process is required to prepare the battery for fast charging. Then, based on the fast charging speed recommended by the battery manufacturer, typically 1C, the charger charges the battery at a constant current, causing the battery voltage to rise slowly. Once the battery voltage reaches the set termination voltage (typically 4.1V or 4.2V), constant current charging terminates, the charging current rapidly decreases, and charging enters the full charge process. During full charge, the charging current gradually decreases until the charging rate drops below C/10 or the full charge time is exceeded, at which point it enters the top-off charging phase. During top-off charging, the charger replenishes the battery with a very small charging current. After a period of top-off charging, charging is then switched off.
Lithium-ion battery protection circuit design
Due to the chemical characteristics of lithium-ion batteries, during normal use, positive chemical reactions occur inside the battery, converting electrical energy into chemical energy. However, under certain conditions, such as overcharging, over-discharging, and overcurrent, side chemical reactions can occur inside the battery. If these side reactions intensify, they can severely affect the battery's performance and lifespan, and may produce a large amount of gas, causing the internal pressure of the battery to increase rapidly and eventually explode, leading to safety issues. Therefore, all lithium-ion batteries require a protection circuit to effectively monitor the battery's charging and discharging status and, under certain conditions, shut down the charging and discharging circuits to prevent damage to the battery.
Lithium-ion battery protection circuits include overcharge protection, overcurrent/short circuit protection, and over-discharge protection. Overcharge protection requires high precision, low power consumption of the protection IC, high voltage withstand capability, and zero-volt rechargeability. The following article will analyze the principles, new functions, and characteristic requirements of these three types of protection circuits, providing valuable reference for engineers designing and developing protection circuits.
Lithium-ion battery protection circuit design case study
In circuit design powered by lithium-ion batteries, the integration of increasingly complex mixed-signal systems onto a small chip inevitably presents challenges to low-voltage and low-power consumption for digital and analog circuits. Finding the optimal design approach within the constraints of power consumption and functionality is a current research hotspot in power management (PM) technology. On the other hand, the use of lithium-ion batteries has also greatly promoted the design and development of corresponding battery management and protection circuits. Lithium-ion batteries require complex control circuits to effectively prevent overcharging, over-discharging, and overcurrent conditions.
This paper discusses a method for designing charging and discharging protection circuits for lithium-ion batteries in electric bicycles, utilizing the ultra-low power consumption and high-performance MSP430F20X3 chip, in light of the energy transition trend in electric bicycles. The method details the entire design process, from system architecture and charging/discharging circuits to testing and protection circuit design, providing a comprehensive reference for electric bicycle power supply designers.
