How to set up a safe lithium battery protection circuit

According to statistics, the global demand for lithium-ion batteries has reached 1.3 billion, and this figure is increasing year by year as the application field continues to expand. As a result, with the rapid increase in the use of lithium-ion batteries in various industries, the safety performance of batteries has become increasingly prominent, requiring not only lithium-ion batteries to have excellent charging and discharging performance, but also higher safety performance. So why do lithium batteries catch fire or even explode? Are there any measures to avoid and eliminate this?

　　Laptop battery explosion is not only related to the production process of the lithium battery cells used in them, but also to the battery protection board encapsulated in the battery, the charge and discharge management circuit of the laptop computer, and the heat dissipation design of the laptop computer. The unreasonable heat dissipation design and charge and discharge management of the laptop computer will cause the battery cells to overheat, thereby greatly increasing the activity of the cells and the chances of explosion and combustion.

　　Analysis of Lithium Battery Material Composition and Performance

　　First, let's understand the material composition of lithium batteries. The performance of lithium-ion batteries mainly depends on the structure and performance of the internal materials used in the battery. These internal battery materials include negative electrode materials, electrolytes, separators and positive electrode materials. The selection and quality of positive and negative electrode materials directly determine the performance and price of lithium-ion batteries. Therefore, the research on cheap and high-performance positive and negative electrode materials has always been the focus of the development of the lithium-ion battery industry.

　　Negative electrode materials are generally made of carbon materials, which are relatively mature at present. The development of positive electrode materials has become an important factor restricting the further improvement of lithium-ion battery performance and the further reduction of prices. In the current commercially produced lithium-ion batteries, the cost of positive electrode materials accounts for about 40% of the total battery cost. The reduction of positive electrode material prices directly determines the reduction of lithium-ion battery prices. This is especially true for lithium-ion power batteries. For example, a small lithium-ion battery used in a mobile phone only needs about 5 grams of positive electrode materials, while a lithium-ion power battery used to drive a bus may require up to 500 kilograms of positive electrode materials.

　　Although there are many types of positive electrode materials that can be used for lithium-ion batteries in theory, the main component of common positive electrode materials is LiCoO2. When charging, the potential applied to the two poles of the battery forces the positive electrode compound to release lithium ions and embed them into the carbon with a layered structure of the negative electrode molecules. When discharging, the lithium ions are precipitated from the carbon with a layered structure and recombined with the positive electrode compound. The movement of lithium ions generates current. This is the working principle of lithium batteries.

　　Lithium battery charge and discharge management design

　　When a lithium battery is charged, the potential applied to the two poles of the battery forces the positive electrode compound to release lithium ions and embed them into the carbon of the negative electrode molecules arranged in a sheet structure. When discharged, lithium ions are precipitated from the carbon sheet structure and recombined with the positive electrode compound. The movement of lithium ions generates current. Although the principle is very simple, in actual industrial production, there are many more practical issues to consider: the positive electrode material needs additives to maintain the activity of multiple charges and discharges, and the negative electrode material needs to be designed at the molecular structure level to accommodate more lithium ions; the electrolyte filled between the positive and negative electrodes, in addition to maintaining stability, also needs to have good conductivity to reduce the internal resistance of the battery.

　　Although lithium-ion batteries have the advantages mentioned above, they have high requirements for protection circuits. Overcharging and overdischarging should be strictly avoided during use, and the discharge current should not be too large. Generally speaking, the discharge rate should not be greater than 0.2C. The charging process of lithium batteries is shown in the figure. In a charging cycle, lithium-ion batteries need to detect the battery voltage and temperature before charging to determine whether they can be charged. If the battery voltage or temperature exceeds the range allowed by the manufacturer, charging is prohibited. The allowable charging voltage range is: 2.5V~4.2V per battery.

　　When the battery is in deep discharge, the charger must have a pre-charge process to make the battery meet the conditions for fast charging; then, according to the fast charging speed recommended by the battery manufacturer, generally 1C, the charger charges the battery with constant current, and the battery voltage rises slowly; once the battery voltage reaches the set termination voltage (generally 4.1V or 4.2V), the constant current charging is terminated, the charging current decays rapidly, and the charging enters the full charging process; during the full charging process, the charging current gradually decays until the charging rate drops below C/10 or the full charging time expires, and then it enters the top cut-off charging; during the top cut-off charging, the charger replenishes the battery with a very small charging current. After a period of top cut-off charging, the charging is turned off.

　　Lithium battery protection circuit design

　　Due to the chemical characteristics of lithium-ion batteries, during normal use, a positive chemical reaction of mutual conversion between electrical energy and chemical energy occurs inside the battery. However, under certain conditions, such as overcharging, over-discharging and overcurrent, chemical side reactions will occur inside the battery. When the side reactions intensify, they will seriously affect the performance and service life of the battery, and may produce a large amount of gas, causing the internal pressure of the battery to increase rapidly and then explode, leading to safety problems. Therefore, all lithium-ion batteries require a protection circuit to effectively monitor the charging and discharging status of the battery, and shut down the charging and discharging circuits under certain conditions to prevent damage to the battery.

　　Lithium-ion battery protection circuits include overcharge protection, overcurrent/short circuit protection, and over-discharge protection, which require high precision overcharge protection, low power consumption of protection IC, high withstand voltage, and zero-volt rechargeability. The following article will introduce the principles, new functions, and feature requirements of these three protection circuits in detail, which will be of reference value to engineers designing and developing protection circuits.

　　Lithium battery protection circuit design case sharing

　　In the circuit design using lithium batteries as power supply, it is required to integrate more and more complex mixed signal systems into a small chip area, which inevitably poses low voltage and low power consumption problems for digital and analog circuits. Under the constraints of power consumption and function, how to obtain the best design solution is also a research hotspot of current power management technology (Power Management, PM). On the other hand, the application of lithium batteries has also greatly promoted the design and development of corresponding battery management and battery protection circuits. When using lithium batteries, complex control circuits must be used to effectively prevent overcharging, over-discharging and over-current conditions of the battery.

　　Based on the energy transformation trend of electric bicycles, this paper discusses the design of lithium battery charging and discharging protection circuits for electric bicycles using ultra-low power consumption and high performance MSP430F20X3. This solution discusses the entire design process from every detail of system architecture, charging and discharging circuits, detection and protection circuit design, providing a relatively comprehensive reference for designers of electric bicycle power supplies.