How to Set Up Safe Lithium Battery Protection Circuit

According to statistics, the global demand for lithium-ion batteries has reached 1.3 billion, and with the continuous expansion of the application field, this data is increasing year by year. Because of this, with the rapid surge of lithium ion batteries in various industries, the safety performance of batteries has become increasingly prominent. It not only requires lithium ion batteries to have excellent charge and discharge performance, but also requires higher safety performance. Why did lithium battery fire or even explode in the end, what measures can be avoided and put an end to it?

The explosion of the notebook battery is not only related to the production process of the lithium battery core used therein, but also related to the battery protection plate enclosed in the battery, the charging and discharge management circuit of the notebook computer, and the heat dissipation design of the notebook. The unreasonable heat dissipation design and charge and discharge management of notebook computers will cause the battery core to overheat, which will greatly increase the activity of the core and increase the probability of explosion and combustion.

Analysis on the Composition and Properties of Lithium Battery Materials

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

Negative materials generally use carbon materials, and the current development is relatively mature. The development of positive materials has become an important factor that restricts the further improvement of the performance and price of lithium ion batteries. In the current commercialized lithium ion batteries, the cost of cathode materials accounts for about 40 % of the total battery cost. The reduction of the price of cathode materials directly determines the price of lithium ion batteries. This is particularly true for lithium-ion power cells. For example, a small lithium-ion battery for a mobile phone requires only about 5 grams of positive material, while a lithium-ion power battery for a bus may require up to 500 kilograms of positive material.

Although there are many types of positive electrode materials that can theoretically be used as lithium-ion batteries, the common positive material is LiCoO2. When charged, the potential added to the two poles of the battery forces the positive compound to release lithium ions and embed the negative polar molecules in the layer structure. Carbon. When discharged, lithium ions precipitate from the carbon of the lamellar structure and recombine with positive compounds. The movement of lithium ions produces electric currents. That's how lithium batteries work.

Management Design of Lithium Battery Charging and Discharge

When the lithium battery is charged, the potential added to the two poles of the battery forces the positive compound to release lithium ions, embedded in the carbon in which the negative molecules are arranged in a laminated structure. When discharged, lithium ions precipitate from the carbon of the lamellar structure and recombine with positive compounds. The movement of lithium ions produces electric currents. Although the principle is very simple, however, in actual industrial production, there are many more practical problems that need to be considered: positive materials require additives to maintain multiple charge and discharge activities, and negative materials need to be designed at the molecular structure level. To accommodate more lithium ions; In addition to maintaining stability, the electrolyte filled between the positive and negative poles needs to have good conductivity and reduce the internal resistance of the battery.

Although lithium-ion batteries have all the advantages mentioned above, their requirements for protective circuits are relatively high. During the use process, over-charging and over-discharge should be strictly avoided, and discharge currents should not be too large. In general, The discharge rate should not be greater than 0.2 C. The charging process for lithium batteries is shown in the figure. During a charging cycle, lithium-ion batteries need to test the battery's voltage and temperature before charging begins to determine whether it is charged or not. Charging is prohibited if the battery voltage or temperature exceeds the manufacturer's permission. The voltage range allowed to charge is: 2.5 V to 4.2 V per cell.

When the battery is in deep discharge, the charger must be required to have a pre-charge process so that the battery meets the conditions for rapid charging; Then, according to the fast charging speed recommended by the battery manufacturer, it is generally 1C. The charger charges the battery with constant current, and the battery voltage slowly rises. Once the battery voltage reaches the set termination voltage(usually 4.1 V or 4.2 V), the constant current charging terminates, the charging current rapidly decays, and the charging enters the full charging process; During the full charge process, the charging current gradually decays until the charging rate decreases below C/10 or when the full charge time expires, it is transferred to the top to stop charging; When the top stops charging, the charger supplements the battery with minimal charging current. After the top stops charging for a period of time, turn off charging.

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