How should a safe lithium battery protection circuit be set

According to statistics, the global demand for lithium-ion batteries has reached 1.3 billion, and with the continuous expansion of application areas, this figure is increasing year by year. Because of this, with the rapid surge in the use of lithium-ion batteries in various industries, the safety performance of the battery is increasingly prominent, requiring not only excellent charging and discharging performance of lithium-ion batteries, but also requires a higher level of safety performance. That lithium batteries in the end why fire and even explosion, what measures can be avoided and eliminated?

Lithium battery material composition and performance analysis

First of all, 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 of the batteries used. These internal battery materials include negative electrode material, electrolyte, diaphragm and positive electrode material. Among them, the choice and quality of positive and negative materials directly determine the performance and price of lithium-ion batteries. Therefore, the research of cheap and high performance positive and negative electrode materials has been the focus of the development of lithium-ion battery industry.

The negative electrode material is generally selected as carbon material, and the development is relatively mature at present. The development of cathode materials has become an important factor limiting the further improvement of lithium-ion battery performance and price reduction. In the current commercial production of lithium-ion batteries, the cost of cathode material accounts for about 40% of the overall battery cost, and the reduction of the price of cathode material directly determines the reduction of the price of lithium-ion batteries. This is especially true for lithium-ion power batteries. For example, a small lithium-ion battery for a cell phone requires only about 5 grams of cathode material, while a lithium-ion power battery for driving a bus may require up to 500 kg of cathode material.

Although there are theoretically many types of materials that can be used as the positive electrode of Li-ion batteries, the main component of the common positive electrode material is LiCoO2. When charging, the electric potential added to the two poles of the battery forces the compound of the positive electrode to release lithium ions, which are embedded in the carbon of the negative electrode with a lamellar structure. When discharged, the lithium ions precipitate out of the lamellar structure of the carbon and recombine with the compound at the positive electrode. The movement of lithium ions generates an electric current. This is the principle of how lithium batteries work.

Li-ion battery charge and discharge management design

Although the principle is simple, in actual industrial production, there are much more practical issues to consider: the material of the positive electrode needs additives to maintain the activity of multiple charging and discharging, and the material of the negative electrode 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 electrical conductivity and reduce the internal resistance of the battery.

Although the lithium-ion battery has all the above-mentioned advantages, but its requirements for the protection circuit is relatively high, in the use of the process should be strictly to avoid over-charging, over-discharge phenomenon, the discharge current should not be too large, in general, the discharge rate should not be greater than 0.2 C. The charging process of lithium batteries is shown in the figure. In a charging cycle, lithium-ion batteries need to detect the voltage and temperature of the battery before charging begins to determine whether it can be charged. If the battery voltage or temperature is outside the range allowed by the manufacturer, charging is prohibited. The allowable charging voltage range is: 2.5V~4.2V per battery.

In case 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 fast charging; then, according to the fast charging rate 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 and the charging current Once the battery voltage reaches the set termination voltage (generally 4.1V or 4.2V), the constant current charging terminates, the charging current decays rapidly and the charging enters the full charging process; during the full charging process, the charging current decays gradually until the charging rate decreases to below C/10 or the full charging time is overrun, then it turns into 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 cutoff charging, the charge is turned off.

Post time: Nov-15-2022