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 with the continuous expansion of application areas, this data is increasing year by year. For this reason, 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 18650 battery catch fire or even explode? Are there any measures to avoid and eliminate them?

Laptop battery explosions are 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 charging and discharging management circuit of the laptop computer, and the heat dissipation design of the laptop computer. The unreasonable heat dissipation design and charging and discharging management of the laptop computer will cause the battery cell to overheat, thereby greatly increasing the activity of the cell, and at the same time increasing the probability of explosion and combustion.

Analysis of the composition and performance of lithium battery materials

The negative electrode material generally uses carbon materials, and the current development is relatively mature. The development of positive electrode materials has become an important factor restricting the further improvement of lithium-ion battery performance and further reduction of prices. In the current commercial production of lithium-ion batteries, the cost of positive electrode materials accounts for about 40% of the total battery cost. The reduction in the price of positive electrode materials directly determines the reduction in the price of lithium-ion batteries. This is especially true for lithium-ion power batteries. For example, a small lithium-ion battery for a mobile phone only needs about 5 grams of positive electrode materials, while a lithium-ion power battery for driving 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 as 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 sheet structure of negative electrode molecules. When discharging, lithium ions are precipitated from the carbon with a sheet structure and recombined with the positive electrode compound. The movement of lithium ions generates current. This is the working principle of 18650 battery.

Lithium battery charge and discharge management design

When charging a lithium battery, 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 sheet structure of negative electrode molecules. During discharge, lithium ions are precipitated from the carbon in the 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 relatively high requirements for protection circuits. Overcharging and over-discharging 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 18650 battery is shown in the figure. In a charging cycle, lithium-ion batteries need to detect the battery voltage and temperature before charging begins to determine whether they can be charged. If the battery voltage or temperature exceeds the range allowed by the manufacturer, charging is prohibited. The voltage range allowed for charging 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.

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