Industry insiders explain 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 the scope of use, this data is increasing year by year. For this reason, with the rapid increase in the amount of lithium-ion batteries used in various industries, the safety performance of batteries has become increasingly prominent. Not only lithium-ion batteries are required to have excellent charging and discharging performance, but also higher safety performance. Then why did the lithium-ion battery catch fire or even explode, and are there any measures to guard against and prevent it?

The explosion of a notebook battery is not only related to the processing technology of the lithium-ion battery cells used in it, but also to the battery protection board packaged in the battery, the charge 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 overheat the battery cells, which greatly increases the activity of the battery cells and increases the probability of explosion and burning.

Lithium-ion battery material composition and performance analysis

First, let's take a look at the material composition of lithium-ion batteries. The performance of lithium-ion batteries depends on the structure and performance of the internal materials used in the battery. These battery internal materials include negative electrode materials, electrolytes, separators, 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 on cheap and high-performance positive and negative electrode materials has always been the focus of the development of the lithium-ion battery industry.

The negative electrode material is generally made of carbon material, and the current development is relatively mature. The development of cathode materials has become an important factor restricting the further improvement of lithium-ion battery performance and the further reduction of price. In the current commercialized lithium-ion batteries, the cost of the cathode material accounts for about 40% of the entire battery cost, and the reduction in the price of the cathode material directly determines the reduction in the price of the lithium-ion battery. This is especially true for lithium-ion power lithium-ion batteries. For example, a small lithium-ion battery for a mobile phone only needs about 5 grams of positive electrode material, while a lithium-ion power lithium-ion battery for driving a bus may require up to 500 kilograms of positive electrode material.

Although there are many types of cathode materials that can theoretically be used for lithium-ion batteries, the main component of common cathode materials is LiCoO2. When charging, the potential applied to the two poles of the battery forces the compounds of the cathode to release lithium ions, and the molecules of the embedded anode are arranged in a lamellar structure. in carbon. During discharge, lithium ions are precipitated from the carbon in the lamellar structure and recombine with the compound of the positive electrode. The movement of lithium ions creates an electric current. This is how lithium-ion batteries work.

Li-ion battery charge and discharge management design

When the lithium-ion battery is charged, the potential applied to the two poles of the battery forces the compound of the positive electrode to release lithium ions, which are embedded in the carbon whose molecules of the negative electrode are arranged in a lamellar structure. During discharge, lithium ions are precipitated from the carbon in the lamellar structure and recombine with the compound of the positive electrode. The movement of lithium ions creates an electric current. Although the principle is very simple, in actual industrial processing, there are many more practical problems to be considered: the positive electrode material needs additives to maintain the activity of multiple charging and discharging, and the negative electrode material needs to be designed at the molecular structure level to accommodate more In addition to maintaining stability, the electrolyte filled between the positive and negative electrodes must also have good conductivity and reduce the internal resistance of the battery.

Although the lithium-ion battery has the advantages mentioned above, it has relatively high requirements on the protection circuit. During use, it should be strictly guarded against overcharge and overdischarge, 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 a lithium-ion battery is shown in the figure. In a charging cycle, the lithium-ion battery should test the voltage and temperature of the battery before charging to determine whether it is rechargeable. Charging is prohibited if the battery voltage or temperature is outside the manufacturer's approved range. The voltage range allowed for charging is: 2.5V~4.2V per cell.

According to statistics, the global demand for lithium-ion batteries has reached 1.3 billion, and with the continuous expansion of the scope of use, this data is increasing year by year. For this reason, with the rapid increase in the amount of lithium-ion batteries used in various industries, the safety performance of batteries has become increasingly prominent. Not only lithium-ion batteries are required to have excellent charging and discharging performance, but also higher safety performance. Then why did the lithium-ion battery catch fire or even explode, and are there any measures to guard against and prevent it?

The explosion of a notebook battery is not only related to the processing technology of the lithium-ion battery cells used in it, but also to the battery protection board packaged in the battery, the charge 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 overheat the battery cells, which greatly increases the activity of the battery cells and increases the probability of explosion and burning.

Lithium-ion battery material composition and performance analysis

First, let's take a look at the material composition of lithium-ion batteries. The performance of lithium-ion batteries depends on the structure and performance of the internal materials used in the battery. These battery internal materials include negative electrode materials, electrolytes, separators, 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 on cheap and high-performance positive and negative electrode materials has always been the focus of the development of the lithium-ion battery industry.

Low temperature lithium iron phosphate battery 3.2V 20A
Low temperature lithium iron phosphate battery 3.2V 20A
-20℃ charge, -40℃ 3C discharge capacity≥70%
Charging temperature: -20~45℃
-Discharge temperature: -40~+55℃
-40℃ support maximum discharge rate: 3C
-40℃ 3C discharge capacity retention rate≥70%

Click for details
The negative electrode material is generally made of carbon material, and the current development is relatively mature. The development of cathode materials has become an important factor restricting the further improvement of lithium-ion battery performance and the further reduction of price. In the current commercialized lithium-ion batteries, the cost of the cathode material accounts for about 40% of the entire battery cost, and the reduction in the price of the cathode material directly determines the reduction in the price of the lithium-ion battery. This is especially true for lithium-ion power lithium-ion batteries. For example, a small lithium-ion battery for a mobile phone only needs about 5 grams of positive electrode material, while a lithium-ion power lithium-ion battery for driving a bus may require up to 500 kilograms of positive electrode material.

Although there are many types of cathode materials that can theoretically be used for lithium-ion batteries, the main component of common cathode materials is LiCoO2. When charging, the potential applied to the two poles of the battery forces the compounds of the cathode to release lithium ions, and the molecules of the embedded anode are arranged in a lamellar structure. in carbon. During discharge, lithium ions are precipitated from the carbon in the lamellar structure and recombine with the compound of the positive electrode. The movement of lithium ions creates an electric current. This is how lithium-ion batteries work.

Li-ion battery charge and discharge management design

When the lithium-ion battery is charged, the potential applied to the two poles of the battery forces the compound of the positive electrode to release lithium ions, which are embedded in the carbon whose molecules of the negative electrode are arranged in a lamellar structure. During discharge, lithium ions are precipitated from the carbon in the lamellar structure and recombine with the compound of the positive electrode. The movement of lithium ions creates an electric current. Although the principle is very simple, in actual industrial processing, there are many more practical problems to be considered: the positive electrode material needs additives to maintain the activity of multiple charging and discharging, and the negative electrode material needs to be designed at the molecular structure level to accommodate more In addition to maintaining stability, the electrolyte filled between the positive and negative electrodes must also have good conductivity and reduce the internal resistance of the battery.

Although the lithium-ion battery has the advantages mentioned above, it has relatively high requirements on the protection circuit. During use, it should be strictly guarded against overcharge and overdischarge, 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 a lithium-ion battery is shown in the figure. In a charging cycle, the lithium-ion battery should test the voltage and temperature of the battery before charging to determine whether it is rechargeable. Charging is prohibited if the battery voltage or temperature is outside the manufacturer's approved range. The voltage range allowed for charging is: 2.5V~4.2V per cell.

Low temperature high energy density 18650 3350mAh
Low temperature high energy density 18650 3350mAh
-40℃ 0.5C discharge capacity≥60%
Charging temperature: 0~45℃
Discharge temperature: -40~+55℃
Specific energy: 240Wh/kg
-40℃ discharge capacity retention rate: 0.5C discharge capacity≥60%

Click for details
When the battery is in deep discharge, the charger must have a pre-charging process to make the battery meet the conditions of fast charging; then, according to the fast charging speed recommended by the battery manufacturer, generally 1C, the charger will charge the battery with constant current, The battery voltage rises slowly; once the battery voltage reaches the set termination voltage (usually 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 Attenuate until the charging rate drops below C/10 or when the full charge time is overtime, switch to the top cut-off charging; when the top cut-off charging, the charger supplements the battery with a very small charging current. After the top end of charging for a period of time, it is closed for charging.

Lithium-ion battery protection circuit design

Due to the chemical characteristics of the lithium-ion battery, during normal use, the internal chemical reaction of the mutual conversion of electrical energy and chemical energy is carried out, but under certain conditions, such as overcharge, overdischarge and overcurrent will cause the battery A chemical side reaction occurs inside. When the side reaction is intensified, it will seriously affect the performance and service life of the battery, and a large amount of gas may appear, which will cause the internal pressure of the battery to rapidly increase and cause safety problems. Therefore, all lithium-ion batteries must be A protection circuit is used to effectively monitor the charging and discharging state of the battery, and shut down the charging and discharging circuit under certain conditions to guard against damage to the battery.

The lithium-ion battery protection circuit includes overcharge protection, overcurrent/short circuit protection and overdischarge protection, requiring high precision overcharge protection, low power consumption of the protection IC, high withstand voltage, and zero volt rechargeability. The following article will specifically analyze the principles, new functions and feature requirements of these three protection circuits, which are of reference value for engineers to design and develop protection circuits.

Li-ion battery protection circuit design case sharing

In the circuit design with lithium ion battery as the power supply, it is required to integrate the more and more complex mixed-signal system into a small area chip, which inevitably raises the problem of low voltage and low power consumption for digital and analog circuits. In the constraints of power consumption and function, how to obtain the best design method is also a research focus of current power management technology (powerManagement, pM). On the other hand, the use of lithium-ion batteries has also greatly promoted the design and development of corresponding battery management and battery protection circuits. Lithium-ion batteries must have complex control circuits to effectively guard against overcharge, overdischarge and overcurrent states of the battery.

From the energy transition trend of electric bicycles, the method of using ultra-low power consumption and high performance MSp430F20X3 to design the lithium-ion battery charging and discharging protection circuit of electric bicycles is discussed. This method discusses the whole process of design from every detail of system architecture, charging and discharging circuit, testing and protection circuit design, and provides a more comprehensive reference for designers of electric bicycle power supplies.