How to set up a safe lithium battery protection circuit

2019-04-11 10:01:55 0

According to statistics, the global demand for lithium-ion batteries has reached 1.3 billion, and this data is increasing year by year as the application field continues to expand. For this reason, with the rapid increase in the usage of lithium-ion batteries in various industries, the safety performance of batteries has become increasingly prominent, requiring not only lithium-ion batteries with excellent charging and discharging performance, but also higher safety performance. Why did the lithium battery explode or even explode in the end, what measures can be avoided and eliminated?

The laptop battery explosion is not only related to the production process of the lithium battery cells used therein, but also related 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 the notebook computer will overheat the battery cells, thereby greatly increasing the activity of the battery cells and increasing the probability of explosion and combustion.

Analysis of the composition and performance of lithium battery materials

First, let's take a look at the material composition of lithium batteries. The performance of lithium-ion batteries depends mainly on the structure and properties of the materials used in the batteries used. These battery internal materials include a negative electrode material, an electrolyte, a separator, a positive electrode material, and the like. The choice and quality of the positive and negative materials directly determine the performance and price of the lithium ion battery. Therefore, the research of low-cost, high-performance positive and negative materials has always been the focus of the development of the lithium-ion battery industry.

The anode 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 further price reduction. In the current commercial production of lithium-ion batteries, the cost of the cathode material accounts for about 40% of the total battery cost, and the reduction in the price of the cathode material directly determines the price of the lithium-ion battery. This is especially true for lithium-ion power batteries. For example, a small lithium-ion battery for a mobile phone requires only about 5 grams of positive electrode material, and a lithium-ion battery that drives a bus may require up to 500 kilograms of positive electrode material.

Although theoretically it can be used as a cathode material for lithium ion batteries, the main component of the common cathode material is LiCoO2. When charging, the potential applied to the two poles of the battery forces the cathode compound to release lithium ions, and the embedded anode molecules are arranged in a sheet structure. In the carbon. At the time of discharge, lithium ions are precipitated from the carbon of the sheet structure and recombined with the compound of the positive electrode. The movement of lithium ions produces a current. This is the principle of working lithium batteries.

Lithium battery charge and discharge management design

When the lithium battery is charged, the potential applied to the two poles of the battery forces the compound of the positive electrode to release lithium ions, and is embedded in the carbon in which the negative electrode molecules are arranged in a sheet structure. At the time of discharge, lithium ions are precipitated from the carbon of the sheet structure and recombined with the compound of the positive electrode. The movement of lithium ions produces a current. Although the principle is very simple, in actual industrial production, there are many practical problems to be considered: the material of the positive electrode needs additives to maintain the activity of multiple charge and discharge, and the material of the negative electrode needs to be designed at the molecular structure level to accommodate more. More lithium ions; the electrolyte filled between the positive and negative electrodes, in addition to maintaining stability, also needs to have good electrical conductivity, reducing the internal resistance of the battery.

Although lithium-ion batteries have all the advantages mentioned above, they have higher requirements for protection circuits. In the process of use, overcharge and overdischarge should be strictly avoided, and the discharge current should not be too large. Generally, the discharge rate Should not be greater than 0.2C. The charging process of the lithium battery is shown in the figure. During a charging cycle, the lithium-ion battery needs to detect the voltage and temperature of the battery before charging begins to determine whether it is chargeable. Charging is prohibited if the battery voltage or temperature is outside the range allowed by the manufacturer. The voltage range that allows charging is: 2.5V~4.2V per battery.

In the case that the battery is in deep discharge, the charger must be required to have a precharge process to make the battery meet the condition of fast charging; then, according to the fast charging speed recommended by the battery manufacturer, generally 1C, the charger performs constant current charging on the battery. The battery voltage rises slowly; once the battery voltage reaches the set termination voltage (typically 4.1V or 4.2V), the constant current charge is terminated, the charging current is rapidly attenuated, and the charging enters the full charge process; during the full charge process, the charging current gradually Attenuation, until the charging rate drops below C/10 or when the full charging time expires, turn to the top cut-off charge; when the top is turned off, the charger replenishes the battery with a very small charging current. After the top end is turned off for a while, the charging is turned off.

Lithium battery protection circuit design

Due to the chemical characteristics of lithium-ion batteries, during normal use, the internal chemical reaction of electrical energy and chemical energy is mutually converted, but under certain conditions, such as overcharging, overdischarging and overcurrent, the battery will be caused. Chemical side reactions occur internally, which will seriously affect the performance and service life of the battery, and may generate a large amount of gas, causing the internal pressure of the battery to rapidly increase and explode, resulting in safety problems. Therefore, all lithium ion batteries are required. A protection circuit for effectively monitoring the state of charge and discharge of the battery, and turning off the charge and discharge circuits under certain conditions to prevent damage to the battery.

Lithium-ion battery protection circuits include over-charge protection, over-current/short-circuit protection, and over-discharge protection. They require over-charge protection for high precision, low IC protection, high withstand voltage, and zero-volt charge. The following article will detail the principles, new features and feature requirements of these three protection circuits, and have reference value for engineers designing and developing protection circuits.

Lithium battery protection circuit design case sharing

In the circuit design of the lithium battery as the power supply, it is required to integrate the increasingly complex mixed signal system into a small area chip, which inevitably brings low voltage and low power consumption problems to the digital and analog circuits. In terms of power consumption and function constraints, how to obtain the best design solution is also a research hotspot of current Power Management Technology (PM). On the other hand, the application of lithium batteries has greatly promoted the design and development of battery management and battery protection circuits. Lithium battery applications must have complex control circuits to effectively prevent overcharging, overdischarging, and overcurrent conditions in the battery.

From the trend of electric bicycle energy transition, the scheme of charging and discharging protection circuit for lithium battery of electric bicycle with ultra low power consumption and high performance MSP430F20X3 is discussed. The program discusses the whole process of design from every detail of system architecture, charge and discharge circuit, detection and protection circuit design, and provides a comprehensive reference for the designers of electric bicycle power.