Lithium power battery circuit protection related knowledge

With the continuous advancement and development of science and technology, more and more portable electronic products such as mobile phones, notebook computers, PDAs, and digital video cameras are constantly emerging, which greatly enrich and facilitate people's lives. Nowadays, these portable electronic devices have become an indispensable part of people's lives, and have a very broad market prospect and development space. As these electronic products are moving toward miniaturization, lightweight, and portability, higher power requirements are imposed on the power supplies used. Therefore, the demand for a secondary battery having a small volume, a lightweight, and a high energy density is quite urgent. Compared with nickel-cadmium and nickel-hydrogen batteries, lithium batteries have become a lithium power battery because of their small size, light weight, high energy density, no memory effect, long service life, wide application range, high working voltage, and low self-discharge rate. The most widely used secondary battery.

Table of Contents

Premise background and significance

Solution

Prerequisite background and significance of low power lithium power battery protection circuit

Due to the high energy density of the lithium power battery, in the overcharged state, the energy will be excessive after the temperature of the battery rises, so that the electrolyte is decomposed to generate gas, which tends to increase the internal pressure and cause spontaneous combustion or rupture; otherwise, in the state of excessive discharge, The electrolyte is degraded to deteriorate battery characteristics and durability, reducing the number of chargeable times and shortening the battery life. Therefore, the protection of lithium batteries is very important. In battery applications, battery protection chips must be used to prevent overcharging, over discharging and overcurrent of the battery.

In summary, the design of the lithium power battery protection circuit is very important. However, the lithium power battery protection circuit will increase the additional loss of battery energy and reduce the application time of the battery, which requires the lithium power battery protection circuit to achieve low power consumption with high precision. In addition to the basic functions of overcharge protection, over discharge protection and overcurrent protection, a lithium power battery protection chip must meet the following requirements - this is also the goal of the chip designed in this paper.

(1) Ultra low power consumption. When the lithium power battery protection circuit is in operation, the power consumption consumed is the loss of the battery. Therefore, we must minimize the power consumption of the lithium power battery protection circuit.

(2) High precision detection voltage. In order to make the lithium power battery protection circuit respond correctly to different working states of the battery, the protection circuit must be able to accurately detect voltage parameters such as overcharge protection voltage and over-discharge protection voltage.

(3) Working correctly under a large voltage range. Since the supply voltage of the lithium power battery protection circuit is the battery voltage, and the battery voltage can float in a large range, the lithium power battery protection circuit is required to operate correctly within the voltage range.

lithium power battery protection circuit scheme

Among the various types of batteries currently used, lithium batteries (also known as lithium ion secondary batteries or lithium ion batteries) are a new type of power source that has been developed in the past decade. Lithium batteries are different from ordinary chemical power sources. The charging and discharging process is realized by the insertion and deintercalation of lithium ions in the positive and negative electrodes of the battery. The negative electrode of the lithium power battery is a carbon material such as graphite; the positive electrode is a lithium-containing transition metal oxide such as cobalt lithium oxide (LiC002). Moreover, the positive and negative materials of the lithium power battery are lithium ion intercalation compounds having a layered structure in which lithium ions can be freely embedded and desorbed, and lithium ions are interposed between the layers, and an electrochemical reaction occurs in an appropriate electrolyte. During charging, lithium ions are removed from the positive electrode lattice by the external electric field and are inserted into the negative electrode lattice through the electrolyte. The process of discharging is just the opposite. Lithium ions return to the positive electrode, and electrons pass through an external circuit to reach the positive electrode and recombine with lithium ions.

Compared with commonly used nickel-cadmium and nickel-hydrogen batteries, lithium batteries have many superior characteristics, mainly in the following aspects:

(1) The power supply voltage of the lithium power battery is high, generally 3.6V, which is about three times that of the nickel-cadmium battery and the nickel-hydrogen battery. For electronic equipment with high supply voltage requirements, the number of batteries required for battery packs can be greatly reduced. Therefore, the lithium power battery used in combination is easy to obtain a higher voltage.

(2) The specific energy is higher, that is, the lithium power battery of the same weight provides higher energy than other batteries. The specific energy of a lithium power battery is generally 2 to 3 times that of a nickel-cadmium battery or a nickel-hydrogen battery. Therefore, the portable electronic device is small and lightweight.

(3) No memory effect. Nickel-cadmium batteries and nickel-hydrogen batteries all have a memory effect and must be periodically discharged, otherwise, the battery will fail due to memory effects. The lithium power battery has no memory effect, and it does not need to pay attention to the amount of residual power, and can be directly charged. This allows the lithium power battery performance to be fully utilized.

(4) Long service life. The lithium power battery uses a carbon negative electrode, and the carbon negative electrode does not generate metallic lithium during charging and discharging, thereby preventing the battery from being damaged by the internal metal lithium short circuit. At present, the lithium power battery has a cycle life of more than 5,000 times, which is much higher than other types of batteries.

(5) The working environment has a wide temperature range, generally working between 30 °C and 0 °C, and has excellent high and low-temperature discharge performance.

(6) The self-discharge rate is low. The self-discharge rate, also known as the charge retention rate, refers to the amount of automatic discharge when the battery is not in use. The self-discharge rate of lithium batteries is 2% to 5%, the nickel-cadmium batteries are between 25% and 30%, and the nickel-hydrogen batteries are between 30% and 35%. Therefore, the lithium power battery retains the charge for the longest time in the same environment.

(7) lithium power battery does not contain any toxic elements such as mercury and cadmium and is a truly green battery.

Based on the above advantages, lithium batteries are widely used in portable electronic devices. On the other hand, lithium batteries have a high energy density, making it difficult to ensure battery safety. Specifically, in an overcharged state, the electrolyte is decomposed, causing the temperature and pressure inside the battery to rise; in the overdischarged state, the electrolytic material in the negative electrode, copper, melts to cause an internal short circuit, causing the temperature to rise: When the external circuit is short-circuited or discharged, the internal power consumption of the battery increases and the temperature rises, which may cause oxidation or decomposition of the electrolyte, resulting in a shortened life of the lithium power battery. In addition, if the lithium power battery is excessively discharged, the electrolyte in the battery will change, and the number of cycles of charging can be reduced, thereby affecting the service life of the lithium power battery.

Since lithium batteries have the above-mentioned disadvantages, a protection circuit must be added in the application of lithium batteries. The basic functions of the protection circuit also correspond to the above shortcomings, so we require the lithium power battery power protection chip to achieve the following basic functions: overcharge protection, over discharge protection, overcurrent protection and short circuit protection. According to the application requirements of the above lithium power battery, in order to improve the service life of the lithium power battery and ensure the safe use of the battery, the lithium power battery protection circuit needs to have the following functions:

(1) If the charging voltage exceeds the maximum allowable value of the battery, a battery discharge circuit can be provided.

(2) If the discharge voltage is lower than the minimum allowed by the battery, a battery charging circuit can be provided. Then cut off the connection between the battery and the external circuit, and cut off the connection between the battery and the external circuit.

(3) If the charge and discharge current of the battery is greater than the limit value, disconnect the battery from the external circuit.

(4) When the battery returns to the normal state, the protection circuit should be able to release the protection state accordingly, so that the battery can continue to work normally.

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