Analysis: Lithium battery equalization circuit based on TL431

Nickel-metal hydride batteries are alkaline batteries. Due to the relatively low voltage of the cells and the "memory effect", regular large-scale discharge is necessary, which greatly aggravates the task of the power management system. Second, it also has the disadvantage of high self-discharge rate (10% to 15%).

Compared with other batteries, lithium-ion batteries have high power density (800W/Kg), high single-cell voltage (average voltage of 3.6V), no pollution to the environment, and low self-discharge rate (about 3% to 5%). The "memory effect" and other characteristics are an ideal power battery, so it is widely used in mobile power, hybrid vehicles, backup power supply in meddle and low voltage switchgear, and space flight energy storage devices.

1. Defects of existing lithium battery protection circuits in China

The average voltage of the lithium battery cell is only 3.6V, and the discharge current also has certain requirements. In order to improve the current and voltage levels of the system, in some power applications, a large current and large voltage lithium battery pack is generally used in parallel as an energy system. Since the lithium battery is very sensitive to voltage, the battery pack generally requires a certain protection circuit when it is used.

2, improved lithium battery protection circuit principle

The equalization circuit refers to the artificially added hardware circuit, which can maintain the consistency between the upper limit voltage of the whole battery pack or the lower limit voltage of the cell, thereby effectively protecting the upper limit charging voltage and the lower limit discharge voltage of the battery, fundamentally Reduce the impact of the battery on the system to achieve improved battery performance and extended battery life. It includes two circuits, upper equalization and lower equalization.

This paper designed two power supply systems for hybrid vehicles: 3-string lithium battery protection system (maximum voltage is 12.75V, average discharge voltage is 10.8V) and 10-string lithium battery protection system (maximum voltage is 42.5V, average discharge voltage is 36V), the discharge current is 10A and 40A respectively. The system principle is shown in Figure 1. The upper and lower equalization circuits are added to the basic protection circuit (overvoltage/undervoltage/overtemperature/overcurrent/short circuit protection).

Lithium battery equalization circuit based on TL431

Figure 1 improved protection circuit

2.1, TL431 equalization circuit

In this paper, the upper equalization circuit based on its characteristic design is shown in Figure 2. Adjust the resistance values of R1, R2, and R3. When the power supply voltage exceeds a certain set value, turn on the TL431, and reduce the voltage of the battery through the power resistor R* to reduce it to a fixed point (equalization point). Through the experimental results of the equalization circuit equipped for domestic electric vehicles and electric motorcycles, when the equilibrium point is 4.20V, the values of the resistance are: R1=68k! , R2=100k! , R3 = 4.3k! .

Lithium battery equalization circuit based on TL431

Figure 2: Equalization diagram on TL431

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