Lithium-ion power battery pack management system solution
Lithium-ion batteries have been widely used for more than ten years, but in the early days they were mainly used in a series of small mobile electronic products such as mobile phones, notebook computers, camcorders, DVDs, etc. These occasions are often used in a single string, with low load current and high safety factor. . In the past two years, lithium-ion batteries have been rapidly applied in the fields of electric bicycles, electric tools and power toys due to their lightness, high energy density, and pollution-free characteristics, and have gradually been applied to the fields of hybrid vehicles and electric vehicles. However, the safety of power lithium-ion batteries is still the most concerned issue at present, so its protection is very important. In addition to ensuring the continuous improvement of the safety of lithium-ion batteries, the battery management system must be studied at the same time, so that the battery and its applications can develop in a balanced manner. Li-ion battery protection mainly includes overcharge protection, overdischarge protection, overcurrent and short circuit protection, etc.
1 Function of protection circuit
1.1 Overcharge protection
For lithium-ion batteries, the maximum voltage of a single cell after charging must not exceed the specified value, otherwise the electrolyte in the battery will be decomposed, causing the temperature to rise and produce gas, reducing the service life of the cell, and even causing an explosion in severe cases Therefore, the protection circuit must ensure that it is absolutely not overcharged, and the terminal voltage of each battery in the battery pack must be monitored. When the voltage of the battery cell exceeds the set value, the overcharge protection function is activated, and the protection circuit cuts off the charging Circuit, stop charging. The protection can be stopped when the cell voltage returns to the allowable voltage and the overcharge lock mode is released. Lithium-ion batteries of different materials have different specified values for their protection voltage and release voltage.
In addition, you must also pay attention to the misoperation caused by noise. In order to prevent misjudgment and misoperation, the overcharge protection delay must be set, and the delay time cannot be shorter than the duration of the noise. When the voltage continues to exceed the overcharge detection voltage for more than a certain period of time, the overcharge protection will be triggered.
1.2 Over-discharge protection
Excessive discharge of lithium-ion batteries will also shorten their service life, and the damage to the battery is often irreversible. In order to prevent the over-discharge of the lithium-ion battery, when the voltage of the lithium-ion battery is lower than its over-discharge voltage detection point, the over-discharge protection is activated, the discharge is stopped, and the battery is kept in the standby mode with low quiescent current. Charge protection.
1.3 Over current/short circuit protection
The maximum discharge current of lithium-ion batteries has a certain limit. Excessive discharge current will also cause irreversible damage to the lithium battery and affect its service life.
The function of short-circuit protection is actually an extension of over-current protection. If a high current discharge is caused by an external short circuit, the discharge must be stopped immediately, otherwise it may cause serious damage to the lithium battery itself and external equipment.
The delay time of over-current protection is generally at least several hundred microseconds to milliseconds, while the delay time of short-circuit protection is microseconds. The circuit is cut off almost at the moment of short-circuit, which can avoid huge damage to the battery caused by short-circuit. .
As far as power tools are concerned, the setting of the protection current value and delay time must also be combined with the parameters of the power tool itself, otherwise it will affect the output torque of the tool and the life of the motor.
1.4 Battery balance
Power lithium-ion batteries generally need several strings, dozens of strings, or even hundreds of strings or more. In the production process, the battery must go through many processes from the beginning of the film to the finished product. Even after strict inspection procedures, each group of power supplies The voltage, resistance, and capacity of the battery are the same, but after a period of use, the internal resistance, voltage, capacity and other parameters of the battery will fluctuate and form an inconsistent state, which will cause differences in one way or another. This difference is reflected in the different voltages between battery cells connected in series when the battery pack is fully charged or discharged. In this case, during the charging process of the battery pack, the battery cell with too high voltage triggers the battery pack overcharge protection early, and the battery cell with the low voltage during the discharge process causes the battery pack over-discharge protection, thus making the whole battery pack The capacity is significantly reduced. The capacity of the entire battery pack is the capacity of the worst-performing battery cell in the battery pack, and it is prone to overcharging and overdischarging during use, and it is not easy to find, leading to premature failure. Therefore, the protection circuit is required to complete the balancing operation of the battery cells to extract excess current from the battery with higher voltage, consume the excess power, achieve battery balance, maximize the utility of the power lithium battery, and extend the battery life. , Increase security. At present, the commonly used equalization methods include energy storage equalization and resistance equalization.
Energy storage equalization is the use of batteries to charge and discharge energy storage elements such as inductors or capacitors, and realize the switching of energy storage elements between unbalanced batteries through relays or switching devices to achieve energy transfer between batteries. This balanced charging method generally has a complex control network and high safety management requirements. In use, you should pay attention to the charge and discharge time of the energy storage element. Its biggest advantage is that it can balance each unit during charging and discharging (working) use. The function of the battery, and does not consume the power of the lithium-ion battery pack.
Resistance equalization is generally performed by the controller to control the on-off of the resistor network to balance the battery pack. This method can balance multiple batteries at the same time, and the control is simple. However, if the resistance is selected too large during the balancing process, the balancing current is too small, and the effect is small; if the resistance is selected too small, the resistance power is large, the system energy loss is large, the balancing efficiency is low, and the system has higher requirements for thermal management. , The need for temperature detection and control.
The principle of resistance equalization is that in the process of charging the battery pack, when a battery is charged faster and the voltage is higher than other batteries, the system controls the conduction and shunt of the equalization resistance by controlling the switch to reduce the charging speed of the battery to reach each battery. The purpose of battery equalization charging.
2 Realization of protection function
There are two main protection methods for lithium-ion batteries: microcontroller control and integrated circuit protection chip.
2.1 IC control
At present, there are many chips that can realize the protection function of lithium-ion batteries. There are many types of chips to choose from abroad, Taiwan, and the mainland. At present, Japanese Ricoh and Seiko have adopted more solutions. The solutions are mature and the peripheral circuits are simple, but the price is relatively expensive.
The functions implemented by various protection ICs are almost the same, and their protection modes are similar to those of external circuits. In actual applications, different ICs can be selected according to the needs. When choosing an IC, you need to consider many ways. Different types of ICs have different overcharge protection voltages, ranging from 4.25V to 4.35V, as well as the power consumption of the IC, whether the peripheral circuit is simple enough, and whether the accuracy of each parameter of the protection IC is To meet the requirements, consider whether the volume is small enough.
In addition to the perfect protection function of the protection board, low power consumption is also an important parameter. To prevent over-discharge, the protection IC must detect the battery voltage. Once it reaches the over-discharge detection voltage, it must turn off the power MOSFET to stop the discharge. But at this time, the battery itself still has natural discharge and current consumption of the protection IC, so it is necessary to minimize the current consumption of the protection IC. In the protection state, its quiescent current consumption must be less than 0.1uA.
In addition, the power lithium-ion battery pack will generate high voltage instantly when it is working or charging, so the protection IC should meet the requirements of high voltage resistance.
2.2 MCU control
Some existing integrated circuit protection chips are mainly aimed at the protection of battery packs with less than 4 cells. For battery packs with more than 4 cells, multiple single-level protection chips can be connected in series or several multi-level protection chips can be connected in series. Way. However, the circuit that uses multiple protection chips in series to protect the battery pack with more than 4 cells has poor scalability. At the same time, integrated protection chips often only target one or one type of battery characteristics, lack flexibility, and often cost relatively high. For this reason, combined with the charging and discharging characteristics of lithium-ion power batteries, in many occasions, the power lithium-ion battery protection circuit adopts a design scheme with MCU (microprocessor) as the core.
With the microprocessor as the core of various function control, in addition to providing overcharge, overdischarge, and overcurrent protection for the lithium-ion battery pack, it effectively provides dynamic balance and temperature for the charging and discharging of each single-cell lithium battery in the lithium-ion battery pack. In addition to protection and short-circuit protection, it can also provide functions such as capacity prediction, communication, and identification.
3 Hardware anti-interference measures
As a part of an application system, the power lithium-ion battery management system is often subject to various electromagnetic interferences. The actual working environment is relatively harsh. It is necessary to take certain anti-interference measures in the hardware design and PCB board wiring.
4 Other requirements
Because the battery is mainly used to power the main application, the BMS is required to have very low power consumption.
The monitoring of power lithium-ion battery packs is a relatively new topic. Its management system will comprehensively monitor and protect the technical design ideas. It has basic functions such as static, charging, discharging, management, and automatic maintenance of the battery pack, so as to be practical and reliable. Requirements.