6F22 carbon battery.Detailed introduction to equalization of series connected lithium-ion battery packs

　　Detailed introduction to equalization of series connected lithium-ion battery packs

　　As a high-power lithium battery pack that provides power for electric vehicles, in order to ensure the capacity of the entire battery pack, balanced management of the battery is necessary. Various battery management ICs on the market provide voltage equalization management functions, which users can achieve by just connecting some external components. Unfortunately, most battery management ICs use resistor shunting/power consumption to achieve balance between batteries. This method is acceptable in low-power battery packs, such as laptops, handheld devices, or electric bicycle batteries. But for batteries with hundreds of amp hours, it is really a waste to consume the power in vain, and for electric vehicles that promote "environmental protection/energy saving", it is somewhat unjustifiable. In fact, in the lead-acid battery era, various methods have emerged for balancing the battery pack. The following mainly introduces several feasible balancing methods for lithium battery packs. 1. Resistor shunting/energy dissipation/bleeding/shutTing is the most common and simplest method on the market. The protective plates for lithium battery packs exhibited at the 2009 Electric Bicycle Exhibition, including those for electric vehicles, are balanced by resistors. The reason is also very simple: it is easy to implement, and there is no better way. In fact, it is also the cheapest way. The circuit principle is as follows: The principle is very simple. The controller will monitor the voltage of each battery in real time. If the voltage of a certain battery is found to be too high and the condition for balanced opening is reached, the switch corresponding to the battery will be turned on. At this time, the battery Current will flow through the corresponding resistor. If it is charging at the time, the current flowing through the battery will decrease, the charging speed of the battery will slow down, and eventually the voltage will be the same as other batteries; if it is discharging or idle, then the battery's power will be on the corresponding resistor consumed, and eventually the voltage drops to the same level as other batteries. Of course, you can set the balancing conditions yourself, or you can set the balancing function to only be performed during charging. The biggest advantages of this method are: low cost, simple structure and easy to implement. For small companies without R&D capabilities, this is the only existing method. The disadvantages are equally obvious: waste of energy, heat generation, and low equilibrium efficiency, because it takes time to consume energy through resistance. In the case of high-power battery packs, this must also be considered. In fact, many large companies, such as TI and Infineon, have implemented more advanced balancing methods, making energy use more efficient and balancing faster. TI has developed a dedicated IC in this regard—bq78pL114. The next section will introduce the active balancing used by this IC: powerpump cell balancing.

　　Last time I introduced the most widely used and simplest lithium-ion battery balancing method. Of course, other balancing methods have always existed, but because large-capacity lithium batteries have not been promoted, and many manufacturers do not have the strength to commercialize these technologies, so Rarely seen on the market. Fortunately, TI has integrated one of the active balancing methods into a dedicated IC. Although it can also be achieved using CPU, the reliability of software is not as reliable as hardware after all, and productization becomes simpler in this way. Here is a brief introduction. The following is the schematic diagram in bq78pL114ApplicationNote: In fact, the principle is very simple and has been introduced in many Ti documents. When V3 is higher than V2, the IC pin connected to p3S outputs pWM. When pWMOFF, Q1 is turned on, and V3 forms a loop with the inductor through Q1 (as shown in the red line in the figure). The power of V3 is consumed. At this time, the current in the inductor rises and the energy is stored in the inductor. At pWMON, Q1 is turned off. At this time, V2 forms a freewheeling circuit through the diode and inductor in Q2 (as shown in the blue line in the figure). At this time, the current stored in the inductor decreases and the energy is transferred to V2. At this point, the process of energy transfer from V3 to V2 is completed, until the pWM is turned off when the voltages of the two are equal. The figure below is the waveform when balancing a set of 2Ah lithium batteries. Using pWM with a fixed duty cycle, we can see the current changes in the inductor and the balanced battery voltage ripple. During the charging stage, the voltage rises.

　　For the selection of pWM frequency, duty cycle, and inductor capacity, the three are related to each other. The recommended frequency of Ti's IC is 200kHz, which can reduce the inductance and thereby reduce costs. The number of pWMs required for this method is 2×(n-1), where n is the number of batteries. There are relatively many. And energy transfer can only be transferred from one battery to its adjacent battery. If the number of batteries is large, the control algorithm is still quite complex. Of course, now that Ti has integrated the algorithm into IC, the key issue is just cost. But with bq78pL114+BQ76pL102, up to 8 batteries can be managed. If an electric car requires 80 cells, then 10 such battery packs are needed to manage them separately. In this case, the cost increases again. Therefore, this IC should be more suitable for handheld devices, laptop batteries, or electric bicycle batteries. There is also a method of using capacitors to store electricity for energy transfer, but the control logic is more complicated because more switches are used, and short circuits are likely to occur if the control is wrong, so I will not introduce it. The simplest way in terms of circuit and logic control is to use a transformer, which is easy to understand. The key is that the design and manufacturing costs of transformers are relatively high and the size is also larger. If the active balancing method of large-capacity lithium-ion batteries is developed, it should be the inductor and transformer method introduced in this article. Infineon currently has an article on the Internet that introduces it, which you can refer to.

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