Brief analysis of the active charge balance method of lithium battery pack

Analysis of the active charging balance method for lithium batteries The automotive system engineering department of Infineon Technologies in Munich recently received a task to develop electric vehicles. E-cart is a drivable vehicle, which is mainly used to demonstrate the electric performance of hybrid vehicles. The car will be powered by a huge lithium battery pack, and the developers know that a balanced battery is necessary. In this case, it is necessary to choose automatic energy transfer between batteries instead of the traditional simple charging balance method. The self-charge balancing system they developed can provide superior functions at the same cost as the mandatory plan. Battery structure For many years, nickel-cadmium batteries and nickel-hydrogen batteries have dominated the battery market. Lithium batteries have only recently entered the market, but their market share is rising rapidly because their performance has been greatly improved. Lithium batteries have impressive storage capacity, but even so, the capacity of a single battery is too low, no matter whether it is voltage or current, it can not meet the needs of hybrid engines. Multiple batteries in parallel can increase the current supplied by the battery, and multiple batteries in series can increase the voltage supplied by the battery. Battery packers usually use abbreviations to describe their battery products, such as 3P50S, which means that the battery pack has three parallel battery cells and 50 battery cells connected in series. The modular structure is ideal for handling batteries that include multiple series of battery cells. For example, in a 3P12S battery array, every 12 batteries are connected in series to form a block. These cells can be controlled and balanced by an electronic circuit centered on a microcontroller. The output voltage of this battery module depends on the number of batteries connected in series and the voltage of each battery. The voltage of a lithium battery is usually between 3.3v and 3.6v, so the voltage of the battery module is approximately between 30V and 45V. The driving of the hybrid vehicle requires a direct current power supply of 450 volts. In order to compensate for changes in battery cell voltage according to the state of charge, it is appropriate to connect a dc-dc converter between the battery pack and the engine. The converter can also limit the current output of the battery pack. In order to ensure that the dc-dc converter works in the best condition, the battery pack voltage is required to be between 150V and 300V. Therefore, 5 to 8 battery modules are connected in series. The necessity of balance If the voltage exceeds the allowable size, the lithium battery is easily damaged (see Figure 2). If the voltage exceeds the upper and lower limits (nano-phosphate lithium batteries, the lower limit is 2v and the upper limit is 3.6v), the battery may suffer irreparable damage. The result, at least, is to accelerate the self-discharge of the battery. The output voltage of the battery is stable in a wide state of charge (SOC) range, and the risk of voltage violation of the safety range is small. But at both ends of the safe range, the charging curve is relatively steep. Therefore, in order to prevent it, it is necessary to monitor the voltage closely. If the voltage reaches a critical value, the discharging or charging process must be stopped immediately. With the help of a robust balance circuit, the voltage of the relevant battery can be returned to a safe scale. But to do this, the circuit must be able to transfer energy between the batteries when the voltage of any one of the batteries starts to be different from the other batteries. Charge balance method 1. Traditional compulsory method: In a general battery processing system, each battery cell is connected to a load resistor through a switch. This forcing circuit can discharge individually selected cells. However, this method can only suppress the voltage increase of the strongest battery in the form of charging. In order to limit power consumption, this kind of circuit usually only allows discharge with a small current of about 100mA, resulting in a charge balance that may take several hours. 2. Automatic balancing method: There are many automatic balancing methods in related materials, which require a storage element to carry energy. If a capacitor is used as a storage element, connecting it to any battery cell will require a huge array of switches. A more effective way is to store energy in a magnetic field. The key component in the circuit is the transformer. The prototype was developed by the Infineon development team in cooperation with Vogt Electronic Components Co., Ltd. Its functions are: A. Transfer energy between battery cells B. Connect multiple single battery voltages to the ADC input with a base voltage. The circuit is constructed based on the principle of a reverse scan transformer. This kind of transformer can store energy in a magnetic field. Disclaimer: Some pictures and content of articles published on this site are from the Internet. If there is any infringement, please contact to delete. Previous: Dry and hot air impact drying process for lithium batteries