Research status and development trends of 18650 1800mah battery management systems

　　Affected by factors such as the energy crisis and environmental protection, pure electric vehicles, as an emerging means of transportation, have become an important research field in the automotive industry due to their advantages of high energy utilization, no emissions, low noise, and diversified energy sources. Traditional electric vehicles have shortcomings such as limited cruising range, short battery life, and limitations in battery size and quality. Lithium-ion batteries have high energy density, high operating voltage, no memory effect, long cycle life, no pollution, and light weight. , small self-discharge and other characteristics, can well solve the above problems, so electric vehicles based on lithium-ion batteries have attracted more and more people's attention, and lithium-ion batteries have become one of the candidate energy sources for pure electric vehicles. Currently, the world's major automakers including Honda, Toyota, and General Motors have plans to launch lithium-ion electric vehicles in 2010. Domestic BYD, Geely, and Chery also have similar plans.

　　The voltage requirements of pure electric vehicles are generally above 100V. In order to achieve this operating voltage, dozens or even hundreds of single cells must be connected in series and parallel. For example, the Wanxiang electric vehicle reported in [5] uses 84 Only lithium-ion battery cells are connected in series. Electric bicycles generally require a voltage of around 48V, so lithium-ion battery packs used for electric bicycles generally consist of 12 to 13 single cells.

　　Due to the limitations of the production process of lithium-ion batteries, there are inconsistencies in capacity, voltage, internal resistance, etc. between lithium-ion battery cells, even in the same batch of batteries. In addition, even if the battery pack has good consistency when it leaves the factory, during subsequent use, this inconsistency will increase as the number of cycles of the battery pack increases.

　　In lithium-ion battery packs, monomer inconsistency will cause the battery pack to be unable to exert its maximum capacity, and will greatly shorten the service life of the battery pack. In addition, due to the particularity of lithium-ion batteries, if overcharge, over-discharge, over-current, etc. occur during use, it will cause irreversible damage to the battery and even cause safety accidents. Therefore, before lithium-ion battery packs are used in electric vehicles, one of the first issues to be solved is the management of the battery pack.

　　1. Research status of power lithium-ion battery management system

　　Early lithium-ion battery management systems generally only had simple functions such as monitoring battery voltage, temperature, current and protection. As lithium batteries are increasingly used in high-power equipment, such as electric vehicles, the requirements for battery management systems are increasing. The higher you go, the more powerful the lithium-ion battery management system becomes. It is generally believed that the lithium-ion battery pack management system should have the following functions: detection of external parameters of the battery pack, judgment of battery status and estimation of remaining power, charge and discharge control of the battery pack, battery power balancing, and the function of providing communication with external devices . At present, the detection technology of battery external parameters has become mature. The current research focus of lithium-ion battery management system is the estimation of remaining battery power and the balance of battery pack.

　　1.1 Detection of battery external parameters

　　The detection of battery external parameters mainly includes the detection of single cell voltage, operating current and battery temperature in the battery pack. The working status of the battery can be judged through these parameters.

　　1.1.1 Voltage detection

　　Among all battery parameters, the voltage of a lithium-ion battery best reflects the condition of the battery. The basis for overcharge and overdischarge of lithium-ion batteries is the terminal voltage of the lithium-ion battery, and the SOC of the lithium-ion battery can also be initially estimated by measuring the terminal voltage. Therefore, it is very important to detect the voltage of lithium-ion batteries in real time. There are four main testing methods for lithium-ion battery packs. The traditional testing method is to use relays and capacitors for isolation processing.

　　The testing principle is: first, the battery voltage is sampled through the capacitor, and then the battery voltage can be obtained by detecting the voltage of the capacitor. The second method is to measure the battery terminal voltage with floating ground technology. During measurement, the window comparator automatically determines whether the current ground potential is appropriate. If it is right, start the A/D for measurement. If it is too high or too low, the controller passes the D/ A is to the ground and controls the floating potential. The third method is the common mode detection method. The common mode measurement is relative to the same reference point. Precision resistors are used to attenuate the voltage at each point in equal proportions, and then subtracted in sequence to obtain the voltage of each cell. The fourth method is the differential mode detection method, which uses an operational amplifier to eliminate the common-mode voltage at both ends of the battery to complete the sampling of the battery voltage.

　　The first measurement method has a simple principle, but the detection accuracy is low and the detection time is long. In floating ground technology measurement, since the ground potential is often changed by on-site interference, the ground potential cannot be accurately controlled, which will affect the measurement accuracy of the entire system. The common mode measurement method has a simple circuit and low measurement accuracy, and is only suitable for situations where the number of batteries connected in series is small or the measurement accuracy is not required. The accuracy of the differential mode measurement method is better than the other three methods. The differential mode detection circuit shown in Figure 1 is suitable for series battery packs of less than 12 cells. When the battery pack has a large number of single cells, 12 cells are generally used as a voltage detection module, and then all modules are connected together through a bus. In the differential mode detection circuit, due to the influence of the leakage current of the voltage measurement circuit, the battery pack close to the negative electrode will consume too much power, resulting in inconsistency in the battery pack.

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