BMS and KEMET METCOM Inductors INTRODUCTION Battery Management System (BMS) is an indispensable part of electric vehicles. It is a vital link that connects on-board batteries and other electric vehicle parts such as the Vehicle Control Unit (VCU). Its main functions are described below. When one of the below functions fails, it will cause fatal harm to the battery. It could even cause the battery to explode or burn, causing accidents or casualties. FUNCTIONS OF BMS Continuously monitor the condition of the battery unit (managing SOC, SOH, SOE, etc.). Prevent the battery cell from aging caused by repetitive operation of the battery. Examples include unbalanced cells caused by repeated charging or discharging, temperature changing, etc. Realize the long-term use of the battery pack and maximize the operating life. Measure other parameters such as voltage and temperature of the whole battery pack or each battery cell. Compensate for the slight inconsistency of each cell (balancing). Report the status and communicate with VCU or other ECUs. Show the battery status on the car display unit and alert the driver if there are any abnormalities. BMS FOR DIFFERENT VOLTAGE AND CAPACITY LEVELS BMS is used in various battery-driven electronic devices, not only for automotive applications but also for a variety of non-automotive applications, from mobile phones to power storage devices. All automotive applications, from electric golf carts to EVs (the focus in this article will be on automotive applications highlighted in gray in the table below), need to use a BMS to ensure that the battery can operate safely. With different voltage and capacitance battery packs used in the products, some parts in the BMS need to be isolated Therefore, different topologies of BMS are required for the specific battery voltage and capacitance values. Table 1 shows some examples of applications that work at different voltage ranges. Applications Other Examples Voltage Range Electric golf carts Electric bicycles, MHEV Under 200 V Electric passenger car (HEV, Mini electric passenger car 200 400 V some PHEV) Electric passenger car (some Special vehicle 400 600 V PHEV, BEV) Electric bus Double-decker electric bus 600 800 V Energy storage system Mobile charging car 800 V and above Table 1 Examples of Applications BATTERY TYPE AND TOPOLOGY OF BATTERY PACK As a power battery for EV, many types of batteries can be used. Each battery has a different output voltage, safety, price, energy density, operation life, etc. Lead-acid battery 1 Lithium-ion battery Nickel metal hydride battery Nickel cadmium battery All-solid-state battery Fuel cell Among the above types of batteries, lithium-ion batteries are most commonly used in EVs because of their excellent energy density characteristics. The battery pack most commonly used in EVs is when a single battery cell connects other single battery cells in series or parallel (usually in series) to form a battery module. It then combines the battery modules in series and parallel to create the final battery pack. For example, a battery pack labeled 330 V might be composed of 12 battery modules in series and 3 in parallel. Each battery module is composed of 12 battery cells in series. Assuming that the voltage of each lithium-ion battery cell is 2.3 V, the battery pack s actual voltage will be 2.3 V x 12 x 12 = 331.2 V. More battery cells in series will increase the battery pack voltage, and more battery cells in parallel will increase the battery pack capacity. BATTERY STRUCTURE A battery pack with a higher voltage or capacitance consists of more battery cells. In addition to the nominal voltage of the battery pack, we may also refer to the battery pack by the number of battery cells in series and parallel. With this concept, the battery pack in the example would be called a 144S (series) 3P (parallel) battery pack (S x P = the number of battery cells). The composition of the battery pack is shown in Figure 1. Figure 1 Battery Pack Structure Example (Image) DISPERSION OF BMS When the number of battery cells is small, the Battery Management Unit (BMU) and Cell Supervisory Circuit (CSC) are placed on the same PCB. But when the number of battery cells that need to be managed increases, the BMU and CSC, need to be placed on different PCBs. Each CSC has a limitation on the number of batteries that can be managed, and it helps reduce the total cable length. Therefore, the degree of dispersion depends on the number of battery cells that need to be managed (for hybrid vehicles, the higher the battery ratio, the larger the battery pack). The BMS products in the market can generally be divided into three different level topologies, which are shown in Figure 2. Their characteristics are discussed below. 2