KEMET application note explains basic Battery Management System (BMS) function, topologies and inductor requirements. Metal composite inductor benefits for BMS and EMI suppression filter are benchmarked versus conventional ferrite core technology.
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.
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.
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.
Nickel metal hydride battery
Nickel cadmium battery
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.
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.
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.
Centralized BMS has the advantages of low cost, compact structure, and high reliability. It is common in small battery systems that they have lower capacitance and lower total voltage battery packs. Centralized BMS is generally used for low-speed vehicles such as electric bicycles, electric motorcycles, electric sightseeing cars, electric patrol cars, electric golf carts, etc., and also used for low battery/gasoline ratio hybrid vehicles as MHEV.
Semi-distributed BMS is between centralized and distributed BMS and manages a medium number of battery cells, with medium capacitance and voltage-rated battery packs. Like distributed BMS, its BMU and CSC are on different PCBs, but the number of battery modules managed by each CSC is more than one. Semi-distributed BMS can also be used in HEV, PHEV, and some EVs in electric vehicles.
Distributed BMS topology can better realize the hierarchical management of the module-level (module) module and the system-level (pack). As the power battery system of passenger cars continues to develop towards higher battery capacitance and higher voltage battery packs, the BMS with distributed topology is mainly used for PHEV and BEV. At present, mainstream mass-produced electric vehicles generally adopt a distributed BMS topology, especially for BEV.
Introduction to BMS and CSC
To achieve the BMS functions, the BMS will operate with two main parts, the main control board (referred to here as the BMU) and the slave board (referred to here as the CSC).
BMS also includes the part that manages high voltage called HVU, but it has been omitted because it is beyond the scope of this article.
The primary function of the BMU is to communicate with other ECUs/VCUs through CAN, process the data collected from CSC, and charge and discharge management. The CSC is responsible for voltage detection, temperature detection, balance management of the cells in the module (some have independent CSU module units), and corresponding diagnosis work.
If the CSC is directly connected to the battery module, CSC is on the high voltage side. So, the signal transmission between the BMU and CSC must be kept isolated typically, a pulse transformer or a capacitor is used as the isolation component) to separate the high voltage and low voltage sides.
Same as other applications, DC-DC is used for the power supply for main chips. Isolated DC-DC is used between high voltage and low voltage sides.
Figure 3 shows an example structure diagram of a distributed BMS.
Isolated and Non-Isolated DC/DC
It can be seen from Figure 3 that non-insulated DC/DC and insulated DC/DC are used in BMS. Usually, their operation is controlled by the DC/DC main chip. A power inductor needs to be used in a non-insulated DC/DC as a voltage transforming component in general solutions. Isolated DC/DC will use a transformer instead of an inductor as the voltage transforming component. Figures 4 and 5 shows isolated and non-isolated DC/DC circuit examples.
The main chip is the most critical component in the circuit, and it is the core of the circuit. Around the main chip, many passive components are used, such as resistors, capacitors, and inductors/transformers. Among 5 them, the inductor/transformer is an essential component. Its importance is next to the main chip in this circuit. The inductor/transformer is connected to the chip’s SW pin. Their function is to temporarily convert electrical energy into magnetic energy, store it in the magnetic core, and then release it in the form of electrical energy to transform voltage. Therefore, the quality directly affects the performance of the entire DC/DC circuit, such as efficiency, power losses, output ripple, EMI, response speed, etc.
In addition, for applications with strict EMI requirements, EMI filters are used at the voltage input. EMI filters are usually made up of inductors, capacitors, and resistors individually or in multiple combinations, depending on the severity and frequency of EMI. The EMI filter of the non-isolated circuit in Figure 4 is mainly composed of two capacitors, and the EMI filter of the isolated DC/DC circuit in Figure 5 is primarily composed of two capacitors and an inductor. Since its shape looks like the symbol of π in mathematics, it is often called π Filter. The π-shaped filter has better filtering characteristics than the filter with only two capacitors in the non-isolated DC/DC circuit. Although EMI filtering is only an optional circuit, for BMS, which are required to work in harsh EMC environments, adding an EMI filter on the DC/DC circuit is highly advisable.