Talking about the Application Protection of Automotive Battery Management System (BMS)
The main function of the Battery Management System (BMS) is to improve battery utilization, prevent overcharging and over-discharging of the battery, extend battery life, and monitor battery status. This paper mainly combines the successful application of over-current over-voltage, and further illustrates the application of AEM FUSE/PPTC/TVS passive protection components around the safety design of battery management system (BMS).
What is a BMS?
The main function of the Battery Management System (BMS) is to improve battery utilization, prevent overcharging and over-discharging of the battery, extend battery life, and monitor battery status.
The BMS system of electric vehicles is more complicated and requires more functional requirements, such as real-time monitoring of power battery parameters of electric vehicles, fault diagnosis, SOC estimation, mileage estimation, short circuit protection, leakage monitoring, display alarm, charge and discharge mode selection, etc. And through the CAN bus way to interact with the vehicle integrated controller or charger to ensure efficient, reliable and safe operation of the electric vehicle, and to ensure the safety in the use of the vehicle. In short, it is the three basic functions:
1) Accurately estimate the state of charge of the power battery pack;
2) Dynamically monitoring the working state of the power battery pack;
3) Equilibrium between single cells.
This paper mainly combines the successful application of over-current over-voltage, and further illustrates the application of AEM FUSE/PPTC/TVS passive protection components around the safety design of BMS.
BMS technical schematic
Figure 1: BMS block diagram
BMS unit circuit protection application
As shown in Figure 1, the general electric vehicle BMS includes a data acquisition, equalization unit, control unit, CAN communication unit and display unit.
At the beginning of product design, it must comply with the ISO26262 standard for vehicles and pass the EMC test. In order to avoid short circuit, overload, surge and electrostatic shock between the unit modules, it is necessary to select FUSE, TVS, PPTC and other components in each functional unit to protect the function and safety of the line.
1) Power input protection of the main control unit BMU
Figure 2: Overcurrent protection at DC in position
Currently commonly used specifications are 1~3A, and SolidMatrix HI Series Fuse protection is recommended. Some manufacturers use one PTC for overcurrent protection.
2) Protection from the controller BCU
Since the scheme technologies of the BMS manufacturers are different, the BCU module includes a data sampling unit and an equalization circuit. This part is basically connected directly to the power battery pack, similar to the following connection diagram:
Figure 3: Pin to pin connection between test board and battery module
A direct short circuit of any line will cause a direct short circuit of the battery, which is harmful to the vehicle and personal safety, so the safety of the function is very high.
The common protection scheme in the industry mainly connects one fuse to each voltage detection line and equalization line of the sampling board to prevent short circuit between lines and lines due to various uncontrollable factors.
For example, the following picture:
Figure 4: The usage of a single BMU is related to the number of battery cells it controls.
At present, the demand for battery packs in the market for passenger cars is more than 300V to drive the motor. If a single battery cell is about 3.3V, that is, a battery pack composed of at least 100 batteries must meet the requirements. Corresponding to each battery has two detection lines, so fuse*2 about 200 / car usage; by analogy, the electric bus BMS is similar to this protection, its driving voltage is between 500 ~ 600V, fuse usage exceeds 400 pcs/car.
When selecting fuse, it is known that the sampling current is only mA level. The commonly used current specifications are 1206 FA 0.5A/ 0603 HI 1A. The equalization line has active and passive points, and the current varies. Specifications 1~5A are available. .
We believe that in the actual selection, we should fully consider the influence of temperature, the breaking capacity of fuse, the ability to withstand short-circuit in the bundle, the influence of the internal resistance of the fuse DCR on the sampling line.
In general, the BMS is placed inside the electric vehicle and connected to the battery pack. The summer space temperature will have a great influence on the working state of the PTC, and the IT current will become smaller and trigger the action. AEM SolidMarix ceramic fuses have good temperature resistance and are suitable for overcurrent protection in locations such as boards.
3) Overcurrent, surge and static protection of CAN bus
In the BMS, communication between each unit module completes the transmission of information. Mainly adopt CAN bus interface protocol. In order to prevent the CAN bus from accidentally appearing in the actual application, the power supply and ground or load short circuit and transient voltage interference phenomenon, in the design process, some customers use fuse or PPTC series and TVS parallel to form a protection circuit to protect each In the case of short-circuit and other overcurrent conditions, considering the versatility of the CAN bus, the operating current is usually several tens of mA. Generally, the low amperage current specification can be used to meet the requirements, and the AEM TF series low amperage specification fuse can also be recommended.
Figure 5: Protection of the CAN bus interface
Among the above protection devices, HSP (AEM high-energy protector) can replace the common GDT, anti-surge protection, commonly used as HSP1206SN/HSP1210 SN series, which is quite cost-effective.
TVS devices consider the requirements of data transmission rate, and the advantages of TS04021C series low capacitance are very suitable for CAN line data transmission.
Fuse application anti-surge type, commonly used F1206HI 0.5~1A.
in conclusion
Concerns about serious environmental pollution and climate change in today's society, countries around the world have developed renewable energy and new energy as an important part of the future energy strategy. Due to the pressure of the environmental situation and the support of national policies, electric vehicles have broad prospects for development in terms of energy diversification, power electrification, and emission cleanliness.
AEM products are fully surface-applied. In the actual application of BMS, the line interface position is also the main one, which is very suitable for the customer's operation requirements, and has a very high cost performance advantage in protecting the function and safety of the customer BMS products.
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