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Battery management system
 
Battery Management System monitoring of each monomer batteries in the Battery status, the Management of the Battery pack use
process, maintain consistency monomer batteries in the Battery status, to ensure the security of the Battery, enhance the role of Battery life.
BMS has become one of the key components of modern electric vehicles.

 

       Battery management system (BMS) monitoring of each monomer batteries in the battery status, the management of the battery pack use process, maintain consistency monomer batteries in the battery status, to ensure the security of the battery, enhance the role of battery life. Both domestic and foreign electric vehicles (including pure electric and hybrid vehicles) are equipped with BMS, and BMS has become one of the key components of modern electric vehicles.

 


•  Battery cell voltage, current, temperature signal acquisition
•  Insulation resistance detection
•  The uniformity of battery cells is balanced
•  SOC/health SOH estimation of battery state
•  Overcurrent, overpressure, overheat protection
•  动力母线预充电控制
•  Intelligent charging control
•  Total voltage and current signal collection of battery pack
•  Battery pack heat management

 


       This scheme USES the master slave BMS system architecture, which is the mode of a master control module + several collection modules. The main control module and the acquisition module CAN communicate through CAN, as shown in the following figure. The master-slave BMS layout is flexible, easy to repair, convenient for daily maintenance, suitable for all kinds of pure electric cars and hybrid cars.

 
 

 


       Due to the diversity and complexity BMS function, BMS software is complex, in order to adapt to different models and upgrading of the same model, software also need to modify repeatedly, the software developers to put forward a challenge. If the development mode of traditional manual programming is used, the development workload is huge and the maintainability of the software is poor, resulting in the uncontrollable development cycle and development cost. This scheme USES the automatic code generation software development mode, the controller software code are made by MATLAB/Simulink/ECUCoder automatic code generation tool to generate, the whole process of software development of the controller to realize, in the form of graphical modeling users need manual programming, no need manual code integration, also need not code transplantation.

       Fully automatic code generation can effectively improve the efficiency of development and greatly reduce the workload of controller software development. In most applications, automatic code generation can reduce the development cycle by at least 50% and reduce development costs by 80%.

The advantages of using MATLAB/Simulink/ECUCoder as a BMS software development solution include:
       •  Automatic code generates both basic and application software without manual integration
       •  A powerful GUI interface that can access and configure the entire underlying software directly from the model
       •  Code is reliable, code readability and execution efficiency good compromise
       •  At the same time, the chip level module library and controller level module library are provided to support the controller hardware developed independently by users

       In BMS software, the basic state of BMS is divided into power, readiness, high voltage shutdown, high voltage precharging, high voltage power, failure, etc. The transformation logic of various states is shown in the following figure:
 


BMS basic state transformation logic diagram

 


Battery balance:
       Due to reasons such as battery technology, different battery monomers such as electrolyte density, electrode equivalent resistance between the differences, these differences lead to even the same series of each monomer battery charge and discharge current, also can make the capacity of each monomer to produce different, thus affecting the whole battery pack. In the worst case, in a battery, a monomer of the residual capacity is close to 100%, the residual capacity of another monomer is 0, then the battery cannot charge or discharge, completely unable to use. Therefore, the balance of battery capacity is very important, especially in the case of a large number of battery cells in the electric vehicle.
 
       Battery balanced approach has a lot of kinds, according to the use of different components can be divided into resistance balance, balance of capacitance, inductance is balanced, transformer equilibrium and DCDC equilibrium, according to the energy to different can be divided into passive equilibrium and active equilibrium.
 
       The passive equilibrium efficiency is low, but the system complexity is low and the cost is low, which is suitable for the balance of the small and medium-sized battery pack. The active equilibrium efficiency is high, but the system is high in complexity and high in cost, which is suitable for the equilibrium of large capacity battery pack. The scheme USES passive equilibrium or active equilibrium based on the capacity, equilibrium efficiency and cost of the battery pack used by the user. Passive equalization more the capacity battery consumption by resistance to reach equilibrium, active balancing transfers more battery capacity to achieving equilibrium of the battery capacity is less, two kinds of balanced way such as shown below:

对比项目 被动均衡 主动均衡
均衡元器件 电阻 电容、电感、变压器、DCDC
均衡方式 能量消耗 能量转移
均衡效率
复杂度
成本
RapidECU 支持 支持
均衡电流 100mA-500mA 1A-10A

The comparison between passive equilibrium and active equilibrium
 
       The passive equilibrium efficiency is low, but the system complexity is low and the cost is low, which is suitable for the balance of the small and medium-sized battery pack. The active equilibrium efficiency is high, but the system is high in complexity and high in cost, which is suitable for the equilibrium of large capacity battery pack. The scheme USES passive equilibrium or active equilibrium based on the capacity, equilibrium efficiency and cost of the battery pack used by the user.

SOC Estimation:
       The battery's charge state SOC describes the battery's remaining power, which is one of the most important parameters in the battery's use. It is estimated that the SOC can prevent the battery from overcharging or overfiring, effectively prolongs the life of the battery, and can predict the mileage in the driving of electric vehicles. Because SOC estimation by temperature, aging, charge and discharge rate, self-discharge, and the influence of such factors as makes for battery in practice is highly nonlinear, leads to the precise SOC estimation difficult. Some features of lithium battery are shown in the figure:



The relationship between the SOC and the voltage of lithium battery


Lithium battery discharge characteristics


Aging properties of lithium batteries
 


SOC has many methods, including:
•  Discharge test
•  A time measurement method
•  Open circuit voltage method
•  Load voltage method
•  Internal resistance method
•  Fuzzy logic method
•  Neural network method
•  Kalman filter
       The above methods have some defects in the application of electric vehicles: the discharge test method needs to interrupt the normal charging and discharging of the battery; The error accumulates in the time measurement method. The dynamic error of open circuit voltage is larger. The internal resistance method is affected by the temperature. Fuzzy logic method depends on engineering experience; Kalman filtering relies on a precise battery model.

       The scheme adopts a kind of based on the Ann when measurement, using the method of open circuit voltage of the battery under stationary state to eliminate the accumulated error of SOC estimation method, the key lies in the accurate judgment the battery charge and discharge state and static state. The test results show that the SOC estimation error is within 5%.

SOC estimation method

Communication protocol:
This scheme USES the CAN bus based communication mode, which mainly includes 3 parts:
       •  The CAN communication between the main control module and the acquisition module in BMS;
       •  The CAN communication of power system control unit such as BMS and vehicle control unit;
       •  CAN communication between BMS and non-car charger.

 
 
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