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Lithium Ion Battery Management In Electric Vehicles

Author: Source: Datetime: 2016-10-13 10:58:00
As more and more lifepo4 batteries used for electric cars which is search for high energy density, high-security cell addition, the importance of battery energy management system is also increasing. Different battery of a different nature, even if the nature of the same type of battery there are inconsistencies in the course of expansion appears likely to cause accidents. So the battery power system for effective management to ensure the safety of electric vehicles is very important, but also the need to ensure the performance of the battery system to extend battery life and improve battery efficiency.

Structure and Principles of The Battery Management System

Lithium battery management system (BMS), like 48v 100ah lifepo4 namely, Battery Management System, to determine the status of the entire battery system by detecting the battery status of each single cell, and according to their state of battery power system to control and adjust the corresponding implementation strategies to achieve on battery power system and each single charge and discharge management to ensure that the battery power system safe and stable operation.
E-bike Batteries Pack
Typical battery management system topology structure is divided into the main control module and the control module from the two blocks. Specifically, the central processing unit (control module), the data acquisition module, data detection module, a display unit module, the control means (fuse means, a relay), and the like. Typically by using the internal CAN bus technology for data communication between modules.

Based on the function of each module, BMS real-time detection of battery voltage, current, temperature and other parameters, to achieve power battery thermal management, balance management, high voltage and insulation testing, and can be calculated battery remaining capacity, charge and discharge power and SOC & SOH state.

Basic Functions of The Battery Management System

Basic functions of the battery management system can be divided into the detection, management and protection of these three blocks. Specifically, data acquisition, monitoring, balance control, thermal management, security and other functions.

(A) Data Acquisition

As a battery management system and other functions on the basis of the premise, accuracy and speed of data collection to reflect the merits of the battery management system. Other features management systems such as SOC state analysis, balance management, thermal management functions to collect all the data obtained as the basis for analysis and processing.

Object data typically collected voltage, current, temperature. In actual use, different battery electrochemical properties at different temperatures, resulting in battery energy released is different. Lithium-ion battery voltage and temperature sensitive, so when the battery SOC must be considered to evaluate the effect of temperature.

(Ii) State Analysis

Analysis of the state of the battery is the main battery residual quantity and age of battery these two aspects, namely SOC and SOH assessment evaluation. SOC allows the driver to get direct information, understand the impact on the remaining battery life mileage. Many research stage focused on the analysis of the SOC, and constantly enhance its accuracy. SOC analysis will be affected SOH of the battery SOH by temperature, current and other lasting effects during use and requires constant analysis to ensure the accuracy of the SOC analysis.

In the analysis of the SOC, the main charge metering method, the open circuit voltage, Kalman filtering method, artificial neural network algorithms and fuzzy logic method. In this brief charge measurement method and the open circuit voltage of these two methods.

(1) Charge Measurement Approach

Charge metering method is through a period of time the battery charge and the charge released statistics that current in the time to calculate the cumulative SOC. Although it is most commonly used measurement methods, however, can be affected by many factors, including the accuracy of data is self-discharge problems. For example, due to the lack of precision current sensor for current exists between the real value of the integral calculation error, so that the SOC deviation is growing. Therefore, when using the charge metering method need to use some correction algorithm to correct a variety of factors, reduce error calculation analysis results.

(2) Open Circuit Voltage

Open circuit voltage in the battery is allowed to stand in a state of the battery open circuit voltage measurements to calculate the battery's SOC. But it should be noted that when using the open circuit voltage is generally believed SOC and EMF have a linear relationship, any one of the SOC value corresponds to only a force value. In using the open circuit voltage must take into account the rebound effect of the voltage, the voltage does not spring back to the calculated value of the SOC will be smaller when stabilization. Compared with the charge metering wears, open circuit voltage when the battery can not be used in normal operation, which is its biggest problem solar powered portable generator.

In fact, at this stage of the SOC to be very accurate measurements there is a big difficulty, eg due to electromagnetic interference caused by sensor accuracy and sampling data inaccuracies deviation status analysis. In addition, the battery inconsistencies, historical data, use conditions of uncertainty is also a great deal of influence on SOC calculations.

(C) The Balance Control
Lead-acid Batteries
Due to manufacturing and the working environment will cause the inconsistency of the battery cell, there is a difference in the nature of voltage, capacity, internal resistance, resulting in each cell in the actual process of using the effective capacity and charge-discharge capacity is not the same of. Therefore, to ensure the overall system performance and extend the life of the battery, in order to reduce differences between the cells and the battery equalization control is very necessary.

Balanced management helps maintain battery capacity and discharge control depth. If there is no battery balance control, since the protective function of the battery management system is provided, there will be some other batteries battery cell or other battery cells is not fully a single battery charge and discharge the minimum cut-off when fully charged yet reaches the discharge cutoff limit phenomenon. Once the battery overcharge or over-discharge, the internal battery will be some irreversible chemical reactions leading to the nature of the battery is affected, thus affecting the battery life.

Equalization circuit in accordance with the management structure and control to summarize these two aspects, the former is divided into centralized and distributed balanced equilibrium, which is divided into active and passive equalization equalization. It refers to centralized equilibrium within the battery cells of the battery pack all share an equalizer that equalizes a controlled, balanced and is distributed a plurality of battery cells or a specific equalizer. The former communication simple and direct, equalized speed. But harness arrangement of the battery cell and equilibrium between the complex and the number of monomers is not suitable for multi-cell system. The latter aspect can solve the problem of the former wire harness, the disadvantage is the high cost. Active balance, also known as non-dissipative equilibrium, the image that is to be the energy transfer between the battery cells. The transfer of energy in the battery cell with high energy to low energy monomers in order to achieve the purpose of energy balance. Passive is dissipative balancing, parallel resistance, etc. will use high-energy energy consumption monomer with other monomers to a state of equilibrium. Active equalization high efficiency, energy transfer instead of being consumed, but the complex structure to bring rising costs.

(D) Thermal Management

Cell system under different operating conditions due to its own have a certain resistance, a certain amount of heat in the output power and energy while producing heat buildup of the battery temperature, such that different spatial arrangement of the battery temperature is not consistent throughout. When the battery temperature is outside its normal operating temperature range, the power must be limited to work, otherwise it will affect battery life. In order to ensure the electrical system battery life and performance, battery power system is generally designed with thermal management system. Battery thermal management system is used to ensure that the battery system works within a suitable temperature range of a management system, mainly by the battery box, heat transfer media, monitoring devices and other components.

The main function of battery management system on the thermal management of the battery temperature is accurately measured and monitored, when the battery pack temperature is too high for effective cooling and ventilation to ensure uniform distribution of the battery pack temperature field. At low temperatures, can be rapidly heated to the battery pack reaches the normal work environment.

(E) Security

Security as the most important function of the entire battery management system is based on the preceding four functions carried out. Including over-current protection, overcharge and over discharge protection, over temperature protection and insulation monitoring.

(1) Over-current Protection

Since the batteries are have some resistance when the battery current is at work over the General Assembly cause the battery internal heat, increase the heat accumulation caused by the battery temperature rises, causing the thermal stability of the battery decreases. For lithium ion batteries, lithium ion deintercalation capacity of the positive and negative material is certain, when the charge and discharge current is greater than the deintercalation capacity will lead to polarization of the battery voltage increases, leading to reduce the impact of the actual capacity of the battery battery life, even affect safety of the battery. Battery management system determines whether the current value exceeds the safe range, the appropriate security measures will be taken more than once.

(2) Overcharge and Over-discharge Protection

During charging, the charging voltage exceeds the cut-off voltage of battery to be charged, the positive electrode will cause destruction of the lattice structure, resulting in smaller battery capacity. And will in turn result in positive and negative terminal explosion hazard when the voltage is too high. Overcharge is strictly prohibited. BMS system will detect single battery voltage, when the voltage exceeds the charge limit voltage, BMS disconnects the battery charging circuit to protect the system.

During discharge, the discharge voltage is lower than the battery discharge cut-off voltage, battery current collector metal on the negative electrode will be dissolved, to cause irreversible damage to the battery. There will be an internal short circuit to charge the over-discharged battery or the possibility of leakage. When the voltage exceeds the discharge limit voltage, BMS disconnects the discharge circuit to protect the battery system.

(3) Over-temperature Protection

For over-temperature protection, thermal management requires a combination of the above functions. Battery active vary at different temperatures. A long time in a high temperature environment, the structural stability of the battery material will deteriorate shorten battery life. Battery active at low temperatures will result in limited available capacity is reduced, in particular the charge capacity will become low, and may have security implications. Battery management system capable of battery temperature exceeds the temperature limit or below the low limit value, will prohibit charging and discharging.

(4) Insulation Monitoring

One of the insulation monitoring function is to ensure the safety of battery systems important functions. Voltage battery systems are usually several hundred volts, once the leakage appears to be a risk to personnel, so the insulation monitoring function becomes very important. BMS will be real-time monitoring of the total positive and total negative grounding of the insulation resistance of the body, if there is insulation resistance below the safe range, it will report the fault and disconnect the high voltage.

System Design and Technical Requirements

Performing battery management system design, the first pre-determined according to the vehicle's design requirements BMS functionality, and then determine its topology, followed by software and hardware design of the main contents of the work, after the completion of the above basic unit testing work to be carried out and BMS overall test battery power. Before hardware and software design, charge and discharge capacity, resistance, and other characteristics of the cells need to be tested in order to better protect circuit design, algorithm design.

In conjunction with the hardware design requirements for the software algorithms, the circuit board development and design of components Note voltage insulation, anti-electromagnetic interference, electromagnetic compatibility, communication and other isolation and ventilation. General software design features include a voltage detection, temperature measurement, current detection, insulation testing, SOC estimation, CAN communication, discharge equalization system self-test function, the system detection, charge management, thermal management.

Related hardware design software design features are supported, such as MCU module is used to collect, analyze data, send and receive control signals; a current detection module is a collection of the battery pack during charging and discharging of the charge and discharge currents.

Sum Up

As the core of electric vehicles - battery power system management technology slao used in storage battery for portable solar power generator, battery management system in a lot of features, there are still insufficient. Electric vehicles to flourish the moment, you can accumulate a lot of data and technical features of the battery management system for continuous improvement. Achieve a complete battery management system function design requires constant efforts of all parties, including the optimization of the hardware design, improve the adaptability of the software and low-power design.

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