Basic functions of electric vehicle battery management system

2021-11-11 08:48:23 0

In different application scenarios, the battery management system has different functions. The following describes the basic functions of the electric vehicle battery management system:

1. Condition monitoring

2. Status analysis

3. Security protection

4. Energy control

5. Information Management

1 Battery status monitoring

Battery status monitoring generally refers to the monitoring of parameters such as voltage, current, and temperature. The temperature monitoring is not only for the battery cell, but also includes the battery ambient temperature and the temperature information of the thermal management system. Battery status monitoring is the most basic function of the battery management system, and it is the premise and foundation of each function.

2 Battery status analysis

Battery status analysis includes battery remaining capacity assessment and battery aging assessment, that is, SOC (State of Charge) and SOH (State of Health)

2.1 State of charge (SOC)

The remaining power of an electric car is equivalent to the remaining fuel of a traditional car. The SOC status is often expressed as a percentage. In order to make it easy for the driver to understand, the system often evaluates the corresponding remaining time or remaining mileage based on the algorithm, but these estimated values have certain errors. SOC evaluation is an important function of the battery management system, and it is also the most challenging function.

2.2 Assessment of Aging Degree (SOH)

At present, the industry mainly evaluates the aging degree of batteries based on two indicators: battery capacity and DC internal resistance. Regarding the service life of electric vehicle batteries, the state stipulates that after 8 years of use / 120,000 kilometers, the battery capacity shall not be lower than the standard of 80% of the initial capacity. In addition to national standards, electric vehicle manufacturers also have regulations on the service life of their own batteries.

SOH is related to the working temperature, charging and discharging current, number of cycles and working time during the use of the power battery. It needs to be continuously evaluated and updated during use to ensure that the driver obtains more accurate information.

3 Battery safety protection

Battery safety protection is the most important function of the electric vehicle management system. The two functions of state monitoring and state analysis are the premise of the battery safety protection function. The safety protection mainly includes three items: overcurrent protection, overcharge and overdischarge protection, and overtemperature protection.

3.1 Overcurrent protection

In charging and discharging overcharging, if the working current exceeds the safe value, corresponding safety protection measures should be taken. Different manufacturers and different models of power batteries support different overload current ratios and overload durations. For example, a certain type of power battery supports a 3C overload current of no more than one minute.

3.2 Overcharge and overdischarge protection

Overcharge protection: When the SOC is 100%, in order to prevent battery damage caused by continuing to charge the battery, protective measures taken

Over-discharge protection: when the SOC is 0%, in order to prevent battery damage caused by continuing to discharge the battery, and take protective measures

In actual situations, when the remaining battery power is insufficient (for example, less than 5%), a warning will be issued to the driver to indicate that the battery is insufficient, and the battery's discharge current will also be reduced. In the actual control process, the over-charge and over-discharge protection can set the cut-off protection voltage for charging and discharging. When it is detected that the battery voltage exceeds the cut-off voltage range, the current is cut off in time to protect the battery.

3.3 Over temperature protection

The power battery is constantly undergoing chemical reactions during use. The chemical reactions are difficult to control at high temperatures, and safety accidents are prone to occur. Because temperature changes require a process, and data collection also has a certain time lag, some advances need to be considered for temperature protection. When it is detected that the ambient temperature or the temperature of the battery box is too high, approaching the maximum operating threshold, corresponding protective measures must be taken in time. Or, when the temperature of a battery cell suddenly rises rapidly, although the maximum operating threshold is not reached, certain protective measures still need to be taken.

4 Energy control management

4.1 Battery charge control management

The charging management system is to perform real-time optimization control of charging voltage, charging current and other parameters during the charging process. The optimization goals include charging time, charging efficiency, and charging fullness.

4.2 Battery discharge control management

The discharge management system controls the discharge current according to the state of the battery during the discharge process. Properly limiting the maximum discharge current of the battery pack is beneficial to prolong the cruising range and service life of the vehicle.

Braking capacity recovery is also one of the important contents of capacity management. In hybrid vehicles, the battery current needs to be maintained between 30% and 80% to provide sufficient battery capacity for braking capacity recovery.

4.3 Battery balance control management

Due to the inconsistency of the cells in the production and use process, the cells in the battery pack will always have different degrees of inconsistency, and the SOCs of the cells are different. In the battery pack, as long as one battery cell has overvoltage or undervoltage, the battery will protect itself. Therefore, the battery balance management system reduces the SOC difference between the battery cell and the battery cell. The cell balancing strategy also includes two types: passive balancing and active balancing.

5 Battery Information Management

Electric vehicles generate a large amount of data all the time. Some data needs to be notified to the driver through the meter, some need to be transmitted to other vehicle controllers or the cloud through the communication network, and some are stored as historical data inside the battery management system.

5.1 Information display

Three major parts of information are displayed in the meter:

Measurement information: voltage, current and temperature

Remaining battery power (SOC)

Warning information: When the battery pack has safety problems, promptly inform the driver through the instrument

5.2 Information exchange

The battery management system needs to transmit the status of the battery to the vehicle controller and motor controller in time:

Measurement information: voltage, current and temperature

Remaining battery power (SOC)

Battery aging degree (SOH)

Available battery power (SOP)

The current working status of the battery

Warning message

With the current development of cloud technology, some long-term battery management functions have been transferred to the cloud, and some battery information needs to be transmitted to the cloud server. The development of cloud processing technology also provides technical conditions for the graded processing of discarded batteries.