Last Updated on: 23rd March 2023, 11:24 am

What’s The Main Function Of EV Battery Management Systems (BMS)?

What’s The Main Function Of EV Battery Management Systems (BMS)?

If you’re in the market to purchase lithium-ion batteries, understanding the basics of a battery management system (BMS) is essential. A BMS works as a monitor and regulator that allows batteries to sustain peak performance over their lifetime by controlling their charging and discharging processes. With its many features, such as setting limits on charge rate, discharge rate and temperature monitoring, it ensures the reliable operation of your battery pack while protecting it from damage caused by overcharging and discharging too much at once. Those functions are the feature of Battery Management System (BMS), an important component better known as “the brain” of the system that regulates charging and discharging of the cells in order to keep them safe and ensure long life performance. Below chapter Bonnen engineer will explain what exactly a BMS does and why it’s important. In this case, we will take the EV as an example of a typical application. Keep reading to find out more!

Overview

1. High voltage power on and off control


2. Charging control


3. Battery state calculation


4. HV interlock monitoring


5. Insulation monitoring


6. Collision monitoring


7. Thermal management


8. Battery failure analysis and online alarm

1. High voltage power on and off control

High-voltage power-on control:
BMS conducts a self-checking, and then waits for the VCU power-on command after passing the test. After receiving the VCU high-voltage power-on command, the BMS close of the main negative and pre-charging relays to do pre-charging. When the MCU input voltage is detected as greater than 95% of the bus terminal voltage, the pre-charging is completed, the main positive relay is closed, and after a period of delay, the pre-charging relay is disconnected, the high-voltage power-on is completed, and it enters the BMS working mode.

When KL15 (KL15 represents the ignition signal of the engine and the signal to start the vehicle) is in the OFF state, or when the BMS receives the VCU/TBOX power-off command, BMS disconnects the main positive relay and the main negative relay successively. Then feeds back the status of the controller to the VCU.

High-voltage power-off control:

In the normal power-off process, the driver’s normal power-off command is detected, and the VCU sends a power-off command. After receiving the command, BMS will successively disconnect the positive and negative relays of the high-voltage circuit. Meanwhile, vehicle speed, working current, and other vehicle status information should also be considered when BMS processing the power-off command.

Emergency high voltage power off: In case of a serious failure of the vehicle, such as a collision, BMS shall immediately cut off the high voltage and the power output.

2. Charging control

Based on the battery temperature and SOC, the BMS calculates the maximum allowable charging current using the charging power MAP. During charging, BMS interacts with the charging equipment (charging pile or on-board charger) regarding the maximum cell voltage, maximum total voltage, the highest temperature of the battery system, and maximum current, nominal energy, state of charge, and current battery voltage allowed to charge, so that the battery system can charge according to the appropriate charging voltage, charging current, and charging method, and display charging information. In addition, SOC and charging current are used to estimate remaining charging time.

There are various types of charging technologies, such as slow charging, fast charging, wireless charging, reservation charging, and battery swapping.

3. Battery state calculation

SOC: The residual capacity of the battery is mostly estimated through the ampere-hour integration method and the Kalman filter algorithm, combined with the correction strategy (static correction of open-circuit voltage, full charge correction, full discharge correction, dynamic correction, capacity correction at different temperatures and SOH, etc.).

SOH: By using a small current charging, the BMS is able to calculate the degradation coefficient in relation to mileage and running time, and the ratio of the theoretical charged capacity to the actual charged capacity as the SOC changes.SOE: BMS obtains the available charge and discharge power SOE of the battery by looking up the temperature and SOC table, taking into account the migration from peak charge and discharge to continuous charge and discharge power.

4. HV interlock monitoring

A BMS monitors the failure of the high-voltage interlock on the battery connector. When a high-voltage interlock failure occurs, it will alarm, limit power, and shut down high-voltage power according to the vehicle’s operating state (fast driving or stationary/charging).

5. Insulation monitoring

The BMS monitors the insulation failure of the battery connector. When an insulation failure occurs, it will alarm, limit power, and power off high-voltage processing according to the operating state of the vehicle (fast driving/stationary state/charging state). Or BMS report the fault to the VCU.

BMS

6. Collision monitoring

BMS monitors the collision failure of the whole vehicle, and carries out emergency power off high-voltage process in the event of a collision fault.

7. Thermal management

Thermal management during fast and slow charging: Close the charging relay and heating relay when the battery’s minimum temperature is below threshold value, and start the heating system. If the minimum temperature of the battery is higher than the normal threshold, disconnect the heating relay and resume normal charging. Heat should be stopped when the temperature difference in the heating process is too large.

Thermal management before and during driving: BMS controls battery temperature. When the battery is over temperature, turn on the water pump and cooling fan to cool the battery. When the battery is under temperature threshold value, turn on the battery PTC to heat.

Battery balancing management: When the max and min SOC thresholds are greater than a certain value, BMS determines whether to enable equalization based on operating current, PCB temperature, SOC value, vehicle status and other conditions, and the equalization time is calculated based on the designed equalization current.

8. Battery failure analysis and online alarm

An example of a trigger condition is:

(1) The min cell voltage is lower than a certain threshold for a period of time.

(2) The highest battery temperature is over than a certain threshold.

(3) The rising rate of the cell temperature is over than a certain threshold.

(4) The temperature difference between the highest temperature and the lowest temperature is over than a certain threshold.

When any two of the above conditions appear at the same time, the thermal runaway fault flag will be set immediately, and a signal will be sent to the vehicle to remind the driver. The vehicle’s power output will be immediately cut off if a thermal runaway fault is triggered. It will also send an alarm signal to the cloud, monitor the status of the vehicle in the background, and carry out follow-up rescue measures.

As a result, Battery Management Systems (BMS) are essential for lithium-ion battery-powered applications, such as electric vehicles, electric boats, and energy storage systems. BMS core functions, such as setting limits on charge rate, discharge rate and temperature monitoring, help maintain reliable operation of your battery pack while protecting it from damage caused by overcharging or discharging too much at once.

At Bonnen, our team of experts understands how important the BMS is for successful operations and is here to offer advice on which type of BMS might best fit your needs. Rely on us to provide optimal grade protection for your whole battery system! Connect with us now to find out more about designing and building custom battery packs with a complimentary BMS.

If you have any more questions about EV Battery Management Systems (BMS) or want suggestions on which might be the best choice for you, get in touch with us today!

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