REPORT ON BALANCING OF BMS ALGORITHM MODEL

FAULT ANALYSIS ON BMS CLOSED-LOOP MODEL

BMS CLOSED-LOOP HARNESS DASHBOARD:

• The BMS closed-loop harness dashboard consists of test sequence variant, BMS closed-loop, BMS state request, and fault alarm sensors.
• The test sequence will tell about the sequence in which the BMS model is running and state requests will tell about the state in which the BMS model is present.

• The fault sensors will give the mall functioning of the model in different states such as charging, discharging, driving.
• It is used to run all the possible conditions that are used to run the battery model without any damage absorbed by the battery.
• The BMS model has different types of fault sensors such as system, high temperature, over-voltage, under-voltage, charger or inverter connected, voltage sensor.
• The BMS closed-loop model is mainly consisting of 2 important parts such as BMS ECU and PLANT.
• The BMS ECU which controls over the state machine, SOC calculations, balancing logic and current and power limitations block.
• The plant consists of the hardware equipment such as battery’s, relays, temperature sensors, scopes, signal builder block etc.
• This model is used to perform various tasks as per user-defined:
• State of Charge estimation using Extended Kalman Filter, Unscented Kalman Filter
  2. Passive Battery Cell Balancing
  3. State Machine for Pre-charging and Contactor Management
  4. Fault Management - Over/Under Voltage, Over Current, Over Temperature, etc.
  5. Charge and Discharge Current Limit Calculations
• To design and test these algorithms, the project also includes files for
  Li-ion Battery Cell Parameter Estimation
  2. Battery Pack with 6 cells in series and 96 cells in series
  3. Simulink Test files with test cases to test State Machine Logic
  4. Linking requirements to Models

BMS ECU:

• The BMS ECU block is used to control the battery by using logics and algorithm which are present in the microcontroller or microprocessor.
• The BMS ECU has 4 sub-blocks

1. Current power limitation block.
2. State machine block.
3. Soc estimation block.
4. Balancing logic block.

CURRENT POWER LIMITATION BLOCK:

• This block is mainly used to limit the current and power to be flown inside and out of the battery pack.
• The function of the block is to calculate the charge current limit and discharge current limit.
• These limits are based upon the battery lower and upper voltage of the cell.

STATE MACHINE BLOCK:

• The state machine block is called as the heart of the BMS ECU where all the algorithms are present in this block.
• Depending on the state the subsystems inside the block are used to operate.
• The state machine having different subsystems which are operated by the test sequence and limit control block.
• The state machine can have the access of discharge current limit, charge current limit, cell voltage, pack voltage, cell temperature, voltage charger, voltage inverter, pack current, state request, maximum cell voltage, minimum cell voltage.
• They are used to sense the changes, according to the state and testing sequence, the state machine block will activate the subsystems such as BMS state, charge current regulator, positive contact charge mode, pre-charge relay charge command, negative contactor charge command, positive charge invertor command, pre-charge invertor command, negative charge invertor command, faults.
• These are the subsystems that are available in the state machine block.

SOC ESTIMATION BLOCK:

• The SOC estimation block comprises of coulomb counting, UKF-EKF, state transition function, and measurement function.
• The coulomb counting is the integration of the current over some time.
• The UKF-EKF block which estimates the state of charge by considering all the disturbances.

BALANCE LOGIC BLOCK:

• Balancing block is used to balance the voltage of the cell.
• It is also having the state machine algorithm which is used to operate the system whenever the voltage of the cell fluctuates.
• The block is used to require in BMS because it takes care of the cell voltage or the pack voltages.
• When the fault state is achieved by the model, the battery pack is disconnected from the load by BMS.

PLANT:

• The blocks inside the plant consist of a battery pack and a pre-charge circuit and charger load.
• The blocks are connected with the charge positive line and negative charge line throughout the plant.

BATTERY PACK:

• The pack consists of two types of battery pack system:

1. 6 cell series 1 modules.
2. 96 cell series of 16 modules.

6 CELL SERIES OF 1 MODULE:

• In these 6 cells 1 module battery pack, the cells are connected in series and the positive line is connected to the positive terminal of battery 1.
• The negative line is connected to the negative terminal of the battery 7.

• The battery pack is parallelly connected with the temperature sensor and is used to regulate the temperature of the cell in the module respectively.
• These temperature sensors are used to indicate the faulty cell when the cell is thermally running out.
• Each module is connected with a cell management in which each cell is monitored and is equipped with a balancing circuit.
• A current regulator block and voltage regulator blocks are attached in series and parallel where the current and voltage are measured for every module.

96 CELL SERIES OF 16 MODULE:

• In this type, the 16 modules consist of 6 cells are connected in series.
• Each module of the negative terminal is connected with the positive terminal of the next module.
• The signal output from each block is monitored by the cell management system.

PRE-CHARGE CIRCUIT:

• The pre-charge circuit consists of two important types of commands such as: charge commands and invertor commands.
• The pre-charge resistor charger, which controls the flow of current into the battery pack from the charge circuit connected to the source.
• The pre-charge resistor invertor is used to relate the flow of current from the battery pack to the load.

CHARGER LOAD BLOCK:

• The charger load block consists of 2 circuits for the load and the charger system.

• When the driving load is activated, the drive load circuit is acted upon which creates a control current supply to the load which is connected.
• When the charging load is activated, the charger circuit is acted and the battery pack is charged with the help of a charger circuit.

DISCUSSION OF RESULTS:

DRIVING STATE:

• The results based upon the pack current, cell voltage, SoC, balancing command, cell temperature, BMS state.
• Initially, when the BMS state is driving, the cell voltage gradually decreases down according to the drive cycle which is given the signal builder block.
• The pack current is varied according to the drive cycle load where the current is drawn from the battery pack.
• In this driving BMS state, the cell temperature is rising because of the internal resistance of the cell and the heat released at the time of the reaction caused inside the cell.
• The SoC of the battery pack module is dropped when the driving state is turned on because the charge is removed from the battery pack.
• When the BMS state is turned into charging state, the load circuit is switched pf and the charger circuit is connected with the source.3
• At this point, the charging is done in two types: constant current charging and constant voltage charging.

CHARGING STATE:

• In constant current charging, the SoC of the battery is increased at a particular rate in a very short period.
• wherein constant voltage charging, the voltage of the battery pack is stabilized and SoC of the battery almost remains constant for some time.
• from the plot, the range of 0.4 to 0.7 where the SoC is increased rapidly at a constant rate is done because of the constant current method.
• The cell voltages increase from 3.8 to 4.2 volt when the charging state is switched.
• The cell temperature starts to rise the future because of the charge transfer inside the cells

STAND BY STATE:

• In this state, the battery SoC and cell voltage and pack current reaches to the capacity where there is no change in the plot.
• At this stage, the vehicle will be in ideal condition where there is no load applied by the user.
• At this stage, the vehicle is not connected to the charge circuit.
• The cell temperature in the stand by the state is coming down, because there is no usage of the battery pack.

FAULT STATE:

• This state occurs when the cell is overcharged or overheated or the thermal drain of the cell occurs.
• In this driving state, one of the cells is in a false state, where overcurrent is discharged from the cell.

• Initially, the cell is in charging where the cell voltage is increased and the cell temperature reached the maximum limit.
• At this point, the BMS state is changed to fault mode because of the rise in temperature of the cell and there is a sudden drop in voltage because of the thermal runaway due to the rise in temperature of the cell.
• The current is zero, because of the fault mode is on, the BMS cuts of the cells of from connecting the load.