Control Principle of Pure Electric Vehicle Power Battery Management System (BMS)

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Control Principle of Pure Electric Vehicle Power Battery Management System (BMS) [Copy link]

A battery is a device that stores the electrical energy obtained in the form of chemical energy and can convert chemical energy into electrical energy. It can be charged and discharged repeatedly. A power battery is a battery that provides energy for the power system of an electric vehicle. Its function is to store and release electrical energy. The power battery system consists of four parts: a battery box shell, a battery pack, a battery management system (BMS), and auxiliary components. This article introduces the composition and control principle of BMS.

Power battery system

1—Housing 2—Auxiliary components 3—BMS 4—Module

1Battery Management System

(1) Battery management system components

The battery management system includes hardware and software. The hardware consists of a master control box, a slave control box, a high-voltage monitoring box, a voltage acquisition line, a current sensor, a temperature sensor, and a battery internal CAN bus, as shown in Figure 1. The software consists of programs such as monitoring voltage, monitoring current, monitoring temperature, monitoring insulation resistance, and estimating the state of charge (SOC).

Figure 1 Block diagram of battery management system

(2) Function of the main control box

As shown in Figure 2, the main functions of the main control box are: ① Receive the real-time module voltage and module temperature sent from the control box, and calculate the maximum and minimum values; ② Receive the total voltage and total current sent by the high-voltage monitoring box; ③ Communicate with the vehicle control unit (VCU), charger, etc. through the new energy controller area network (CAN), and communicate with the DC charging pile and data acquisition terminal through the fast charging CAN; ④ Control the charging and discharging current (the executive components are the on-board charger, DC fast charging pile and motor controller); ⑤ Control the heating of the power battery.

Figure 2 Main control box function

(3) Function of slave control box

The slave control box is also called the battery information collection box, as shown in Figure 3. The main functions of the slave control box are: ① real-time monitoring of the voltage of each module; ② real-time monitoring of the temperature of each module; ③ monitoring the SOC value; ④ transmitting the above monitored data to the master control box.

Figure 3 Function of slave control box

The main control box is mostly installed in the battery box, and some are installed outside the battery box. The one installed in the battery box adopts 1 master and N slaves, which is called distributed ; the master and slave are combined into one , which is called centralized , as shown in Figure 4. If the centralized type is damaged, it is easy to cause safety hazards, which will cause the BMS to short-circuit or even burn out.

Figure 4 Centralized BMS

(4) Function of high voltage monitoring box

As shown in Figure 5, the main functions of the high-voltage monitoring box are: ① monitoring the total voltage of the power battery, including four monitoring points inside and outside the main relay (inside the main positive relay, outside the main positive relay, inside the main negative relay, and outside the main negative relay); ② monitoring the charging and discharging current; ③ monitoring the insulation of the high-voltage system; ④ monitoring the high-voltage connection status; ⑤ transmitting the above monitored data to the main control box.

Figure 5 High voltage monitoring box function

2BMS Function

BMS is the core component for protecting and managing batteries, which is equivalent to the human brain. It not only ensures the safe and reliable use of batteries, but also fully utilizes the battery's capabilities and prolongs its service life.

(1) Control pre-charge relay and main positive relay

By closing and opening the relay contacts, the power battery's pre-charging, charging, power-on, and power-off procedures are completed.

(2) Data collection

①The high-voltage monitoring box collects the total voltage and current of the power battery; ②The control box collects the voltage of each cell (module) and the temperature of each module.

(3) Status analysis

① Battery remaining capacity (SOC) assessment , which allows the driver to understand the driving range. Methods include charge measurement method, circuit breaker voltage method, Kalman filter method, artificial neural network method, and fuzzy logic method; ② Battery health (SOH) assessment , which assesses the battery health (aging) level and the impact of temperature on current for reference in evaluating SOC.

(4) Thermal management

①Heating the battery pack under low temperature conditions ; ②The battery itself has internal resistance, the flow of current generates heat, the heat accumulates and the temperature rises. When the temperature exceeds the normal level, it will affect the electrical performance and life. The BMS monitors the temperature of each module and dissipates heat through coolant circulation or ventilation .

(5) Security protection

① Overcurrent protection : when the current exceeds the safe range, safety protection is adopted; ② Overcharge protection : when the charging voltage is higher than the upper limit, the BMS disconnects the charging circuit; ③ Overdischarge protection : when the discharge voltage is lower than the lower limit, the BMS disconnects the discharge circuit; ④ Overtemperature protection : when the temperature is higher or lower than the normal range, charging and discharging are prohibited; ⑤ Insulation monitoring : the BMS monitors the insulation resistance between the high-voltage positive and negative voltages and the vehicle body ground in real time. If it is lower than the safe range, the high-voltage power is disconnected and a warning is issued.

3 Charging methods

(1) Fast charging

BMS connects to the DC fast charging pile, data acquisition terminal, and diagnostic interface through the fast charging CAN . After the fast charging gun is plugged in, BMS transmits the charging demand to the DC fast charging pile, which adjusts the charging current . The fast charging process takes 30~45 minutes (normal temperature 25℃, SOC from 20%→80%).

(2) Slow charging

BMS connects VCU, drive motor controller, on-board charger, DC/DC controller, heater (PTC) controller, electric compressor controller, and diagnostic interface through the new energy CAN . The BMS of some early models is connected to the on-board charger and data acquisition terminal through the slow charging bus. After plugging in the slow charging gun, the VCU wakes up the BMS and switches it from sleep mode to working mode. The VCU connects the main negative relay in the battery box. The BMS first connects the pre-charge relay, then connects the main positive relay and disconnects the pre-charge relay. The BMS adjusts the charging current according to the total voltage of the power battery, the module voltage, and the module temperature. The slow charging process takes 8~10h (normal temperature 25℃, SOC from 0%→100%).

4Heating before charging

The slave control box measures the real-time temperature of each module and feeds it back to the master control box. If it is lower than the set value, the master control box instructs the heating relay to close, and high-voltage current passes through the fuse and the heating film.

(1) Slow charging heating circuit

The slow charging heating circuit is shown in Figure 6: AC charging pile → on-board charger → high voltage + → heating relay contact → fuse → heating film → high voltage - → on-board charger → AC charging pile.

Figure 6 Slow charging heating circuit

(2) Fast charging heating circuit

The fast charging heating circuit is shown in Figure 7: DC charging pile → high voltage + → heating relay contact → fuse → heating film → high voltage - → DC charging pile.

Figure 7 Fast charging heating circuit

5 Pre-charge

The slow charging pre-charging circuit is shown in Figure 8: AC charging pile → on-board charger → high voltage + → pre-charging relay contact → pre-charging resistor → battery pack → maintenance switch (with a fuse inside) → battery pack → current sensor → main negative relay contact → high voltage - → on-board charger → AC charging pile.

Fast charging and pre-charging are powered by a DC charging station.

Figure 8 Slow charge pre-charge circuit

6. Charging

(1) Slow charging circuit

The slow charging circuit is shown in Figure 9: AC charging pile → on-board charger → high voltage + → main positive relay contact → battery pack → maintenance switch (with a fuse inside) → battery pack → current sensor → main negative relay contact → high voltage - → on-board charger → AC charging pile.

Figure 9 Slow charging circuit

(2) Fast charging circuit

The fast charging circuit is shown in Figure 10: DC charging pile → high voltage + → main positive relay contact → battery pack → maintenance switch (with a fuse inside) → battery pack → current sensor → main negative relay contact → high voltage - → DC charging pile.

Figure 10 Fast charging circuit

7 Power on

(1) Pre-power-on circuit

When the ignition switch is turned on, the VCU receives signal No. 15 to wake up the BMS; the BMS performs self-checking and initialization, and reports the results to the VCU; the VCU sends current to the main negative relay, and the main negative relay contacts close. Because there are capacitors in the motor controller and the electric compressor controller, the BMS first pre-discharges the capacitors and then closes the pre-charge relay.

The pre-power-on circuit is shown in Figure 11: power battery +→pre-charging resistor→pre-charging relay contact→high voltage +→load→high voltage -→main negative relay contact→current sensor→power battery -.

Figure 11 Pre-power-on circuit

(2) Power-on circuit

When the capacitor voltage is equal to the power battery voltage , the BMS closes the main positive relay and disconnects the pre-charge relay.

The power-on circuit is shown in Figure 12: power battery + → main positive relay contact → high voltage + → load → high voltage - → main negative relay contact → current sensor → power battery -.

Figure 12 Power-on circuit

8Insulation monitoring

The insulation monitoring circuit is shown in Figure 13: ① Battery positive monitoring circuit, power battery +→insulation monitoring resistor R1→main positive insulation monitoring relay S1→ground; ② Battery negative monitoring circuit, power battery -→insulation monitoring resistor R2→main negative insulation monitoring relay S2→ground.

The BMS instructs S1 and S2 to close respectively, and measures the voltages U1 , U2 , and the total high-voltage voltage Utotal respectively . These three are substituted into the formula to calculate the insulation resistance between the high-voltage + and the ground, the insulation resistance between the high-voltage - and the ground, and then determines whether the insulation performance is normal.

Figure 13 Insulation monitoring circuit

R1, R2—Insulation monitoring resistors U1 , U2 —Monitoring voltage

R—voltage sensor U total—high voltage total voltage

Excerpted from "Structure and Principle of Pure Electric Vehicles (Color Illustrations)" published by China Machinery Industry Press