Archives July 2022

Before the power battery energy management system handles the fault, it first analyzes the fault to identify what kind of fault it is and the fault level that occurs. For fault identification, fault condition thresholds must be designed in advance. Generally, the state range of parameters or components within a certain period of time is specified, and the failure level is set according to the threat of the failure to components and personnel. If the parameters are outside the threshold range within the specified time, for example, if the voltage of the Tycorun Lithium Battery cell is more than 4.3V for more than 500ms, the BMS determines that the fault is that the cell voltage is higher than the upper limit value, and immediately disconnects the main relay and reports a fault warning. According to the severity of the fault, the fault level is defined as follows:

①Fault level 0: No fault.

②Fault level 1: Power reduction mode, the maximum charging power available for the battery is halved, and the discharging power is not affected.

③Fault level 2: Immediately disconnect the charging circuit relay, and do not specifically limit the charging and discharging power.

④Failure level 3: Serious failure mode, the maximum charging power is 0kW; if the vehicle speed is higher than 10km/h, the maximum discharge power is 0kW; if the vehicle speed is lower than 10km/h, the maximum discharge power is set to 10kW travel power.

⑤Failure level 4: Serious failure mode, disconnect the main relay immediately.

⑥Fault level 5: Dangerous fault mode, but do not automatically power off (if there are other level 4 faults at the same time, it should first report a level 4 fault for 1s to trigger a high-voltage power failure, and then continue to report a level 5 fault).

The power battery energy management system has safety protection functions, and corresponding measures should be taken in case of failure:

(1) Overvoltage protection. When it is detected that the highest cell voltage of the battery pack exceeds the level 2 fault threshold, the maximum available charging power of the battery drops to 0 within 2s; when the highest cell voltage exceeds the level 3 fault threshold, the BMS requests the VCU to disconnect the high-voltage relay. Get a timely response, and after 2s, the BMS will cut off the high-voltage relay by itself, and the high-voltage will be powered off.

(2) Low voltage protection. When it is detected that the highest cell voltage of the battery pack is lower than the level 2 fault threshold, the maximum available discharge power of the battery drops to 5kW within 2s; when the lowest cell voltage exceeds the level 3 fault, the BMS requests the VCU to disconnect the high-voltage relay. Respond in time. After 2s, the BMS automatically cuts off the high-voltage relay and powers off the high-voltage.

(3) Discharge overcurrent protection. When it is detected that the discharge current of the battery pack is greater than the level 2 fault threshold, the maximum available discharge power of the battery is reduced to 5kW within 2s.

(4) Charge overcurrent protection. When it is detected that the charging current of the battery pack is greater than the level 2 fault threshold, the available maximum charging power of the battery drops to 0kW within 2s.

(5) High temperature protection. When it is detected that the temperature of the power battery pack is higher than the level 2 fault threshold, the maximum available discharge power of the battery is reduced to 5kW within 2s; when it is detected that the temperature of the battery pack is higher than the level 2 fault threshold, the BMS requests the VCU to disconnect the high-voltage relay. Get a timely response, and after 2s, the BMS will cut off the high-voltage relay by itself, and the high-voltage will be powered off.

(6) Emergency power off. When the VCU sends a power-off command or the insulation resistance is low, the BMS sends a level 4 fault to the VCU. When the vehicle speed is not zero, the relay continues to be closed, and the main relay cannot be closed until the key off/on is again.

(7) High Voltage Interlock (HVIL). The HVIL of all high voltage connectors, MSD or cover switch (if any) must be connected in series to the vehicle HVIL circuit. The battery system BMS should be able to monitor the HVIL state, and when the HVIL state is abnormal, the battery system should be able to immediately disconnect the high-voltage circuit.

(8) Collision protection. When a collision occurs, the BMS can detect the collision signal, and should cut off the high-voltage output in time to ensure the safety of personnel.

There is a capacitive load in the high-voltage circuit of the electric system. If there is no pre-charging design during power-on, the main positive relay is directly closed, and the instantaneous capacitive load is closed, which is equivalent to an instantaneous short circuit. This kind of high-voltage shock will cause damage to high-voltage electrical equipment and may bring danger. In order to avoid the transient impact damage to the high-voltage electrical equipment when the high-voltage is powered on, a pre-charging process should be designed for the high-voltage circuit system before the main positive relay of the high-voltage power-on is closed.

1. Precharge principle and precharge model

1.1 The principle of precharging
As shown in Figure 1, the high-voltage circuit system precharge circuit.
In Figure 1, K1 and R1 are the pre-charging relay and pre-charging resistor respectively, K2 and K3 are the main positive relay and the main negative relay, respectively, R2 and C are the equivalent resistance and equivalent capacitance of the high-voltage system load, with Vb and Rb is the voltage and internal resistance of the power battery.

In the high-voltage circuit system, the high-voltage electrical equipment can be equivalently represented by a resistor and a capacitor, while the power battery can be simply represented by voltage and internal resistance. If there is no pre-charging design, the high-voltage main relay is directly closed when the high-voltage is powered on, which means that the high-voltage electrical equipment is directly connected through the capacitor, and the high-voltage components are also short-circuited. , it will generate a large transient current, and the transient current will cause transient impact on high-voltage electrical equipment, which will cause dangerous situations.

If the pre-charging process is introduced before the high-voltage power-on, that is, the pre-charging relay is closed before the main relay is closed, and the pre-charging resistor is connected, it will have a protective effect on the high-voltage electrical equipment. When the load high voltage reaches a certain precharge threshold, for example, the load high voltage is equal to or exceeds 90% of the total battery voltage during the precharge process as the conditional threshold for judging the success of the precharge. After the precharge is successful, the main relay is closed and the precharge is disconnected. Relay, the high voltage is powered on normally, and the high voltage electrical equipment works normally. Under normal circumstances, the high voltage on the main positive relay will be closed only when the pre-charging is successful.

2. Precharge Model

The pre-charging circuit is simplified below, a pre-charging model is established, and then the equivalent load resistance and capacitance are calculated and deduced. In the precharge circuit, compared with the resistance value of the precharge resistor and the resistance value of the high voltage load, the internal resistance of the power battery is very small and can be ignored; The high-voltage load resistance is extremely large, and the high-voltage load resistance is negligible relative to the precharge resistance.

After simplification, the precharge model as shown in Figure 2 can be obtained.
In Figure 2, Vb is the power battery voltage, K1 and Rb are the pre-charging relay and pre-charging resistor, C is the high-voltage load equivalent capacitor, Vr is the voltage across the pre-charging resistor, Vc is the high-voltage load equivalent capacitor The voltage across the capacitor, i is the current of the high voltage loop. According to Kirchhoff’s law, we can get

3. Selection of precharge resistors
   The precharge resistance is derived from the precharge model:

In the actual calculation, C is determined by the charging capacitor of the motor controller, and t is determined by the completion time of the pre-charging requirement in the power-on process. Usually, the bus voltage Vc outside the load reaches 90% of the total voltage Vb of the power battery as the condition for judging the successful pre-charging. . The selection of the pre-charge resistor is to select the appropriate pre-charge resistor according to the power-on time requirements of the pre-charge, the specification of the charging capacitor of the motor controller and the voltage requirements. The result obtained by deriving the load outside bus voltage:

The precharge process is shown in Figure 3.

For example, the charging capacitor at the motor controller end is 1100μF, and the total precharging time does not exceed 500ms. After removing the influence of factors such as the self-check time of the BMS and the action time of the relay, remove 200ms and leave the precharging process time no more than 300ms. The bus voltage reaches 90% of the total voltage of the power battery as the condition for judging the success of pre-charging, r=RC=55ms. Relay outside bus voltage:

The overload multiple of the precharge resistor is selected according to experience and cost, as long as the designed rated calorific value under the designed peak power is greater than the theoretically calculated calorific value.
If U=0.9Ubattery is used as the condition for judging successful precharge, then the precharge time t≈2.69, r=147.95ms; if U=0.8Ubattery is used as the condition for judging successful precharge, there is precharge time t≈1.98, r=108.9ms.
Considering the ability of the precharge resistor to pass the peak current, lmax=Ubattery/R=6A, and the peak power of the precharge resistor Pmax=I²maxR=1.8kW (the calorific value at the peak power is 9000J), so 50Ω100W (peak power delivery) is selected. Heat 15000J) metal precharge resistors, as shown in Figure 4.

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1. Normal high voltage power-on sequence

The normal high-voltage power-on requirement starts from the key on/Crank and is completed within 1s. The DC/DC should be required to enter the working mode (buck) after the high-voltage power-on is completed, and the integrated power unit (IPU) should be required to enter the working mode (TorqControl) after the high-voltage power-on is completed. Table 1 shows the requirements for each time point of high-voltage power-on, and Figure 2 shows the normal high-voltage power-on sequence.

2. Normal high voltage power-off sequence

The normal high-voltage power-off process starts from the key off, and each node completes the high-voltage power-off process within a few seconds. Table 3 and Table 4 shows the requirements for each time point, and Figure 5 shows the normal high-voltage power-off sequence.

3. Emergency high voltage power-off sequence

Table 6 shows the requirements for each time point of emergency high-voltage power-off, and Figure 7 shows the emergency high-voltage power-off sequence.

It should be noted that, in the T6 time period, when the VCU requests the IPU to enter the emergency discharge mode, if the motor speed is less than 100rpm, the IPU enters the emergency discharge mode within 50ms; if the motor speed is not lower than 100rpm, when the motor speed drops to After 100rmp or less, the IPU enters the emergency discharge mode within 50ms.