The battery management system (BMS) controls the operating status of the battery through a finite state machine, responds to external commands, and detects various internal conditions of the battery system. The following are several common operating modes and their characteristics:
1. Low power (sleep) mode
In this mode, the battery is disconnected from the load by opening the contactor, and the battery current is zero. The system aims to minimize energy consumption from high-voltage battery packs and control power sources. Due to the disconnection of the battery system, there is no need to monitor battery voltage and temperature. All monitoring circuits and ICs are in a high impedance state, the communication bus is idle, the microprocessor is powered off, and as many circuits as possible are turned off.
Application scenarios:
When turning off the ignition switch of an electric vehicle.
Function:
Regularly wake up from sleep mode for battery state estimation and balance checks.
Real time clock or timer circuit is required in the hardware, supporting the "alarm clock" function, set through I2C or SPI bus.
2. Idle (standby) state
In this mode, the battery is still disconnected from the load, but the monitoring circuit is effective. The battery voltage and temperature are being measured, and fault detection algorithms and charge state estimation algorithms are running. This state allows for verifying the condition of the battery cells and the entire system while the battery system is disconnected and preventing charging and discharging.
Application scenarios:
Ensure that the system is safe before closing the contactor during system startup and shutdown.
Function:
Communication bus activity involves exchanging information between batteries, loads, and other devices on the network.
Unit balancing can be performed in this state.
3. Pre charging or soft start mode
In this mode, other devices on the high-voltage bus should expect the bus voltage to rise to the battery voltage, but must not consume any current from the bus to prevent pre charging failure. The closing sequence of the contactor will be triggered by an external command connected to the battery.
Application scenarios:
When starting an electric vehicle, ensure that the high voltage bus voltage rises steadily.
Function:
This state will end when the contactor closing sequence is successfully completed or when a fault is detected during the attempt.
4. Online status
In this mode, the battery is connected to the load or charging device. Many applications, such as grid storage and hybrid vehicles, use the same device network to charge and discharge batteries, so there is no distinction between charging and discharging modes. Other systems with separate loads and chargers may use different modes to connect each device.
Application scenarios:
The charging and discharging status of battery electric vehicles.
Function:
There may be an error state where the system can be queried for fault codes and diagnostic routines executed, but the system cannot close the contactor until a clear command is executed to exit the error state.
5. Error status
In this mode, due to issues with the battery system, the battery remains idle when the contactor is open, but does not respond to certain commands.
Application scenarios:
When the battery system malfunctions.
Function:
The system can be queried for fault codes and diagnostic routines executed.
The system cannot close the contactor until a clear command is executed to leave the error state.
Matters needing attention:
Avoid endless cycles between incorrect states and attempts to connect the battery.
If certain battery faults are automatically cleared, triggering an attempt to return to online status, this situation may occur.
Epilogue
The operation mode of the battery management system covers various situations from low power consumption to online status, ensuring that the battery system can operate safely and reliably under various conditions. Through reasonable state management and fault detection, battery life can be effectively extended, and the overall performance and reliability of the system can be improved.