Battery system architecture
Nickel-cadmium batteries and the subsequent nickel-hydrogen battery technology have dominated the market for many years. Lithium batteries have only entered the market in recent years. However, with its outstanding superior performance, its market share has risen rapidly. Lithium batteries have amazing energy storage capacity, but the voltage and current of a single battery are too low to meet the needs of hybrid motors. In order to increase the current, multiple batteries are connected in parallel. In order to obtain a higher voltage, a plurality of batteries are connected in series.
Battery manufacturers usually use abbreviations like "3P 50S" to describe the arrangement of the batteries. "3P 50S" means that three batteries are connected in parallel and 50 batteries are connected in series. For multiple batteries in series, the modular structure is ideal for battery management. For example, up to 12 cells are connected in series to form one of the 3P 12S arrays. The charge of these batteries is managed and balanced by an electronic circuit with a microprocessor. The output voltage of the battery block is determined by the number of batteries connected in series and the battery voltage. The voltage of a single lithium battery is generally between 3.3 and 3.6V, so the output voltage of the corresponding battery block is between 30 and 45V.
Hybrid car drives require a DC supply voltage of around 450V. In order to compensate for the difference in battery voltage due to the different state of charge, a DC/DC converter is connected between the battery pack and the motor drive. The converter can also limit current. In order for the DC/DC converter to reach the ** working state, the voltage of the battery pack should be kept between 150 and 300V. To do this, 5 to 8 battery blocks need to be connected in series.
The necessity of balance
Once the voltage is outside the allowable range, the lithium battery is easily damaged. If the upper and lower limits of the voltage are exceeded (for example, the upper and lower voltage limits of the nanophosphate lithium battery are 3.6V and 2V, respectively), the battery may be irreversibly damaged, at least increasing the self-discharge rate of the battery. The output voltage can be kept stable over a fairly wide range of state of charge, so the probability of exceeding the safe range under normal conditions is relatively small. However, in areas close to the upper and lower limits of the safe range, the curve is very steep. As a precautionary measure, careful monitoring of voltage levels is necessary.
When the battery voltage approaches a critical value, the discharge or charging must be stopped immediately. The function of the balancing circuit is to adjust the voltage of the corresponding battery to keep it in a safe area. In order to achieve this, when the voltage of any of the batteries in the battery pack is different from that of the other batteries, energy must be transferred between the batteries.
Charge balance
1 Traditional passive balancing method
In a conventional battery management system, each battery is connected to a load resistor through a switch. The passive balancing circuit can discharge the specified battery separately, but this method can only suppress the voltage rise of the battery in the charging mode. In order to limit power consumption, a small current of 100 mA is generally used, which may cause hours to complete the charge balance.
2 active balance
Several active charge balancing methods are described in the existing literature, which use energy storage elements to transfer energy. If a capacitor is used as the energy storage element, many switching elements are required to connect the storage capacitor to all of the batteries. Relatively speaking, the use of magnetic fields to store energy is more efficient. The core component of this circuit is the transformer. The Infineon project team developed a prototype by working with VOGT electronic Components GmbH, which can be used to: transfer energy between cells; multiplex multiple battery voltages as ground-based voltage The analog-to-digital conversion input is constructed using a flyback converter. This transformer stores energy in a magnetic field with a gap in the core to increase the magnetic reluctance and avoid magnetic saturation of the core material. The transformer has two different windings: the main winding is connected to the battery pack; the secondary winding is connected to the battery.
A viable transformer model can support 12 batteries. The limiting factor is the number of possible connections. The transformer prototype described in this article has 28 pins. The switches use MOSFETs in the OptiMOS 3 family, which have extremely low on-resistance and negligible conduction losses.
Each battery block is controlled by Infineon's 8-bit microcontroller XC886CLM, which has flash memory and 32KB of data memory; two hardware CAN interfaces support communication using the common automotive controller area network (CAN) bus protocol, reducing The load of the processor; the hardware multiplication and division algorithm unit (MDU) increases the speed of the operation.
Balance mode
Since the transformer can be used in both directions, we can use two different balancing methods depending on the situation. The control circuit first detects the voltages of all the batteries one by one, calculates the average value, and then finds the battery whose voltage deviates from the average value**. If the voltage of the battery is lower than the average value, a bottom-balancing method is employed; if it is higher than the average voltage, a top-balancing method is used.
1 Lower limit balance: Each cycle consists of 2 active pulses and 1 interval. The period in this example is 40ms and the corresponding frequency is 25kHz. The design frequency of the transformer should be higher than 20 kHz to avoid noise due to the magnetic resilience of the transformer core. When the state of charge of a battery reaches the lower limit, the lower balance method can extend the operating time of the battery pack. As long as the current flowing out of the battery pack is lower than the average balance current, the vehicle can continue to run until the last battery is exhausted.
2 Upper limit balance: If the voltage of a battery is higher than other batteries, it is necessary to remove excess energy from the battery, which is especially necessary in charging mode. If there is no balance function, you must stop charging immediately after ** batteries are full. The balancing function keeps the voltages of all the batteries at the same level, thus avoiding the above situation. The current and timing in the upper-balanced mode of operation are similar to the lower-limit balance, except that the order of operation and current flow are reversed.
Balanced power
Using the prototype configuration in Infineon E-Cart, the average balance point is 6-bit 5A, which is 50 times higher than the passive mode, and the 5A balance current consumes only 2W in the entire battery block. Therefore, this balancing method does not require special cooling measures and improves the energy balance of the system.
Voltage detection
In order to manage the state of charge of each battery, the voltage of each battery is measured. Since only the No. 1 battery is in the analog-to-digital conversion range of the microcontroller, it is not possible to directly measure the voltage of other batteries in the battery block. One possible solution is to use a differential amplifier array, but this requires maintaining the voltage level of the entire battery block.
The following is a method for detecting all battery voltages with a small amount of hardware. The main function of the transformer is charge balancing, but we can also use it as a multiplexer. In the voltage detection mode, the flyback mode of the transformer is not used. When one of the S1 to SN switches is closed, the voltage of the turned-on battery is transmitted to all windings of the transformer. After a simple pre-processing of a discrete filter, the detection signal is input to the ADC ADC input pin.
The duration of the detection pulse generated when any of the switches S1 to SN is closed is very short, and the actual conduction time may be only 4 μs, so the energy stored in the transformer is not much. When the switch is turned off, the energy stored in the magnetic field will be fed back to the entire battery block through the main transistor, so the energy of the battery block is not affected. After scanning all the batteries, one scan cycle ends and the system returns to the initial state.
Conclusion
The advantages of the new lithium battery can only be fully exploited with the proper battery management system. The performance of an active charge balancing system is significantly better than the traditional passive approach. The creative use of simple transformers effectively reduces material costs.
Nickel-cadmium batteries and the subsequent nickel-hydrogen battery technology have dominated the market for many years. Lithium batteries have only entered the market in recent years. However, with its outstanding superior performance, its market share has risen rapidly. Lithium batteries have amazing energy storage capacity, but the voltage and current of a single battery are too low to meet the needs of hybrid motors. In order to increase the current, multiple batteries are connected in parallel. In order to obtain a higher voltage, a plurality of batteries are connected in series.
Battery manufacturers usually use abbreviations like "3P 50S" to describe the arrangement of the batteries. "3P 50S" means that three batteries are connected in parallel and 50 batteries are connected in series. For multiple batteries in series, the modular structure is ideal for battery management. For example, up to 12 cells are connected in series to form one of the 3P 12S arrays. The charge of these batteries is managed and balanced by an electronic circuit with a microprocessor. The output voltage of the battery block is determined by the number of batteries connected in series and the battery voltage. The voltage of a single lithium battery is generally between 3.3 and 3.6V, so the output voltage of the corresponding battery block is between 30 and 45V.
Hybrid car drives require a DC supply voltage of around 450V. In order to compensate for the difference in battery voltage due to the different state of charge, a DC/DC converter is connected between the battery pack and the motor drive. The converter can also limit current. In order for the DC/DC converter to reach the ** working state, the voltage of the battery pack should be kept between 150 and 300V. To do this, 5 to 8 battery blocks need to be connected in series.
The necessity of balance
Once the voltage is outside the allowable range, the lithium battery is easily damaged. If the upper and lower limits of the voltage are exceeded (for example, the upper and lower voltage limits of the nanophosphate lithium battery are 3.6V and 2V, respectively), the battery may be irreversibly damaged, at least increasing the self-discharge rate of the battery. The output voltage can be kept stable over a fairly wide range of state of charge, so the probability of exceeding the safe range under normal conditions is relatively small. However, in areas close to the upper and lower limits of the safe range, the curve is very steep. As a precautionary measure, careful monitoring of voltage levels is necessary.
When the battery voltage approaches a critical value, the discharge or charging must be stopped immediately. The function of the balancing circuit is to adjust the voltage of the corresponding battery to keep it in a safe area. In order to achieve this, when the voltage of any of the batteries in the battery pack is different from that of the other batteries, energy must be transferred between the batteries.
Charge balance
1 Traditional passive balancing method
In a conventional battery management system, each battery is connected to a load resistor through a switch. The passive balancing circuit can discharge the specified battery separately, but this method can only suppress the voltage rise of the battery in the charging mode. In order to limit power consumption, a small current of 100 mA is generally used, which may cause hours to complete the charge balance.
2 active balance
Several active charge balancing methods are described in the existing literature, which use energy storage elements to transfer energy. If a capacitor is used as the energy storage element, many switching elements are required to connect the storage capacitor to all of the batteries. Relatively speaking, the use of magnetic fields to store energy is more efficient. The core component of this circuit is the transformer. The Infineon project team developed a prototype by working with VOGT electronic Components GmbH, which can be used to: transfer energy between cells; multiplex multiple battery voltages as ground-based voltage The analog-to-digital conversion input is constructed using a flyback converter. This transformer stores energy in a magnetic field with a gap in the core to increase the magnetic reluctance and avoid magnetic saturation of the core material. The transformer has two different windings: the main winding is connected to the battery pack; the secondary winding is connected to the battery.
A viable transformer model can support 12 batteries. The limiting factor is the number of possible connections. The transformer prototype described in this article has 28 pins. The switches use MOSFETs in the OptiMOS 3 family, which have extremely low on-resistance and negligible conduction losses.
Each battery block is controlled by Infineon's 8-bit microcontroller XC886CLM, which has flash memory and 32KB of data memory; two hardware CAN interfaces support communication using the common automotive controller area network (CAN) bus protocol, reducing The load of the processor; the hardware multiplication and division algorithm unit (MDU) increases the speed of the operation.
Balance mode
Since the transformer can be used in both directions, we can use two different balancing methods depending on the situation. The control circuit first detects the voltages of all the batteries one by one, calculates the average value, and then finds the battery whose voltage deviates from the average value**. If the voltage of the battery is lower than the average value, a bottom-balancing method is employed; if it is higher than the average voltage, a top-balancing method is used.
1 Lower limit balance: Each cycle consists of 2 active pulses and 1 interval. The period in this example is 40ms and the corresponding frequency is 25kHz. The design frequency of the transformer should be higher than 20 kHz to avoid noise due to the magnetic resilience of the transformer core. When the state of charge of a battery reaches the lower limit, the lower balance method can extend the operating time of the battery pack. As long as the current flowing out of the battery pack is lower than the average balance current, the vehicle can continue to run until the last battery is exhausted.
2 Upper limit balance: If the voltage of a battery is higher than other batteries, it is necessary to remove excess energy from the battery, which is especially necessary in charging mode. If there is no balance function, you must stop charging immediately after ** batteries are full. The balancing function keeps the voltages of all the batteries at the same level, thus avoiding the above situation. The current and timing in the upper-balanced mode of operation are similar to the lower-limit balance, except that the order of operation and current flow are reversed.
Balanced power
Using the prototype configuration in Infineon E-Cart, the average balance point is 6-bit 5A, which is 50 times higher than the passive mode, and the 5A balance current consumes only 2W in the entire battery block. Therefore, this balancing method does not require special cooling measures and improves the energy balance of the system.
Voltage detection
In order to manage the state of charge of each battery, the voltage of each battery is measured. Since only the No. 1 battery is in the analog-to-digital conversion range of the microcontroller, it is not possible to directly measure the voltage of other batteries in the battery block. One possible solution is to use a differential amplifier array, but this requires maintaining the voltage level of the entire battery block.
The following is a method for detecting all battery voltages with a small amount of hardware. The main function of the transformer is charge balancing, but we can also use it as a multiplexer. In the voltage detection mode, the flyback mode of the transformer is not used. When one of the S1 to SN switches is closed, the voltage of the turned-on battery is transmitted to all windings of the transformer. After a simple pre-processing of a discrete filter, the detection signal is input to the ADC ADC input pin.
The duration of the detection pulse generated when any of the switches S1 to SN is closed is very short, and the actual conduction time may be only 4 μs, so the energy stored in the transformer is not much. When the switch is turned off, the energy stored in the magnetic field will be fed back to the entire battery block through the main transistor, so the energy of the battery block is not affected. After scanning all the batteries, one scan cycle ends and the system returns to the initial state.
Conclusion
The advantages of the new lithium battery can only be fully exploited with the proper battery management system. The performance of an active charge balancing system is significantly better than the traditional passive approach. The creative use of simple transformers effectively reduces material costs.
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