[Guide]The use of large-scale battery arrays as backup and portable energy storage devices is receiving more and more attention, as evidenced by the recent Powerwall system launched by Tesla Motors for home and office applications. In these systems, the battery is continuously charged through the power grid or other power sources, and then the AC power is delivered to the user through the DC/AC inverter when the user needs it.
The use of batteries as a backup power source is not new. Many systems provide everything from basic 120/240Vac and hundreds of watts (for short-term backup of desktop computers) to thousands of watts of backup power (for ships, hybrid vehicles or pure electric vehicles, etc.) Special vehicles), backup power for grid-scale telecommunications and data centers is up to hundreds of kilowatts (see Figure 1). However, although everyone is generally concerned about advances in battery chemistry technology, the battery management system (BMS) part is equally important in terms of practical battery installation solutions.
Figure 1. Battery-based backup power is very suitable for fixed and mobile applications ranging from several kilowatts to hundreds of kilowatts, and can provide reliable and effective power in various applications
There are many challenges in the implementation of energy storage battery management systems. The solutions cannot simply be expanded from small-scale, low-capacity battery packs, but require new and more complex strategies and key support components.
The first challenge is that the measurement of many important battery cell parameters requires high accuracy and reliability. In addition, its subsystems must adopt a modular design, allowing customized configurations according to the specific needs of the application, and considering possible expansion, overall management issues, and necessary maintenance.
The working environment of large storage arrays also brings other major challenges. Despite the presence of high voltage/current inverters and the resulting current peaks, BMS must still provide accurate and consistent data in a noisy, high-temperature electrical environment. In addition, it must also provide a large amount of accurate data and system temperature measurements on internal modules, which are essential for charging, monitoring and discharging, rather than just providing some rough summary values.
Since these power systems undertake basic tasks, their operational reliability is of utmost importance. In order to achieve these goals, BMS must ensure the accuracy and completeness of the data, and at the same time continue to conduct state assessments so that necessary measures can be taken continuously. Achieving reliable design and safety is a multi-level process. The BMS must predict problems, perform self-tests, and perform fault detection on all subsystems, and then perform appropriate operations in standby and operating modes. Finally, due to high voltage, high current, and high power levels, BMS must meet many strict regulatory standards.
Transform concepts into actual solutions through system design
Although the concept of monitoring rechargeable batteries is very simple (just need to set up voltage and current measurement circuits at both ends of the battery), the actual situation of BMS is completely different and much more complicated.
A reliable design must first comprehensively monitor a single battery cell, which places high demands on the analog function. Cell readings need to be accurate to millivolts and milliamps, and voltage and current measurements must be time synchronized to calculate power. The BMS must also evaluate the validity of each measurement value, it needs to improve the data integrity to a greater extent, and it must identify erroneous or suspicious readings. It cannot ignore abnormal readings that may indicate a potential problem, but it also cannot act on erroneous data.
Modular BMS architecture can improve robustness, scalability and reliability. Modularity also helps to use isolation between segments of the data link as needed, to minimize electrical noise and improve safety. In addition, advanced data encoding formats including CRC (Cyclic Redundancy Check) error detection and link confirmation protocol ensure data integrity so that the system management function can be sure that the data it receives is the data it sends.
For example, the scalable, customizable battery management system developed by Nuvation Engineering (Waterloo, California, Ontario, and Sunnyvale) uses the above principles. Practice has proved that the design of grid energy storage system and backup power equipment in Nuvation BMS is very successful, among which reliability and robustness are very important. The core advantage of this off-the-shelf BMS lies in its hierarchical and hierarchical topology (Figure 2) with three subsystems, each of which has unique functions, as shown in Figure 3.
Figure 2. The Nuvation Engineering battery management system is the interface between the AC grid and the battery cell array; it provides advanced battery charge/discharge monitoring and DC/AC inverter functions
Figure 3. The three main subsystems of Nuvation BMS (battery cell interface, battery stack controller, power interface) adopt a modular and hierarchical design, which can achieve scalability, robustness and reliability at various power levels
1. The cell interface strictly manages and monitors each battery cell in the battery stack; the system uses as many cell interfaces as necessary, depending on the number of battery stacks. These interfaces can be connected in a daisy chain according to the number of cells, thereby increasing the stack voltage.
2. The cell interface is connected to a single stack controller, which monitors and manages multiple cell interface units. If needed, multiple stack controllers can be connected together to support large battery packs with many parallel stacks.
3. The power interface connects the stack controller to the high voltage/current line and is also the interface to the inverter/charger. It physically and electrically isolates the high voltage and high current components of the battery stack from other modules. It also supplies power to the BMS directly from the battery stack, allowing the BMS to operate without any external power supply.
The modular layered architecture of Nuvation BMS supports battery pack voltages up to 1250Vdc, using cell interface modules, each module contains up to 16 cells, a battery stack with up to 48 cell interface modules, and contains more A parallel stack of battery packs. From the user’s point of view, the entire array assembly is managed as a single unit.
Build reliable designs from the bottom up
Factors such as modular architecture, hierarchical topology, and error-aware design are indispensable to the integrity and scalability of Nuvation BMS, but these are not enough. Successful implementation requires high-performance functional modules as the physical basis.
This is why the LTC6804 multi-cell battery monitor IC (Figure 4) plays a key role in the implementation of Nuvation BMS. It is specially tailored to meet the needs of BMS system and multi-cell design, and can accurately measure up to 12 battery cells stacked in series. The measurement input is not grounded as a reference, which greatly simplifies the measurement of these units, and the LTC6804 itself can be stacked and used with high-voltage arrays (it also supports various cell chemical characteristics). It provides a maximum error of 0.033% and 16-bit resolution, and it only takes 290μs to measure all 12 cells in the battery stack. This simultaneous voltage and current measurement is essential to produce meaningful power parameter analysis.
Figure 4. The LTC6804 multi-cell battery monitor IC can accurately measure stacked battery cells, which is the starting point for successful BMS implementation
Of course, the actual achievable performance of a good workbench prototyping environment is different from that of a real BMS setup with unfavorable electrical and environmental conditions. The LTC6804’s analog/digital converter (ADC) architecture is designed to suppress and minimize these adverse effects using filters specifically designed for power inverter noise.
The data interface uses a single twisted pair, isolated SPI interface, supports speeds up to 1Mb and distances up to 100 meters. In order to further enhance system integrity, the IC has also undergone a series of subsystem tests. The LTC6804 meets the stringent AEC-Q100 automotive quality standards, further proving its reliability and robustness. This IC can achieve such results because its design pays close attention to the BMS issues and environment, including the unique system-level goals of the application and its many challenges.
Three problems solved
The LTC6804 mainly addresses three aspects that affect system performance, conversion accuracy, battery balance, and connectivity/data integrity considerations:
BMS applications have short-term and long-term accuracy requirements, so a buried Zener conversion reference voltage source is used instead of a bandgap reference voltage source. This can provide stable low drift (20ppm/√kHr), low temperature coefficient (3ppm/°C), low hysteresis (20ppm) primary voltage reference source and excellent long-term stability. This accuracy and stability are very important. It is the basis of all subsequent battery cell measurements. These errors will have a cumulative impact on the credibility of the data obtained, the consistency of the algorithm, and the performance of the system.
Although a high-precision reference voltage source is a necessary function to ensure excellent performance, this function alone is not enough. The analog-to-digital converter architecture and its operation must meet the requirements of the electrical noise environment, which is the result of the pulse width modulation (PWM) transient characteristics of the system’s high current/voltage inverter. Accurate assessment of the battery’s state of charge (SOC) and state of health also requires related voltage, current, and temperature measurements.
In order to reduce system noise before affecting BMS performance, the LTC6804 converter uses a sigma-delta topology and handles the noise environment with the help of six user-selectable filter options. By using the essential characteristics of multiple sampling for each conversion, and the use of averaging filtering function, the sigma-delta method reduces the influence of electromagnetic interference (EMI) and other transient noises.
In any system that uses large battery packs arranged in battery packs or module groups, it is inevitable to achieve battery balancing. Although most lithium battery cells are well matched when first acquired, they will lose capacity as they age. The aging process of different battery cells may be different due to many factors, such as the temperature gradient of the battery pack. Moreover, battery cells that work beyond the upper limit of SOC will age prematurely and lose extra capacity. These differences in capacity and small differences in self-discharge and load current will cause battery imbalance.
In order to solve the problem of battery imbalance, LTC6804 directly supports passive balancing (using a user-settable timer). Passive equalization is a simple, low-cost method to standardize the SOC of all cells during the battery charging cycle. By removing charge from lower capacity cells, passive equalization can ensure that these lower capacity cells are not overcharged. LTC6804 can also be used to control active equalization, which is a more complex equalization technique that transfers charge between cells through charging or discharging cycles.
Regardless of whether the active method or the passive method is used, battery balancing relies on high measurement accuracy. As the measurement error becomes larger and larger, the operating protection level established by the system must also increase, so the effectiveness of the equalization performance will be limited. In addition, as the SOC range is further restricted, the sensitivity to these errors has also increased. The total measurement error of the LTC6804 is less than 1.2mV, which fully meets the system-level requirements.
Connectivity/data integrity considerations
The modular design of the battery pack increases scalability, service capabilities, and flexibility of external dimensions. However, this modularity requires electrical isolation (no resistance path) for the data bus between the battery packs, so the failure of any one battery pack will not affect other parts of the system or apply high voltage to the bus. In addition, the wiring between the battery packs must be able to withstand high levels of electromagnetic interference.
An isolated twisted pair data bus is a viable solution that can achieve these goals in a compact and cost-effective manner. Therefore, LTC6804 provides an isolated SPI interconnect called iso-SPI, which can encode clock, data input, data output and chip select signals as differential pulses, and then couple them through a rugged, mature and reliable isolation component transformer (Figure 5).
Figure 5. LTC6804 supports isolated SPI interface, which can be connected in a daisy chain to form a larger array, so as to achieve reliable anti-electromagnetic interference interconnection, minimize wiring requirements, and reduce the number of isolators
Devices on the bus can be connected in a daisy-chain configuration, which greatly reduces the size of the wiring harness and enables the modular design of large high-voltage battery packs while maintaining high data rates and low EMI sensitivity (Figure 6).
Figure 6. The test results on the LTC6804 and isoSPI interface show that the input radio frequency is 200mA, and there is no data error when isoSPI runs under 20mA signal strength
In order to verify the immunity, a BCI test was also carried out on the LTC6804. Including the coupling of 100mA of radio frequency energy into the battery harness, the sweep of the radio frequency carrier from 1MHz to 400MHz, and the 1kHz amplitude modulation of the carrier. The cutoff frequency of the LTC6804 digital filter is set to 1.7kHz, and an external RC filter and ferrite choke are added. Result: In the entire RF sweep range, the voltage reading error is less than 2mV.
In addition, a series of self-assessment and self-test functions are provided to increase the applicability of LTC6804 to BMS applications. These tests include open circuit detection; the second internal reference source of the ADC clock; self-testing of the multiplexer, and even the measurement of its internal power supply voltage. The device is designed for systems that comply with ISO 26262 and IEC 61508 standards.
Backup and portable power supplies for grid-level systems are extremely attractive. It seems very simple: just keep a set of batteries charged (whether from the AC grid side line, or solar, wind or other renewable energy), and then use the battery with a DC/AC inverter when needed, then It can provide AC power equivalent to line power supply.
In fact, any behavior or performance characteristics of the battery are not simple. It is necessary to carefully control the charge and discharge, and monitor the voltage, current and temperature. As the power level increases, a practical, efficient and safe system is not a small design, so the grid-connected multi-cell BMS is a complex system. Many unique issues need to be understood and resolved in depth, and security is also a major issue.
Successful and feasible system design requires a modular, structured, top-down architecture, supported by optimized components such as LTC6804 from bottom to top. Combined with advanced and safe data acquisition and control software, the high-performance BMS constructed is safe and reliable, requiring only a small amount of operator intervention, and can automatically and reliably run for many years.
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