Automotive and industrial equipment manufacturers typically require battery life in excess of 10 years, and these manufacturers also specify the required available battery capacity. The challenge for battery system designers is how to achieve maximum capacity with the smallest battery pack. To achieve this goal, the battery system must carefully control and monitor the battery with precise Electronic components.
High Power Battery Pack System
High-power battery pack systems for electric vehicles or industrial equipment consist of many cells stacked in series. A typical battery pack may contain as many as 96 cells, producing a total of over 400V for a Li-Ion battery charged to 4.2V.
Although the system treats the battery pack as a single high-voltage battery, charging or discharging the cells in the pack simultaneously, the battery control system must consider the state of each cell independently. If one cell in a battery pack has a slightly lower capacity than the other cells, its state of charge (SOC) will gradually drift away from the rest of the cells over a number of charge/discharge cycles. If the states of charge of this cell and the rest of the cells are not periodically equalized, the cell will eventually enter a deep state of discharge, damaging and eventually causing the battery pack to fail. Therefore, the voltage of each cell must be monitored to determine the state of charge. In addition, precautions must be taken to allow the cells to be charged or discharged individually to equalize the state of charge between cells.
Communicate with surveillance system
An important factor to consider in a battery pack monitoring system is the communication interface. For communication within a printed circuit board (PCB), common choices include the Serial Peripheral Interface (SPI) bus and the Inter-Integrated Circuit (I2C) bus. Both interfaces have low communication overhead and are suitable for low-interference environments.
Another option is the CAN bus, which is widely used in automotive applications. The CAN bus is very reliable with error detection and fault tolerance, but the communication overhead is high and the material cost is high. Although it may be desirable to have an interface from the battery system to the main CAN bus, within the battery pack, SPI or I2C communication is advantageous.
Devices such as Linear Technology’s LTC6802 battery pack monitor IC measure the voltage of battery packs consisting of up to 12 cells. Multiple LTC6802s can be stacked in series from the bottom to the top of the pack. The device also has internal switches. , allowing a single cell to discharge to equalize the capacity of that cell with the rest of the battery pack.
To illustrate this battery pack architecture, consider a system with 96-cell Li-Ion cells. Monitoring the entire battery pack requires 8 battery pack ICs, each operating at a different voltage.
Using 4.2V Li-Ion batteries, the bottom monitoring device monitors 12 batteries with voltages from 0V to 50.4V. The next set of cells has a voltage range of 50.4V to 100.8V, and so on up the stack.
These devices operate at different voltages, and communication between them presents a huge challenge. Various approaches have been considered, each with its own advantages and disadvantages, given the different priorities of system designers.
Requirements for battery monitoring
When determining the architecture of a battery monitoring system, at least 5 main requirements need to be balanced. The relative importance of these requirements varies depending on the needs and expectations of the end customer.
1. Accuracy: In order to fully utilize the maximum battery capacity, the battery monitor must be accurate. However, automotive and industrial systems are noisy, and EMI exists in a wide frequency range. Any loss of accuracy will negatively impact the life and performance of the battery pack.
2. Reliability: Regardless of the power supply used, automotive and industrial manufacturers must meet extremely high reliability standards. In addition, the high energy capacity and potential variability of some batteries are also major safety concerns. A failsafe system that shuts down under conservative conditions is better suited for catastrophic battery failure, although such a system has the unfortunate chance of stranded passengers or halted production lines. Therefore, battery systems must be carefully monitored and controlled to ensure full control over the life of the system. To minimize spurious and real failures, a well-designed battery pack system must guarantee reliable communication, employ failure-minimizing modes, and have failure detection capabilities.
3. Manufacturability: New vehicles contain a wide variety of electronic components and complex wiring harnesses. Adding complex electronic components and wiring to support electric vehicle/hybrid electric vehicle (EV/HEV) battery systems creates additional challenges for vehicle manufacturing. Components and wiring must be minimized to meet stringent size and weight constraints and ensure mass production is practical.
4. Cost: Complex electronic control systems can be expensive. Minimizing relatively expensive components such as microcontrollers, interface controllers, galvanic isolators, and crystals can significantly reduce the overall cost of the system.
5. Power: The battery monitor itself is also a load for the battery. The lower operating current improves system efficiency, while the lower backup current prevents over-discharging the battery when the car or device is shut down.
battery monitoring
Table 1 presents four architectures for battery monitoring systems. Each architecture is designed as an autonomous battery monitoring system and assumes that the system consists of 96 cells, 12 cells are grouped into 8 groups (see Table 1). Each group has a CAN interface to the main CAN bus and is galvanically isolated from the rest of the system.
Table 1: Comparison of battery monitoring architectures
Parallel independent CAN modules (Figure 1)
Each module consisting of 12 cells contains a PC board with an LTC6802, a microcontroller, a CAN interface and a galvanic isolation transformer. The large amount of battery monitoring data required by the system overwhelms the main CAN bus, so the CAN module must be on the CAN subnet. The CAN subnets are coordinated by a master controller that also provides the gateway to the main CAN bus.
Figure 1 Parallel independent CAN modules
Parallel module with CAN gateway (Fig. 2)
Each 12-cell module contains a PC board with an LTC6802 and a digital isolator. These modules have independent interface connections to the controller circuit board, which contains a microcontroller, a CAN interface, and a galvanic isolation transformer. The microcontroller coordinates these modules and provides the gateway to the main CAN bus.
Figure 2 Block diagram of parallel module with CAN gateway
Single Monitoring Module with CAN Gateway (Figure 3)
In this configuration, there are no monitoring and control circuits in the 12-cell module. Instead, a single PC board contains eight LTC6802 monitor ICs, each connected to its battery module. The LTC6802 device communicates through a non-isolated SPI-compatible serial interface. A single microcontroller controls the entire battery monitor via an SPI-compatible serial interface, which is also the gateway to the main CAN bus. The CAN transceiver and galvanic isolation transformer are the last two components of this battery monitoring system.
Figure 3 Block diagram of a single monitoring module with CAN gateway
Serial module with CAN gateway (Fig. 4)
This architecture is similar to a single monitoring module, except that each LTC6802 is on a PC board inside a module consisting of 12 cells. The eight modules communicate via the LTC6802 non-isolated SPI-compatible serial interface, which requires 3 or 4 conductive cables to connect between the battery modules. A single microcontroller controls the entire bank of battery monitors via the bottom monitor IC and acts as a gateway to the main CAN bus. CAN transceivers and galvanic isolation transformers remain the last two components of a battery monitoring system.
Figure 4 Block diagram of serial module with CAN gateway
Battery Monitoring Architecture Selection
The first and second architectures are generally more challenging because parallel interfaces require numerous connections and external isolation. To deal with this increased complexity, designers have resorted to communicating with each monitor device independently. The third (single monitoring module with CAN gateway) and fourth (tandem module with CAN gateway) architectures take a simplified approach with the fewest limitations.
The LTC6802 can meet the needs of these four configurations, leaving the system designer with a choice. Two variants of this device have also been developed, one for series configuration and the other for parallel configuration. The LTC6802-1 is used in a stacked SPI interface configuration. Multiple devices can be connected in series through an interface that sends data back and forth along the battery pack without the need for external level shifters or isolators. The LTC6802-2 allows parallel architectures to be met with a single device. Both variants have the same battery monitoring performance specs and features.
Electric vehicles and high-power industrial equipment place numerous demands on battery packs. Manufacturers expect to meet stringent reliability requirements with affordable battery systems. The latest battery monitoring ICs give system designers the flexibility to choose the best battery pack architecture without compromising performance.
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