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15 Saving Power for the Future | ADI Introduction An energy storage unit has to provide high capacity and the ability to release the energy in a controlled manner. Storage and release of energy, if not properly controlled, can result in a catastrophic failure of the battery and, ultimately, catch fire. Batteries can fail for several reasons, most of them related to inappropriate use. Failure can come from mechanical stress or damage, electrical overstress in the forms of deep discharge, overcharging, overcurrent, and thermal overstress. In order to reach the highest levels of efficiency and safety, a battery monitoring system is required. The main function of the BMS is to keep any single cell of the battery pack inside its safe operating area (SOA) by monitoring the following physical quantities: Stack charge and discharge current, single cell voltage, and battery pack temperature. Based on these quantities, not only can the battery be operated safely, but also SOC and state of health (SOH) can be computed. Another important feature provided by the BMS is cell balancing. In a battery stack, single cells can be arranged in parallel and in series in order to achieve the required capacity and operating voltage (up to 1kV or higher). Battery manufacturers attempt to provide stacks with identical cells, but this is not physically possible. Even small differences lead to different charge or discharge levels, with the weakest cell in the stack disproportionately affecting overall stack performance. Accurate cell balancing is a significant feature in a BMS, enabling safe operation of a battery system at its highest capacity. BMS Architectures An electric vehicle battery consists of several cells stacked in series. A typical stack—with 96 cells in series—when charged at 4.2V can develop a total voltage in excess of 400V. Higher voltages can be reached by stacking more cells. Charge and discharge current are the same for all the cells, but voltages have to be monitored on every single cell. To accommodate the large quantity of cells required for high powered automotive systems, batteries are often divided into modules, and distributed throughout available spaces in the vehicle. With 10 cells to 24 cells in a typical module, modules can be assembled in different configurations to suit multiple vehicle platforms. A modular design can be used as the basis for very large battery stacks. It allows battery packs to be distributed over larger areas for more effective use of space. Analog Devices has developed a family of battery monitors capable of measuring up to 18 series connected cells. The AD7284 can measure 8 cells, the LTC6811 can measure 12 cells, and the LTC6813 can measure 18 cells. Figure 1 shows a typical battery pack with 96 cells, divided into 8 modules of 12 cells each. In this case, the battery monitor IC is the 12-cell LTC6811. The cell measurement range is 0V to 5V, making the IC suitable for most battery chemistries. Multiple devices can be connected in series, permitting simultaneous cell monitoring of long, high-voltage battery stacks. The device includes passive balancing for each cell. Data are exchanged across an isolation barrier and compiled by the system controller, which is in charge of computing the SOC, controls cell balancing, checks the SOH, and maintains the full system inside the safety constraints. To support a distributed, modular topology within the high EMI environment of an EV/HEV, a robust communication system is required. Both isolated CAN bus and ADI's isoSPI™ offer road-proven solutions for interconnecting modules in this environment. While the CAN bus provides a well-established network for interconnecting battery modules in automotive applications, it requires a number of additional components. For example, implementing an isolated CAN bus via an LTC6811's isoSPI interface requires the addition of a CAN 1 A 96 cell battery pack architecture with the 12 channel LTC6811 measurement IC Battery Management System: The Complete Signal Chain