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 0 V to 5 V, 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.1 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 transceiver, a microprocessor, and an isolator. The primary downside of a CAN bus is the added cost and board space required for these additional elements. Figure 2 shows a possible architecture based on CAN. In this case, all modules are parallel connected.
An alternative to a CAN bus interface is ADI’s innovative 2-wire isoSPI interface.1 Integrated into every LTC6811, the isoSPI interface uses a simple transformer and a single twisted pair, as opposed to the four wires required by the CAN bus. The isoSPI interface provides a noise-immune interface (for high RF signals) in which modules can be connected in a daisy-chain over long cable lengths and operated at data rates up to 1 Mbps. Figure 3 shows the architecture based on isoSPI and using a CAN module as a gateway.