Challenges in Battery Management for Hybrid/Electric Vehicles
The battery management system (BMS) plays a defining role in how well a battery stack meets each of these design challenges. At the heart of the BMS is a battery monitoring integrated circuit (IC). This IC measures individual cell voltages which are used to determine state of charge and battery stack health.
The most critical characteristics of a battery monitor IC are accuracy, data robustness, and fault detectability to ensure safety. The accuracy of the monitor IC directly affects system cost, battery pack reliability and lifetime. Each cell has a limited capacity which must be carefully managed. Overcharging can cause safety and reliability issues, while over-discharging can affect the lifetime of the cell. Using a less accurate monitor IC requires that the system designer use larger “guard bands” to protect against overvoltage and undervoltage, therefore limiting the amount of total available capacity for the vehicle. A higher accuracy monitor IC can make use of more or each cell’s total capacity, reducing the total cost of the battery stack system.
For the best accuracy over time and harsh operating conditions, Linear Technology uses a sub-surface Zener voltage reference in the LTC6804 Battery Monitor IC (Figure 1). This results in a guaranteed total cell voltage measurement error of less than 1.2mV. To maintain the best accuracy measurements in the presence of electrical noise and transients from inverters, actuators, switches, relays, etc., the LTC6804 uses delta-sigma ADCs with built-in 3rd order noise filtering.
Ideally, a battery stack would be divided into smaller modules which are distributed throughout the vehicle for better design flexibility and weight distribution. The challenge is that the battery modules need to communicate sensitive measurement data in an electrically noisy and physically harsh environment. CANbus was originally designed to provide robust communications in harsh automotive environments, but the data throughput demands of raw BMS data and component costs have kept CANbus from being adopted in EVs and HEVs. For this reason, Linear Technology has created the isoSPI™ interface to provide low cost robust communication for up to 100m cable lengths.
The isoSPI interface built into the LTC6804 and the companion LTC6820 isoSPI communication interface IC works together with tiny transformers similar to those used in Ethernet networks and a balanced wire pair to provide up to 1Mbps data rates without the costs associated with CANbus. The isoSPI interface was designed for error-free transmission while subjected to the rigors of bulk current injection (BCI) interference testing. In practice, full performance against ultra-harsh 200mA BCI has been demonstrated at Linear Technology and duplicated at several key automotive companies, fully qualifying isoSPI links for chassis-harness vehicle wiring.
Safety is a top priority for EV manufacturers. As electronic components are used more extensively in automobiles, increased attention has been given to the impact their operation has on safety. Linear Technology has been a supplier to major automotive customers for many years and continuously works to improve its excellent quality and reliability levels. In addition, battery stack designers look to IC manufacturers to provide comprehensive fault detection. A battery monitor designed for automotive safety will be compatible with the ISO26262 standard and include redundant circuits, self-tests, watchdog timers, and communication error detection and correction.
As EV and HEVs continue to grow in popularity, battery stack system designers will work to continuously improve cost, performance and safety. Recognizing the key role the battery monitor IC plays in each of these areas, automotive designers will use ICs that provide the highest levels of accuracy, robustness, and fault detection.
About the author:
Brian Black is Product Marketing Manager for Signal Conditioning Products at Linear Technology Corp.
If you enjoyed this article, you will like the following ones: don't miss them by subscribing to :
