How to maximize the full potential of electric vehicle batteries

The rapid adoption of electric vehicles has accelerated innovation in battery technology, including battery management semiconductors. An important aspect of this is integration, which brings advantages such as simplified design, improved security and performance. New advances in this area will help maximize the potential of electric vehicle batteries without compromising battery health and safety.

Safety and energy efficiency:

There are many reasons for battery failure in electric vehicles. Mechanical stress or damage after a collision can puncture the battery pack or damage a single cell. Electrical stresses such as overcharging can also cause safety issues and shorten overall battery life.

Battery management systems (BMS) help monitor the safety and energy efficiency of electric vehicle batteries. The primary function of the system is to ensure that the voltage, current, and temperature at which each lithium-ion cell in the battery pack operates are within the safe operating zone (SOA). Leaving the Safe Operating Zone (SOA) can lead to serious consequences, such as a major battery failure or, more seriously, thermal runaway. Therefore, the battery junction box (BJB) is particularly important.

The battery junction box plays an important role in the system:

The Battery Junction box (BJB) is one of the 3 functional modules in the battery management system (BMS). The junction box measures and records the total battery voltage and the current in and out of the battery to accurately calculate its state of charge (SOC). This enables accurate mileage calculations, allowing the driver to know how many kilometers the battery is capable of driving the vehicle. It also implements safety-critical functions such as contactor and isolation monitoring, as well as overcurrent detection. The BJB needs to meet the ASIL Class C or D current and voltage measurement functional safety requirements for automotive applications.

The challenge for high-voltage system communication is how to isolate the low-voltage semiconductor (12V) from the high-voltage (400V) battery side. In this regard, communication with the BMU used to be via the CAN bus, but Daisy chaining the BJB via the transformer Physical Layer (TPL) is an attractive alternative. The TPL interface is designed for BMS and supports high isolation voltages up to 2kV. This solution offers several advantages, including stronger electromagnetic compatibility (EMC) characteristics, higher high-voltage isolation, faster communication speeds, and synchronous measurement. The result is reduced complexity associated with on-premises software and reduced material costs.

Communication in a functional safety environment:

Functional safety communication can be achieved using the grey channel approach. Grey channel is an abstract term that refers to a secure transmission over an inherently insecure channel. Although it cannot be assigned an ASIL rating, it is considered a Quality Management (QM) rating, which is the lowest safety rating. As a result, grey channels allow for the establishment of ASIL-level communication paths using Quality management (QM) level equipment. To achieve ASIL rating at the system level, it is necessary to ensure that data is not manipulated throughout the communication path. Protect data at the source and decode it at the destination. Because the data is protected at the source and not manipulated, errors can be detected throughout the data transmission process, so what happens during this process is negligible.

Using the grey channel approach in the BJB, information can be transferred from the BJB to the battery management unit (BMU) without additional security work on the communication device.

Integrated solutions help save time and costs:

Emerging semiconductors specifically for BJB applications integrate all the necessary functions in one device. NXP's MC33772C, for example, provides accurate current sensing from milliamps to kiloamps via coulomb counting.

The device offers a variety of advanced voltage and temperature measurement functions, and the built-in balanced transistor with diagnostic functions simplifies BJB applications. It also supports standard SPI and isolated Daisy chain communication with the MCU, capable of handling and controlling up to 63 nodes. Other diagnostic and functional safety features include the detection of internal and external faults, such as open circuits, short circuits, and leaks. This stable and durable device is AEC Q-100 compliant, hot-swappable, supports ISO 26262 and has a safety rating up to ASIL D.