“The rapid rollout of electric vehicles is accelerating innovation in battery technology, including battery management chips. One of the key aspects is that through integration, benefits such as ease of design, security, high performance, etc. can be achieved. New advances in this area help maximize the potential of electric vehicle batteries without compromising battery health and safety.
By Konrad Lorentz, Product Manager, Battery Management Systems, NXP
The rapid rollout of electric vehicles is accelerating innovation in battery technology, including battery management chips. One of the key aspects is that through integration, benefits such as ease of design, security, high performance, etc. can be achieved. New advances in this area help maximize the potential of electric vehicle batteries without compromising battery health and safety.
Ensure safety and efficiency
Electric vehicle batteries can fail for a number of reasons. Mechanical stress or damage after a crash can puncture the battery pack or damage individual cells. Electrical stress, such as overcharging, can also cause safety issues and reduce overall battery life.
A battery management system (BMS) helps monitor the safety and efficiency of EV batteries. One of its primary functions is to ensure that each Li-Ion cell in the battery pack operates within its Safe Operating Area (SOA) defined by voltage, current, and temperature. Operating outside of a fairly stringent SOA can have serious consequences, such as catastrophic failure of the battery, or worse, thermal runaway. This is why the battery junction box (BJB) is particularly important.
The battery junction box plays a key role
BJB is one of three functional modules in BMS. It measures and records the total battery voltage and the current flowing into and out of the battery to accurately calculate its state of charge (SOC). This enables precise mileage calculations and lets the driver know how much battery is left. It also implements key safety functions such as contact checking, isolation monitoring, and overcurrent detection. BJBs are required to meet ASIL C or ASIL D functional safety levels for current and voltage measurements in automotive applications.
One challenge in high voltage system communication is isolating the low voltage semiconductor (12 V) from the high voltage (400 V) battery side. Here, BMU (Battery Management Unit) communication is traditionally via the CAN bus, but daisy-chaining the BJBs 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. The benefits of this solution include stronger electromagnetic compatibility (EMC) characteristics, higher high voltage isolation, better communication speeds and simultaneous measurements. This reduces software-related complexity and reduces BOM costs.
As a standalone module, the BJB requires an onboard MCU with its own software
Communication in a functional safety environment
Functionally safe communication is possible using the grey channel approach.
Grey channel is an abstract term that refers to secure and reliable transmission over a channel that is neither functionally nor physically secure. Although it cannot be assigned an ASIL rating, it is considered a Quality Management (QM) rating, which is the lowest level of safety. Thus, gray channels allow the use of QM-rated devices to establish ASIL-rated communication paths. To achieve ASIL at the system level, you need to ensure that there is no data manipulation in the entire communication path. Data is protected at the source and decoded at the destination. What happens during data transfer is negligible because you can always detect errors because the data is protected at the source from manipulation.
By using the grey channel approach in the BJB, intelligence can be transferred from the BJB to the battery management unit (BMU) without spending extra security on the communication.
Integrated solutions help save time and costs
Emerging semiconductors specifically for BJB applications integrate all necessary functions in a single device. For example, NXP’s MC33772C provides accurate current sensing from milliamps to kiloamps with coulomb counting.
The device offers several advanced voltage and temperature measurement functions, and embedded balancing transistors with diagnostics simplify BJB applications. It also supports standard SPI and isolated daisy-chain communication with the MCU to handle and control up to 63 nodes. Additional diagnostic and functional safety features include detection of internal and external faults such as opens, shorts and leaks.
The rugged AEC Q-100 compliant device is hot-swappable and supports ISO 26262 with up to ASIL D functional safety capability.
MC33772C Block Diagram: 6-Channel Li-Ion Battery Controller IC
This design simplification reduces time-to-market and associated development costs for Tier1 and OEMs, while improving range, safety and battery life.