The rapid rollout of electric vehicles (EV) is accelerating innovation in battery technology, including battery management semiconductors. One key aspect of this is integration, which can offer benefits in terms of ease of design, safety and performance. New advances in this area can go a long way in helping to maximize the potential of EV batteries, without compromising battery health and safety.
EV batteries can fail for a variety of reasons. Mechanical stress or damage following a crash can puncture the battery pack or impair individual cells. Electrical stress, such as over-charging, can also lead to safety issues and reduce the overall battery life.
The battery management system (BMS) helps monitor critical battery parameters for a safe and efficient operation of the EV battery. One of its main functions is to ensure that every Li-Ion cell within the battery pack operates within its safe operating area (SOA) defined by voltage, current and temperature. Operating outside the SOA can result in severe consequences, such as permanent damage to the battery or, worse still, thermal runaway. Helping to keep the battery in safe conditions, the battery junction box (BJB) in particular, is important.
Explore solutions for BMS. Learn more about NXP's battery management system.
The battery junction box (BJB) is a key element of the BMS. Depending on the architecture, it measures the total battery voltage in addition to the charge and discharge current, the latter allowing a precise calculation of the battery’s state of charge (SOC). This enables a more precise range estimation, letting the driver know how many kilometers are left in the battery. It also fulfils safety functions like overcurrent detection as well as contactor and isolation monitoring. In automotive applications, the BJB needs to support up to automotive safety integrity level (ASIL) D on various functions including current and voltage measurements.
One challenge for communication in high-voltage systems is the isolation of low voltage semiconductors from the high voltage battery side. As a standalone module, communication with the battery management unit (BMU) is usually over CAN bus, requiring an onboard microcontroller with embedded software. Integrating the BJB via TPL daisy chain communication bus can be an attractive alternative here. The TPL interface is designed specifically for BMS to comply with automotive EMC and EMI standards while supporting low-cost transformer or capacitive isolation. Furthermore, it also implements specific measures that facilitate the synchronization between cell voltages and pack current measurements. When connecting the BJB via TPL, the result is reduced complexity associated with local software and a lower bill of material (BOM) cost.
Safe communication between cell monitoring ICs and the BJB ICs is enabled by end-to-end protection between the individual devices and the safety MCU. Functional safety communication can be achieved using a grey channel approach. In the context of the SW-free BJB, this refers to a non-safety critical communication line between the two modules BJB and BMU which are regarded as safety-critical. To achieve an ASIL rating at the system level, you need to ensure no data manipulation throughout the communication path. Data is secured on the origin and decoded at the destination. What happens during the data's journey can be ignored because you can always detect errors since the data was secured at the origin and not manipulated.
Newly emerging semiconductors that specifically target the BJB application integrate all the required functions in one single device. One example, the MC33772CTC from NXP, provides precise current sensing from milliamps to kiloamps with coulomb counting.
Want to learn more? Watch our video MC33772C: 6 Channel Li-Ion Battery Cell Controller IC.
The device offers multiple advanced voltage and temperature measurement functions as well as comprehensive diagnostics that simplify the BJB application. For direct connection to an MCU in a standalone module, it also supports standard SPI communication. When using the TPL interface, up to 63 nodes can be connected in the same daisy chain, offering the option to connect the cell monitoring analog front ends as well. However the communication architecture is planned, the device enables end-to-end protection with the functional safety MCU and therefore supports the grey channel approach. Additional diagnostics and functional safety features include detecting internal and external faults like open lines, shorts and leakages. The robust AEC Q-100 compliant device supports ISO 26262 with up to ASIL D capability on system level.
Moving the intelligence from the BJB to the BMU is a simplification in design. This reduces time-to-market and the associated development costs at Tier 1s and OEMs while improving range, safety and battery lifetime, accelerating the transition to CO2 emission free transportation.
Development boards are already available, get started on your next design with battery management systems (BMS) hardware solutions.
Konrad Lorentz holds a Bachelor’s Degree in International Project Engineering from Reutlingen University. He joined NXP through a thesis paper on the Automotive Value Chain in 2020 and now holds a position as Product Manager in the BMS Marketing Team.
