Automotive-Grade MCU Monitors EV Batteries at the Pack Level

Infineon Technologies introduced an automotive-grade MCU to enable safe, secure, and more accurate pack-level monitoring of high-voltage lithium-ion (Li-ion) electric-vehicle (EV) batteries.

The new battery-management IC features a high-precision analog front-end (AFE). The PSoC 4 HVPA-SPM comes with high-resolution, delta-sigma analog-to-digital converters (ADCs), together with digital filtering channels, to accurately measure parameters such as voltage, current, and temperature of the battery pack as a whole. These measurements can then be used to estimate the state of charge (SOC) and state of health (SOH) at any time. The ADCs operate at 16-bit resolution at 8 kSPS and 20-bit resolution at 1 kSPS.

The chip also contains a Cortex-M0+ MCU with 128 kB of code flash, 16 kB of data flash, and 8 kB of SRAM, bringing more intelligence to battery-pack monitoring, which tends to be relatively unintelligent today. Infineon said this allows it to fit into the “zonal” architectures being adopted in more EVs, and it gives companies the ability to customize the system for specific requirements. The chip can also offload the main high-performance MCU that oversees the battery-management system (BMS).

BMS with a Pack Mentality

A BMS must be able to closely and constantly monitor voltages at the cell level as well as the voltages, currents, and temperatures at the pack level. It uses these parameters to estimate the SOH and SOC over the lifetime of the battery and to make sure it’s operating reliably and safely.

Due to the high-voltage design of the BMS, insulation resistance measurement between the high-voltage domain and low-voltage domain is also key to identifying defects in the battery structure and protecting against unsafe conditions.

By monitoring the internal state of the battery, the BMS can do more than ensure the safe and reliable operation of the battery pack. It can also boost the performance of the battery pack, enabling longer runtimes, faster charging, and extended cycle lives. With batteries making up approximately 30% to 40% of the total cost of the EV, battery-management ICs have become a bigger focus for Infineon and other players in the power industry, including Analog Devices, NXP, and TI.

In most cases, the BMS is a three-in-one system. It comprises a battery-management unit (BMU) that takes care of all the functions within the battery pack, calculating the SOC and the SOH and then adjusting the energy in and out to maximize both metrics.

The BMS pairs with a cell-monitoring unit (CMU) that tracks the voltages of each and every cell, along with a battery junction box (BJB) equipped with power contactors that connects the entire pack to the traction inverter or the onboard charger — and disconnects it when in danger.

In a traditional BMS, the BMU measures the conditions inside the battery pack on behalf of the passive BJB. In future architectures, a more intelligent pack monitor such as the PSoC 4 HVPA-SPM will be placed inside the box to measure voltages and currents locally before relaying the parameters to the BMU. This will help reduce the amount of copper wiring within the BMS, reducing costs. It can also improve voltage and current measurements with lower noise, maximizing battery life.

These units monitor the voltage of the battery pack using divided-down resistor strings, while measuring up to thousands of amps of current using shunt resistors or Hall sensors. The box also monitors the temperature of the shunt resistor, primarily so that the main BMU can adjust the temperature to maximize the remaining energy. It’s important to keep tabs on the temperature of the contactors in the junction box, too, to make sure they’re not stressed beyond normal operating conditions.

Smart Single-Chip Battery-Pack Monitor Plus Gateway

The PSoC 4 HVPA-SPM 1.0 is a fully integrated solution for these situations, serving as a smart battery pack monitor and gateway in a single chip. Due to its integrated Cortex-M0+ MCU, the chip can act as a CAN gateway and communicate via a standard CAN interface to zonal controllers in the EV with Infineon’s iso-CAN transceivers. Communication with daisy-chained cell-monitoring ICs happens via iso-UART interfaces.

The ability to monitor the battery pack as well as communicate with the rest of the EV allows Infineons’ customers to reuse existing software stacks and eliminates the need for a separate MCU in the junction box.

The company said the MCU is also “EIS-ready," so it can be adopted in EVs that implement electrochemical impedance spectroscopy (EIS). The BMS can use EIS to more accurately determine the degradation of the battery and, thus, its remaining useful life and residual economic value.

The MCU meets the ASIL D (ISO 26262) standard for functional safety, ensuring robust and reliable operation that’s vital for EV batteries. The chip also comes with internal OCD comparators to detect overcurrent conditions and prevent catastrophic damage that can occur to the battery pack in the event of a short circuit — one of the main causes of thermal runaway. Overcharging, undercharging, or physically damaging the battery can cause overheating as well.

The integrated LDO means the MCU can be supplied directly from the battery without the need for an external power supply.

To reduce the cost and complexity involved with system design, Infineon said it will also bundle the new battery-pack-management IC with smart edge BMS software developed by Munich Electrification.