Season 5 (2018-2019) of the ABB FIA Formula E Championship may have seen a first-of-a-kind lithium-ion Battery Management System (BMS) for an all-electric race car’s fundamental low-voltage (12 V) system [1,2,3].  In the first four seasons of Formula E, the only safety-related battery problems occurred in the 12v system batteries. This lithium-ion BMS development is particularly interesting, as much of the focus of electric vehicles is on the large high-voltage battery bank supplying the vehicle’s electric traction motor.

However, other essential safety and operation systems are alternatively powered by a low-voltage battery. These systems include the brakes, steering, sensors, communications, and other critical electronics. In a race where there are no scheduled pit stops, system reliability is a competitive advantage.

Mahindra Racing car

How the Renesas Low-Voltage Lithium-Ion BMS Benefits Mahindra Racing

The low-voltage battery provides the power the EV needs to start its functions, and also provides high power service to auxiliary functions that the DC/DC converter from the main battery pack isn’t able to power. Moreover, the low-voltage battery provides a critical function and backup system in case of the main battery, battery line, or DC/DC converter failure. Before the Renesas partnership with Mahindra Racing, it is unlikely that other Formula E race cars implemented a critical safety feature for these lithium-ion low-voltage batteries — a BMS.

The low-weight, minimal internal resistance, and high-power density make lithium-ion batteries all around attractive for performance electric vehicles, where every ounce counts. While unnecessary for common lead-acid battery types used in most cars, for lithium-ion batteries a BMS is essential for maintaining the health of a lithium-ion battery. It also ensures the battery doesn’t fail in such a way that could lead to thermal runaway.

In November 2017, Renesas and Mahindra Racing — one of the ten founding teams of the ABB FIA Formula E Championship — decided that this was a key area where Renesas could bring high reliability, automotive-grade semiconductor solutions, and electric vehicle engineering expertise. From this partnership, Renesas designed and developed a BMS module, complete with automotive-grade BMS components, enclosure, and lithium-ion battery. Moreover, the innovative low-voltage BMS module also packs their high-powered processor, the RH850/E2x microcontroller (MCU), with enough extra processing bandwidth to enable future innovations of the low-voltage battery system.

The Renesas Formula E BMS module provides a significant safety improvement over the unmanaged low-voltage lithium-ion battery system and simultaneously enhances the overall efficiency and reliability of the low-voltage system. Though efficiency and safety are important, the design of a BMS for a Formula E race car requires additional considerations. For instance, the vibration, shock, and even g-forces faced by electronics components in an automotive application far exceed the capabilities of most commercial and even industrial grade components or design practices. Renesas had the challenge of designing the complete module from the semiconductor to resolving how the overall module integrates with the Mahindra Racing car.

The module-level approach Renesas took in developing the BMS requires a custom design of circuit schematics, PCB, hardware components, software, and rigorous testing of each component and the overall module. Though the majority of the design effort was done in-house at Renesas, Voxdale designed the enclosure, and A123 provided the lithium-ion battery. The essential elements of the low-voltage BMS module included Renesas RH850/E2x automotive MCU, ISL78714 cell balancing BMS IC, and RH850/E2x demonstration board. Each of these components provides critical functions enabling improved efficiency and performance of the BMS.

The RH850/E2x automotive MCU is an embedded flash 400 MHz lockstep dual-core with FPU device that supports functional safety [4,5]. The RH850/E2x also has integrated A/D converters and a wide range of communication interfaces, including automotive industry standard CAN bus, CAN FD (5), Ethernet, and other serial communication channels. Moreover, as the RH850/E2x comes from a line of MCUs designed for internal-combustion engines, it is incredibly rugged and has an impressive range of integrated peripherals. These include onboard voltage monitoring, temperature sensing, and a separate security core.

The Renesas RH850 Family development environment included the RH850/E2x demonstration board, which was critical in the design and software development of the Formula E low-voltage BMS module [6]. The demonstration board and development environment enable a much faster and more efficient development. In the case of Formula E, the BMS module could benefit from more development effort, leading to higher reliability and better performance.

Though Renesas already makes many of the components to develop the low-voltage BMS, substantial engineering effort and collaboration between Renesas and Mahindra was necessary to make the system Formula E performance ready. This collaboration involved regular communication between Renesas’ engineering team and Mahindra Racing’s principal engineers. Renesas engineers were also present at Mahindra Racing to optimize the new BMS integration. Fortunately, Renesas prepared for this partnership with top engineering talent in the electric vehicle and autonomous vehicle industry after strategically hiring engineers from Tier 1s and OEMs.

Renesas’ Low-Voltage Lithium-Ion BMS & Automotive Processor Benefit the Electric Vehicle Industry

Outside of the benefits to Mahindra Racing’s race car, integrating innovative BMS solutions in an electric race vehicle can also lead to advancements for the broader electric vehicle market. As a proving ground, Formula E enables Renesas engineers to gain valuable insights and on-the-track experience developing electronics for the pinnacle of electric vehicles. The enhanced performance and capabilities of Renesas low-voltage BMS technology could eventually lead to a range of design, development, and even commercial improvements for electric vehicles.

The Renesas RH850/E2x automotive MCU series provides processing power far beyond the essentials for a BMS. Therefore, there is adequate processing power to enable onboard testing and evaluation of an electric vehicle system while the vehicle is operating. This MCU also has several high precision embedded peripherals that can provide a deeper understanding of electric vehicle operation.

Additionally, the other main component of Renesas Formula E low-voltage BMS, the ISL78714 multi-cell balancing BMS IC, provides exceptional voltage accuracy. With greater voltage accuracy, battery cells can be better balanced, and each cell can be charged and discharged to a better degree of precision. This level of accuracy could lead to extended lifespans for lithium-ion batteries and better avoidance of dangerous battery conditions.

In the case of consumer electric vehicles, the use of a high-grade automotive MCU and BMS can offer direct advantages. The stringent performance requirements of internal-combustion engines have led to extremely reliable automotive standards for electronics. An MCU that meets these standards, such as the RH850/E2x line, can provide substantial processing power for electric vehicles. This is paired with an assurance that the MCU will be able to handle the rigors of electric vehicle operation over the vehicle’s lifetime.

Moreover, the level of integration and wealth of embedded features of the Renesas MCU and BMS IC used in the Formula E electric race car can further reduce the bill-of-materials and design complexity of increasingly sophisticated electric vehicle electronics. This could have a direct impact on the cost of these electric vehicles and the manufacturer’s time-to-market.