The first article in this series discussed some of the basic limitations and challenges for designers working with Electric Vehicles (EVs). This article discusses several key components of Battery Electric Vehicles (BEVs) and Formula E race cars, and how they differ. An overview of the valuable technical contribution that Renesas engineers made to the race-winning Mahindra Racing electric race cars is also featured in this article.
How EVs are Made
The basic functions of an EV are some form of electric energy storage, ways of charging and converting that electric energy for use by motor components, electrical motors, power controllers, sensors, and regenerative braking technologies. These facets of EVs all combine to determine EV performance limits, which are constantly tested by Formula E racers .
Electric Motors, Regenerative Braking, & Transmission (Powertrain)
Electric motors convert electrical energy into mechanical rotational energy. An electric motor consists of a stationary stator with large electrical windings that create magnetic flux that works against the distributed magnetics in the rotating rotor component. When electrical energy is applied to the stator coils, the magnetic flux generated by these coils forces the magnets of the rotor to turn. Electric motors also work in reverse, meaning that a moving motor without electrical energy applied generates electrical energy as the motor resists rotation, a method known as regenerative braking for EVs.
There are a variety of types of electric motors, though the most common types used for EVs are 3-phase Alternating Current (AC) electric motors. Depending on the type of electric motor, there are certain differences in how these motors are powered and controlled to achieve optimum performance.
Regardless of the electric motor type, a transmission is used to connect to the rotating motor shaft and efficiently transfer the rotational mechanical energy to the drivetrain. Most electric vehicles use just a single gear, or only a few gears in the transmission gear box, as electric motors used in EVs typically operate efficiently to tens of thousands of RPMs. This is very different from ICE power curves, which have limits between 1500 and 6000 RPM, requiring several complex transmission gears to operate efficiently. This division in efficiency is also compounded as electric motors can reach efficiencies greater than 90% (in conjunction with regenerative braking), while there are practical barriers that typically limit the efficiency of ICE engines to between 25% and 50%.
Electrical Energy Storage & Chargers
Electrical energy storage technology (often regarded as both the biggest enabling and limiting factor for EV performance) is what holds and delivers electrical power to an EV’s motor. There is a wide variety of electrical energy storage technologies, such as ultra-capacitors, chemical batteries, solid-state batteries, among others. Each of these electrical energy storage types demonstrates varying performance in terms of capacity, weight, efficiency, power density, cost, and commercial viability. Among these types of energy storage, Lithium-ion (Li) chemistry batteries currently offer the most practical balance of performance and commercial viability .
The five principal Li battery chemistries now used for EVs are Lithium-Nickel-Cobalt-Aluminum (NCA), Lithium-Nickel-Manganese-Cobalt (NMC), Lithium-Manganese spinel (LMO), Lithium Titanate (LTO), and Lithium-Iron-Phosphate (LFP). There are trade-offs for each lithium-chemistry, such as safety, specific power, power density, cost, and life-span.
An EV battery is generally made up of thousands of tiny battery cells that are combined to form large assemblies. The reason for this is that there are practical limits to the size, operating current, and operating voltage of Li batteries, and creating battery assemblies is one of the most efficient methods of making batteries with much greater power density and operating voltage in the least amount of space and weight. It is desirable to have batteries that charge and discharge at high voltages, as the amount of current needed to deliver the desired power reduces as the voltage increases. The amount of current determines the resistive loss within conductors, and therefore the amount of heat produced during conduction. Hence, most EV batteries are designed to operate in the hundreds of volts.
Just as an EV battery is discharged to power an EV, it must also go through a recharge cycle to be useful for future EV operation. The technology used to charge an EV battery depends on the type of EV charging electronics and the Battery Management System (BMS) of the EV battery. A BMS is a technology designed to ensure that a battery remains safe and balanced at all times. Especially with Li batteries, a BMS is essential for charging safety, as unbalanced or rapid charging can result in uncontrolled charging, imbalanced pack charge, chemical damage, and thermal runaway that can potentially lead to catastrophic battery failure. Generally, EVs either include on-board AC/DC converters for wall charging or are designed to operate with high voltage DC chargers that do the AC to DC conversion external to the EV. Formula E cars, as they aren’t designed to be charged in a common household and every ounce of weight is a concern, don’t include on-board charging equipment and rely on external converters and charging circuitry. However, Formula E Li battery cells are still protected by a BMS.
DC/AC Inverters and AC/DC Converters for EVs are high power electronic assemblies that are designed to withstand the extreme electrical power discharge of an EV battery, or the (potentially) even more extreme power used for EV charging. Charging power for EVs ranges from 1.5 kW from normal US household outlets to 400 kW for extremely fast chargers. Most houses are limited to roughly 7 kW of charging power based on 230-240 VAC 30A two pole outlets. House-based charging (type 2) requires the EV to have an AC/DC converter internal to the vehicle to convert household AC to the appropriate DC voltage that the EV batteries require.
Typical DC fast chargers range from 50 kW to 200 kW, or from 400 VDC to 600 VDC and up to 300 A. These types of chargers require a very sophisticated EV BMS and power electronics in the charger itself. As EV owners may want flexibility with how they charge, there is a Society of Automotive Engineering (SAE) standard for Combined Charging Systems (CCS) that enable both AC and DC charging modes (SAE J1772 CCS).
For 3-phase AC induction motors, common to higher performance EVs, a high power inverter is needed to convert DC battery power to 3-phase AC for the motors. This is the case for Formula E, and it is one of the key areas where a Formula E team can get an advantage by incorporating better-performing electronics. This is a challenging feat, however, as sophisticated control electronics are necessary to optimize the behavior of switching inverters to prevent high current and/or voltage spikes while maintaining high efficiency.
Moreover, there is generally an additional DC/DC converter that converts the high voltage DC from the main EV battery to the low-voltage auxiliary battery. This auxiliary battery operates the basic functions of an EV when the main battery isn’t active. When the main battery is active, the auxiliary battery is part of a critical function that ensures the vehicle safety systems are functional even in the case of main battery failure.
This is a key area where Mahindra Racing benefited from their partnership with Renesas. As an aspect of the partnership, Renesas designed and built a low-voltage EV battery BMS and monitoring module that enhanced the safety and efficiency of the Mahindra Racing Formula E race car and was likely the first-ever such system for Formula E.
Sensors & Embedded Electronics
There are many necessary and beneficial sensor systems that help to ensure safety and enhance the efficiency of EV operation. Some of these sensors include thermal, current, voltage, shock/vibration, magnetic flux position/strength, velocity, acceleration, and many more. The quality of these sensors, as well as their reliability, is a key factor in EV performance. Though an EV could be operated by direct pedal operations, it is more efficient and safer for more complex and intelligent systems to be employed. This level of sophistication enables advanced driving features, such as greater range efficiency, autonomous vehicle features, and pioneering safety features that can prevent accidents and casualties. Hence, it is crucial that these sensors and embedded electronics for control and intelligence features are rated to extreme automotive standards.
This part of the three-part series delved into the internal workings of EVs and compared commercially EVs and Formula E electric race cars. The final article of the series will detail how the essential embedded electronics within EVs make these amazing new vehicles possible.