Technology

Fundamentals of EV power strategies

03 September 2021

Due to growing environmental concerns and evolving regulations, electric vehicles (EVs) have gained significant attraction, particularly within the automobile manufacturing sector. However, as the demand for EVs increases in the market, new challenges about vehicle design efficiencies also emerge.

While the transportation regulations require automotive OEMs to retrofit their production lines to build EVs, energy regulators are also tightening clean energy regulations to address the probable shortage of electricity due to the growing penetration of EVs. With that scenario in mind, the need to optimize vehicle energy consumption while still providing a comfortable driving experience is one of the most significant priorities of automotive OEMs.

Energy management for the EV involves careful examination of different factors ranging from vehicle energy processing to aerodynamic design, operational requirements, driving behaviors and charging infrastructure. The assessment of all these parameters is exhaustive; this article only covers the assessment of factors that can be used to optimize energy storage: regeneration of electrical energy within the EV.

Energy storage

Batteries are among the critical systems within EVs as they are used to store electrical energy necessary to drive vehicles load. Being an electrochemical device, the selection of the right chemistry is an important design decision to optimize energy performance for an EV. While there are numerous chemistries available, lithium-ion (Li-ion), lead-acid and nickel metal hydride (Ni-MH) have been particularly popular in automotive applications. In the early 2000s, Ni-MH batteries were the preferred option for hybrid electric vehicles (HEVs) due to their high energy density and capability of energy recovery through regenerative braking. However, they had a high self-discharge, heat generation rate and weight.

Similarly, lead-acid batteries also had some advantages such as being inexpensive, but they never made their way to EVs as the primary energy storage medium to carry vehicle load due to their excessive weight and higher safety risks due to hydrogen emissions. Moreover, they also deteriorate quickly when discharged deeply and thus cannot be used to operate continuous high power loads, such as electric traction motors, HVAC and other continuous auxiliaries. As a result, their role is primarily restricted to driving the supplemental load in internal combustion engine (ICE)-based cars as starting, lighting and ignition batteries where a short burst of current is required to ignite these engines without deep discharging the battery.

Among all other types, Li-ion batteries have been a promising option for EVs due to their high energy density, better power to mass ratio, and low self-discharge rate. They can also be cycled deeply many times compared to lead-acid without much degradation. Moreover, due to low weight, they also contribute to a better payload for a given gross vehicle weight rating, thus improving the overall vehicle design. However, Li-ion batteries are generally temperature-sensitive and have a range of voltages within which they should operate. To ensure safe operation, extra monitoring and protection are required in the form of a battery management system (BMS) for these batteries to ensure their safe operation. A BMS can monitor battery parameters such as voltage fluctuations, temperature gradients and charge current, and disconnect cells to ensure their safety.

To improve energy efficiency, some EV manufacturers provide Li-ion cells with improved cooling and heating systems to maintain temperature limits. For example, one method to optimize energy can be to incorporate a large number of smaller rated cells in series and parallel combinations compared to fewer higher-rated cells. Vehicle manufacturers such as Tesla incorporate a large number of smaller cells with liquid cooling systems to enhance cooling efficiency. A large number of small cells also permits temperature monitoring on a granular level thus providing accurate temperature assessment for the entire battery pack.

Regenerative braking

Regenerative braking involves recovering a portion of energy that is lost as heat during the braking effort. It is a simple technique that EV manufacturers adopt to extend the drive mileage and improve overall energy efficiency. Unlike ICE-based cars that require translation of linear motion of the piston into rotational force, the EVs are equipped with traction motors that take their power feed directly from batteries via power converters.

A power converter is a system of switches and a controller that regulates the speed and torque of the traction motor. In the case of normal forwarding drive, the traction rotor speed generally lags behind the stator speed represented by a speed slip – positive speed difference between stator and rotor. When the brakes are applied, the stator frequency or speed has to be reduced to slow down the motor. However, since the rotor continues to spin due to its inertia, the rotor speed exceeds the stator speed for some period causing the motor to enter into the negative slip – motor acting as a generator. The bi-directional power flow inverter would then reverse its switching mechanism and allow the reverse flow of power from the induction motor acting as a generator back into the batteries.

The figure below indicates the typical example of ramp-up (positive slip) and ramp-down (negative slip) situations. In the case of ramp-up, the batteries are discharged into traction motors. However, in the case of ramp down, the motor rotor speed will exceed the stator speed causing the power to flow back into the batteries thus charging them.

In the case of ramp-up, the batteries are discharged into traction motors. However, in the case of ramp down, the motor rotor speed will exceed the stator speed causing the power to flow back into the batteries thus charging them. Source: Adobe/dreampictureIn the case of ramp-up, the batteries are discharged into traction motors. However, in the case of ramp down, the motor rotor speed will exceed the stator speed causing the power to flow back into the batteries thus charging them. Source: Adobe/dreampicture

Since regenerative braking involves slowing of a motor due to a change in stator magnetic fields, it has limited capability to quickly bring the car to a complete stop when sudden emergency brakes are applied. To avoid this situation, mechanical frictional brakes are often provided as a separate pedal in some EVs to ensure a safe and complete stop on time. Some EV manufacturers optimize the usage of brake energy by integrating electrical and frictional brakes into a single pedal. Depending upon how deep the pedal is pressed, the controller will translate the brake angle – the deep the pedal is pressed the greater the brake angle – into a voltage that triggers the frictional brake. Tesla's design initiates the regenerative braking as soon as the foot is removed from the throttle paddle causing the brakes to apply and recover braking energy back into the batteries this further improving the energy recovery efficiency.

Conclusion

The power and energy management for EVs requires careful examination of range of factors that covers not only electrical aspects but also vehicle aerodynamic design, passenger comforts and operational requirements. A good energy management strategy provides a good trade-off between improving energy efficiency and meeting local transportation safety regulations and improving passenger comfort levels.

About the author

Muhammad Usman Khalid is an electrical engineer with 10 years of experience in energy systems, energy management, power systems and EVs. He is an author of multiple IEEE conference papers on the same and is a subject matter expert on EVs and energy system projects.



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