|Fig. 1: We are starting to see vehicle charging stations everywhere as electric vehicles are becoming more popular. (Source: Wikimedia Commons)|
Electric cars are becoming more and more popular due to increased concerns about carbon emissions from traditional cars and new technologies that allow them to be made easier. All around Stanford's campus and the surrounding area, Teslas can be seen quietly driving around with their sleek and unique design, which is why we are starting to see more and more charging stations pop up (see Fig. 1). As a matter of fact, over 3% of registered vehicles in California in 2015 were Teslas.  The idea behind these cars is great: the owner feels like he or she is helping save the planet, and they are driving a much more efficient car. But how much more efficient are these cars?
There are three-different types of electric vehicles, each with different electric motor powers, percent of energy saving, and costs. Micro-hybrids (Fiat 500, SMART car, and BMW 1 and 3 series) have an electric motor power of 2.5 kW at 12 V. They have about a 5-10% energy saving for city driving, and they only cost a few percent more than traditional cars. Mild-hybrids have an electric motor power of 10-20 kW at 100-200 V. They have about a 20-30% energy saving for city driving and cost about 20-30% more than traditional cars. Lastly, full hybrids have an electric motor power of 50 kW at 200-300 V. They have a 30-50% energy saving and cost about 30-40% more than traditional cars. 
One of the more interesting parts about electric cars is the regenerative braking technology that they use. It is a method of braking in which energy is extracted from the parts braked, to be stored and reused. Instead of letting kinetic energy convert to heat, the motor converts the kinetic energy to electrical energy used to restore the batteries or capacitors. During this process, the brake controller system monitors the speed of the wheels and determines the torque required to brake as well as the excessive energy that can be converted into electricity and put it back into the battery. 
Different types of this technology are emerging in order to make these electric vehicles more efficient, but today regenerative braking offers an energy efficiency increase of about 18%.  However, one of the new technologies of regenerative braking could increase it even further by about 10% which would lead to an increase the range by about 10%. 
It is predicted that in the future, electric vehicles will become more and more popular. This could happen one of two ways. Companies could focus on making cars as efficient as possible, even though they might be super expensive, or they could make simpler, not quite as efficient cars, but make the cost lower.  As technologies regarding electric vehicles develop, it is expected that costs will go down, and at some point electric vehicles will be affordable for everyone, regardless of how efficient they are.
© Jack Walsh. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 P. Wang, "Impact of Electrical Vehicle Adoption on the Grid," Physics 240, Stanford University, Fall 2016.
 C. C. Chan, "The State of the Art of Electric, Hybrid, and Fuel Cell Vehicles," IEEE 4168013, Proc. IEEE 95, 704 (2007).
 J. Leis-Pretto, "Regenerative Braking," Physics 240, Stanford University, Fall 2017.
 T. Yabe et al., "Efficiency Improvement of Regenerative Energy for an EV," World Electric Vehicle J. 5, 494 (2012).
 C. Lv et al., "Mechanism Analysis and Evaluation Methodology of Regenerative Braking Contribution to Energy Efficiency Improvement of Electrified Vehicles," Energy Convers. Manage. 92, 469 (2015).