Technical Analysis: Ditching Bulky EV Batteries Is Not So Easy

From the May/June 2025 issue of Car and Driver.As good as the batteries of modern electric vehicles are, they still have drawbacks, such as their physical size and heft. Though the cost of EV batteries continues to fall, they remain pricey pieces with a limited working life span. So, why not ditch the bulky battery pack for a capacitor, a simpler electrical storage option that has a long life and is capable of releasing energy almost instantly?Capacitors are common in all manner of electrical circuits. Unlike a battery, which is essentially a chemical device consisting of two electrodes in some sort of electrolyte that allows electrons to flow from one of the electrodes to the other to release energy, a capacitor stores energy by accumulating positive and negative electrical charges on a pair of surfaces separated by an insulator. Capacitors come in a variety of shapes and sizes, though they are typically rather small.Illustration by Chris Philpot|Car and DriverCapacitors consist of a pair of surfaces separated by an insulator. When a voltage is applied, one surface takes on a positive charge while the other takes on a negative one.In recent years, some automakers have employed this sort of energy storage device—Mazda, with its i-ELOOP kinetic-energy-recovery system, and Lamborghini, in the limited-run Sián FKP 37 and Sián roadster (pictured above). However, no EV models use this tech. The reason for this is a capacitor’s lower energy density. Whereas a 100-kWh lithium-ion battery might weigh around 1200 pounds, a capacitor with a similar capacity would hit the scales at roughly 10 times that weight. One way to reduce this mass is to increase the operating voltage, since the energy storage of a capacitor goes up with the square of voltage. But high-voltage capacitors are harder to manufacture, and nobody wants several thousand volts coursing through a car’s electrical system.Moreover, the voltage of a capacitor decreases with energy usage—by about 30 percent when the capacitor is half depleted and by nearly 70 percent when it is 90 percent drained. Accommodating this discrepancy requires more complex power electronics that can transform a capacitor’s varying voltage into the constant voltage an electric motor needs to maintain consistent performance. Plus, when left alone, capacitors discharge faster than batteries. Even an optimized super capacitor loses substantial energy by the week.Illustration by Chris Philpot|Car and DriverBatteries use an electrolyte to move electrons between two electrodes (positive and negative) to release energy.On the other hand, a capacitor charges very rapidly. For instance, fully charging a depleted 100-kWh capacitor could take as little as one minute. However, doing so would use 6000 kilowatts of electricity, equivalent to the power draw of roughly 3500 homes (for that minute). Installing a second capacitor at a charging station would serve as a more practical setup. That would allow the station’s capacitor to charge gradually and then dump its energy into an EV’s capacitor by way of a very heavy-duty coupling or a wireless connection.These foibles make it difficult to imagine a capacitor-powered EV coming to fruition anytime soon. Still, as Mazda and Lamborghini prove, there is room for automakers to take advantage of the benefits of a capacitor. We wager that others will follow suit and employ this technology in specific applications where a capacitor’s strengths outweigh its weaknesses.Csaba Csere joined Car and Driver in 1980 and never really left. After serving as Technical Editor and Director, he was Editor-in-Chief from 1993 until his retirement from active duty in 2008. He continues to dabble in automotive journalism and WRL racing, as well as ministering to his 1965 Jaguar E-type, 2017 Porsche 911, 2009 Mercedes SL550, 2013 Porsche Cayenne S, and four motorcycles—when not skiing or hiking near his home in Colorado. 
Source: caranddriver.com