Power Systems and Battery Chemistries Used in Electric Vehicles

This article provides an introduction to the characteristics and requirements of EV battery systems.

Electric motors are not exotic devices, and the infrastructure for generating and distributing electricity is already in place. Why, then, is it so difficult to convert automobiles from combustion to electric power? From a technological standpoint, the primary obstacle is the battery.

In terms of performance requirements, an EV battery simply cannot be compared to the batteries that power small electronic devices. When I see smartphones in action, I get the impression that designers are winning the battery game: people use these devices constantly and seem minimally inconvenienced by the need for recharging. When it comes to electric vehicles, on the other hand, the battery is a limiting factor.

An EV battery must be capable of delivering very large amounts of power and, to provide adequate range, it must store correspondingly large amounts of energy without being too bulky or too heavy. It must also tolerate extreme environmental conditions and numerous charging cycles. This is a tall order.

 

EV Battery Power Requirements

The amount of power required to accelerate an average-sized vehicle is extremely high compared to, say, the power consumption of a high-performance processor or an LCD monitor. This document (PDF) presents an estimated average power of 61 kW during acceleration of a typical vehicle. And this assumes a flat road with no headwind.

The need for very high peak power is a difficult design requirement: No one wants a car that can’t accelerate properly when driving uphill on a windy day and, consequently, the battery must have internal resistance that is low enough to support the high power delivery that occurs only during occasional “worst-case” conditions.

 

This equivalent circuit for batteries was developed by the National Renewable Energy Laboratory; see this document (PDF), Section 2.6.1, for more information. The series resistance limits the maximum current, and hence the maximum power, that the battery can supply.

 

What Is Energy Density?

The key concept here is energy density, which refers to the amount of energy that a given substance or device can store relative to its physical size or weight. (In a precise scientific context, energy density conveys energy per unit volume and specific energy conveys energy per unit mass.)

Batteries are, in general, far more convenient than fossil fuels such as gasoline and coal; however, the energy density and specific energy are much lower. For example, the specific energy of gasoline or diesel might be 100 times higher than that of a lithium-ion battery and 300 times higher than that of a lead-acid battery.

The battery vs. gasoline comparison is not as bad as it looks, because an electric motor can convert stored energy into vehicle motion far more efficiently than an internal combustion engine (ICE) does. Even after you correct for the difference in efficiency, though, it is still difficult to achieve range that is comparable to what you can get from an ICE vehicle with a full tank of gas.

I’ve seen some impressive range numbers for currently available EVs, but it’s important to remember the downsides:

• These high-capacity batteries are not cheap. They cost thousands of dollars to manufacture and can account for a large portion of the EV’s purchase price.
• Repeatedly charging and discharging the battery causes degradation that is influenced by various factors and thus difficult to predict. I don’t know how much solid data we have on long-term EV battery performance, but I think we’ve all heard of properly maintained ICE vehicles that reach the one-hundred-thousand-mile mark in good operational condition.
• Charging up a high-capacity battery is extremely slow compared to filling up a gas tank. Even Tesla’s “superchargers” make you wait half an hour.

Battery Chemistries for Electric Vehicles

Researchers have explored quite a few battery technologies as power sources for electric vehicles. I found a rather old document from Argonne National Laboratory published in 1994 that included the following chemistries as “candidate electric vehicle battery systems”: lead-acid, nickel/cadmium (NiCd), nickel/iron, nickel/metal hydride (NiMH), sodium/sulfur, zinc/bromine, zinc/air, sodium/nickel chloride, lithium/iron sulfide, and lithium-polymer.

The order of this list is based on the specific energy of each system, with lead-acid having the lowest value (25–40 watt-hours per kilogram) and lithium-polymer having the highest (100–200 watt-hours per kilogram).

Currently, the favored battery chemistry for EVs is lithium-ion. A paper published in 2012 indicates that NiMH was previously the dominant technology for hybrid electric vehicles, though the authors predicted that the use of lithium-ion systems would expand rapidly.

 

This image is from an animation, provided by the U. S. Department of Energy, that depicts the functionality of a lithium-ion battery. If you want to watch the animation, click here.

 

In 2013, NiMH was considered advantageous because it was a “mature” chemistry, but lithium-ion technology eventually made NiMH less appealing—lithium-ion offers higher specific energy, higher power output relative to weight, and fewer problems associated with the memory effect (see Section 1 of this paper from the Technical University of Cluj-Napoca for more information).

The Impact of EVs

You’ve probably noticed that electric vehicles haven’t exactly displaced the old-fashioned gasoline- and diesel-based approach to personal transportation. The percentage of EVs on an average highway seems to be miniscule—though this is in part because I live in the United States, where there are perhaps cultural (and geographical) barriers that come into play. Long-distance travel is an important part of this country’s history and identity, and when it comes to providing a satisfying long-range driving experience, modern batteries still cannot compete with petroleum-based fuels.

If you think that you’d be happier in a place where electric vehicles are more accepted, consider China, where investment in EVs is commonplace. Or, even better, check out Norway:

 

Norway’s EV market share is about 32 times larger than the USA’s—despite the fact that there are 328.8 million people in the United States and only 5.3 million in Norway. The chart is taken from this article published by the International Energy Agency.

 

A large-scale transition to electric vehicles would bring major changes to industrialized societies. It seems to me that most of these changes would be beneficial, though human societies are notoriously complicated, so maybe we shouldn’t stress too much about the painfully slow adoption of electric vehicles in every country except Norway. In any event, battery systems are a major obstacle to the practicality and affordability of electric vehicles, and this means that developments in battery technology will be decisive factors in the ongoing evolution of the electric-vehicle industry.