The Problem with Electric Cars

There's been a lot of hullabaloo recently about electric cars. They were invented over a century ago, but have struggled to achieve any kind of real success. The chronic problem is packing enough electricity into a battery to move something as heavy as a car a useful distance.

Now, with the explosion in battery technology brought about by ubiquitous electronics, some people are thinking that perhaps the time for the electric car has come. With powerful computers to manage power and advanced manufacturing techniques that can make a strong but light vehicle, is it possible that electric cars will soon achieve real commercial success?

A Tesla Model S, probably the most talked about electric car ever.

There are inherent advantages to an electric car. Probably the biggest is efficiency. An electric motor can turn up to 90% of the input power into rotary motion, versus about 35% for an efficient internal combustion engine. Electric motors are also much simpler than engines and should require less maintenance. Side benefits are the impressive acceleration afforded by a motor, and silent operation. Also, electricity is far cheaper than gasoline or diesel fuel. Taken together, these advantages should make an electric car a compelling option for potential customers, even those who are not particularly concerned about environmental issues.

Sensing this, many car manufacturers are piling into electric car research. General Motors and Toyota have developed hybrids with extra large batteries that can be plugged in. Nissan has developed an all electric car and a giant factory to make them. Several start-ups, such as Fisker, Coda, and Tesla, have tried to make viable vehicles. Of these three, Tesla is the only one still above water.

Many billions of dollars have been spent already to try and build useful electric cars. The reasoning is that the first manufacturer who achieves a viable product will have a huge head start over their rivals, and reap huge rewards. One can certainly not accuse them of being timid.

Now for the bad news. There is an inherent problem with an electric car versus one powered by gasoline: energy density. Let's do some math.

There are approximately 37kWh of power in a gallon of gasoline (the value varies somewhat based on the particular blend, but never mind that). Now, the most efficient road going cars today are about 35% efficient. This means that they can turn 35% of the input power into rotary motion to move the car. So, 35% of 37kWh is about 13kWh. This means that a gas powered car extracts the equivalent of 13kWh from a gallon of gasoline to do useful work with.

Now, electric cars in real world situations are about 85% efficient, meaning 85% of the input power in turned into rotary motion. So, in order for the electric car to have the same amount of usable energy as a gas powered car, it will need a battery with approximately 15kWh of power stored. Now, the battery has to last for several years under varying load conditions, so manufacturers do not allow the battery to be depleted entirely, but leave a buffer. If we leave a 15% buffer in battery capacity, the electric car needs an 18kWh battery to equal the same power that the gas car will extract from a gallon of gasoline.

A typical gas powered car has an 18 gallon gas tank. This allows it to store about 666kWh of total energy, and about 260kWh of usable energy. This energy is sufficient to move a gas powered car over 400 miles.

The electric car, allowing for the protection buffer outlined above, will need a battery with about 300kWh of storage in order to provide the same useful energy as the gas powered car is able to access. This is much less than the total energy stored by the gas powered car. Remember, these calculations take into account the much greater efficiency of the electric motor.

An electric car does have another advantage over the gas car. It does not need an engine or transmission, which are the heaviest components of a gas powered car. Taken together, these might weigh 1,000 pounds. Weight is the greatest enemy of fuel efficiency, as a heavier object takes more energy to move. In order for the electric car to pack the same amount of useful energy but be no heavier than it's gas powered equivalent, the battery must weigh no more than 1,000 pounds.

Currently, it is laughably optimistic to believe we can build a 300kWh battery that weighs only 1,000 pounds, is robust enough to survive usage inside a vehicle, and is cheap enough for widespread use. Such a battery simply cannot be built. This does not just include current technologies, but also any currently conceivable technologies. While it is certainly possible we will one day have good enough battery chemistry, "might" is a slender reed to stake one's fortune on.

The electric car is again doomed by the same problem that has plagued it all its life: too little energy can be stored in a battery. The energy contained in chemical bonds is simply to great to ignore.



No Comments Yet.

Leave a comment