Electric cars: it’s not just the battery

Soaring fuel prices are pushing more Australians than ever to consider electric vehicles. Modern electric cars can travel hundreds of miles, with short charging times that fit perfectly with a coffee break on long journeys.

This increased attention will put the spotlight on batteries, as manufacturers fight for better, lighter and range-extending power sources.

But there is more to an electric car than the battery. This shift from internal combustion engines means an overhaul of the fundamental mechanical systems we’ve refined for more than a century. It’s not just a battery issue, it’s also a power issue.

Power on, power off

Let’s talk about power, which, to put it as simply as possible, is what makes things happen. Unlike a stationary device or machine, however, vehicles must carry the power source necessary for their movement.

Essentially, every time you speed up your car, you must also speed up the power source used to make it move.

This is also known as the rocket problem because space rockets need an enormous amount of power to escape gravity. This requires a huge amount of fuel, which makes the rocket heavy – increasing the amount of power needed to lift it. Etc.


Read more: Electric vehicles are on the way, but it’s more than just plug and play


The solution to the rocket problem is fuel density – you want the best value (power) for your money (weight), which is expressed in megajoules per kilogram. Gasoline and other fossil fuels are extremely energy dense – octane boasts an incredibly high 48 MJ/kg, compared to an optimistic 1 MJ/kg for lithium batteries.

But fuel density is only part of the picture. All that power goes into the motor, which turns the axle, which turns the wheels. Engineers refer to these three parts – power source, motor (or electric motor and inverter), and gearbox – as the powertrain.

With a combustion engine, energy is stored, converted into motion, and then dissipated as heat, either steadily through drag and friction or abruptly during braking. Additionally, combustion engines are very inefficient, typically using only 20-30% of the energy stored in fuel, with the rest being dissipated as heat. Compared to a combustion engine, an electric powertrain can be over 95% efficient.

Electric motors, on the other hand, offer different possibilities – such as capturing the energy released by regenerative braking and reusing it – which alter the traditional equations of power input and output for transportation.

So when we reduce the weight or improve the efficiency of part of an electric car, we increase its range with a cumulative effect.

Powertrains (not a transformer)

Converting a high energy density fossil fuel into motion requires an incredibly complex system, precisely controlling thousands of explosions over millions of cycles using a highly flammable fuel supply.

For over a hundred years, developments have reduced these engineering challenges to produce safe, inexpensive and efficient vehicles.

A smaller, lighter and more efficient powertrain allows for much lighter energy storage, as less energy is needed to reach a given speed. This means that physics favors designs that store energy in light, dense forms, and reduce the mass carried as it is consumed.

Although the power reserve is clearly an essential component of any vehicle – and next-generation innovations in cell construction and lithium-sulfur chemistry will further improve batteries – the power conversion system still plays a role. crucial. In fact, it becomes relatively more important as energy storage improves.

Feedback effects are important in electric vehicles, so even in designs with heavy batteries, it is still important to increase efficiency and decrease mass. One percent efficiency equals 1% battery, which represents a significant amount of mass and cost to the vehicle.

Electric powertrains use semiconductors and magnetic motors to convert electrical energy into motion, making energy conversion and distribution a major challenge.

Lighter, smaller and more efficient motors and controllers within the powertrain reduce cooling requirements, internal losses and overall vehicle mass, increasing vehicle range.

And unlike a combustion system, the energy expended is often reused many times over, as electric motors can recover energy through regenerative braking, rather than wasting it as heat in the brake discs.

This means that the same powertrain efficiency is effectively applied multiple times as the vehicle accelerates and decelerates. This makes powertrain efficiency even more critical.

Electric motors can also be powered by a source other than a battery, such as hydrogen fuel cells that consume hydrogen produced in many ways, from natural gas to algae biomass.

What happens afterwards?

Many people are working on these issues, including my colleagues and I at the Monash Energy Institute. My research aims to use new manufacturing methods and technologies to create the next generation of inexpensive, lightweight, and efficient motor controllers and inverters.

Electric vehicles are inherently complex systems, but their performance can be dramatically improved through intelligent system-level design.

For example, a motor and controller developed for a hydrogen fuel cell vehicle in a truck would be vastly different from a motor developed for a battery-powered bus, despite very similar system mass and power requirements.

A truck usually travels at high speed for long periods of time, while buses tend to be more stop-start. This drastically changes where the motor needs to be optimized to minimize energy loss.

A price problem

The power source also changes the price of future upgrades. Hydrogen systems have extremely high initial costs in expensive and heavy fuel cells and components – but as the cost of producing hydrogen drops it will be much cheaper to add power to an engine hydrogen than a battery-powered engine.

Right now, we stand somewhere near the start of the century-old design process that finally gave us the modern combustion engine (albeit with all the benefits of modern technology).

But the vehicles of the future won’t just have batteries where the gas tank used to be – it’s a whole new world of powerful movement.

This article was co-authored with David Klink, Postgraduate Researcher, Department of Electrical and Computer Systems Engineering. Behrooz and David are members of the Monash Energy Institute and the Power Engineering Advanced Research Lab (PEARL) at Monash University.

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