How do electric cars work?

With all the recent argy-bargy about electric vehicles we thought it was time for a back-to-basics primer about just what an EV is.

For the purpose of this exercise we’re dealing with battery electric vehicles, not fuel cell vehicle, plug-in hybrids or mild hybrids.

So, that’s a vehicle which is powered purely by electricity, which it stores in a battery pack and then feeds to at least one electric motor.

We couldn’t have completed this exercise without the help of Xavier Casley. Xavier’s a compliance engineer at Tritium, the Brisbane-based developer and exporter of fast chargers.

So, let’s get into it.

Electricity is the movement of electrons between charged particles. That’s the basics.

A chemical reaction inside a battery creates positive and negative charge and that makes electrons flow between the terminals.

If you’ve got the positive at one end and the negative at the other end you can make a path through something else that the electrons have to flow through.

They can generate heat, operate switches and spin an electric motor. That’s where EVs come in.

Next up: Electricity comes in two forms, alternating current – or AC – and direct current – or DC.

The easiest way to think about alternating current is as a flow of electrons that switches from positive to negative and back again.

AC electricity has several characteristics that make it easier to transmit over distance than DC electricity, hence its household use.

Direct current electricity only flows in one direction and doesn’t ‘alternate’ like AC. It is best used to charge batteries and as a power supply for low voltage electric systems.

“The battery is inherently DC due to the magic of chemistry,” Casley tells us.

“The great thing about DC with the battery source you’ve got is it’s effectively inert until you draw energy from it. They [battery cells] will stay safe and happy, they won’t develop heat, they just sit there until you draw energy from them.”

So, we have DC electricity in the battery pack powering an AC motor, something’s missing here.

Electricity is stored in the EV’s battery as DC electricity. It is then converted back to AC by an inverter so it can run the lectric motor that powers the car.

“It is turning positive DC battery voltage into alternative positive and negative,” confirms Casley. “That’s where the name comes from, it’s inverting over and over again. It’s turning DC in the battery into AC in the motor.”

Crucially, the inverter also acts as a motor controller, regulating the flow of electrons from the battery pack into the electric motor, otherwise it would run flat-out all the time. So the inverter is the mechanism that adjusts power and torque from the motor.

Think of the battery pack as the EV equivalent of a petrol engine’s fuel tank. It’s here where the electricity that will power the motor is stowed until required.

The battery is the most expensive and heaviest component in an EV. Its combination of rare-earth metals and high-tech are much of the reason EV pricing is so expensive.

The battery pack is literally a whole bunch of battery cells clumped together. The more you have, the more electricity you store and the longer range you have.

There is one enormous proviso here and that’s chemistry. A lead-acid battery is nowhere near as efficient as a Nickel-metal hydride battery pack, which in turn won’t match the current/amps or voltage of a lithium-ion battery pack.

Then different types of li-ion chemistry or construction can change that performance level again.

Current, also known as amps, is the flow rate of the battery; how much energy it can push out and how quickly it can push it out.

It doesn’t only apply to EV battery packs but any energy source, including the wall sockets in our homes.

More amps means better response. But it also means the potential to drain the pack sooner, as well as larger cables, more space, more weight and more cost.

Voltage is the electric potential of the battery pack. The more volts you have the more readily you can apply the current to the AC motor.

If you want to use an internal combustion engine analogy, more volts is like having more cubic centimetres.

Voltage also sets the top speed of the electric motor – the more volts the faster you can go.

Most EVs max out at 400 volts, but the forthcoming Porsche Taycan will have an 800-volt system.

Kilowatt hours – shortened to kWh – is the accepted way to describe the size of a battery pack.

It is the measure of energy based on the chemical storage of the battery pack. So, if you can pack more energy into each individual battery cell than a rival manufacturer, the better off your customers are.

“All things being equal, if you have a car with 25kWh and you can go 150km then the same car with a 50kWh battery pack would go twice as far – 300km,” Casley explained

“KWh is how much range you have to drive on.”

Orthodox petrol and diesel-fueled vehicles record their consumption in terms of litres per 100km. For an EV it’s kilowatt-hours per 100km.

And just like a petrol Toyota Corolla versus a petrol Toyota LandCruiser, it takes less energy to move a smaller, lighter EV than a big heavy one.

“The biggest weight in the world of electric cars is the battery,” explains Casley.

“So, it’s this constant battle. Do we want to make the battery bigger, but if we do is that going to negatively or positively impact the range of the car?”

An electric motor is an electrical machine that converts electrical energy into mechanical energy. In the case of an EV, it drives the wheels, just like a petrol engine does in an orthodox car.

Like a petrol engine its outputs are measured in torque – force in a rotational movement – and kilowatts or electrical power, which is current times voltage (internal combustion engine kilowatts are a measure of mechanical power, or how much torque you generate at a certain engine speed)

“In any electric motor you typically have two parts to it,” explains Casley. “You have the stator, which is generating the electrical field and you have the rotor, which is being induced or moved by the electric field.

There are several different types of electric motors in use by major EV players that are variations on this fundamental theme. They are:

Synchronous AC motor: Also known as a permanent magnet motor, this sits the magnets on the outside of the rotor.

Asynchronous motor: Also known as an induction motor, this has no magnets. Instead the stator creates a magnetic field in the rotor which then rotates. Induction motors can briefly produce double their power, typically by over-rating the motor. That’s how the Tesla Model S produces its ‘Ludicrous’ mode.

Interior permanent magnet motor: As the name suggests, the magnets are inside the rotor. In theory this delivers the best of both worlds; the superior efficiency of a magnet motor with an induction motor’s ability to deliver higher rev capability and bursts of neck-snapping power.

“Because electric motors are so efficient, they don’t generate lots of heat so they can operate at big power levels for long periods of time,” explains Casley.

An electric motor can also produce maximum torque from zero revs – which improves acceleration ability – as well as a very broad, stable spread of power.

This is a key reason EVs don’t need gearboxes as ICEs do to maximise the torque and power curves. Many electric motors make do with a single reduction gear that matches outputs with wheel size.

Electric motors have another cool feature. They not only accelerate but can brake too – and store electricity back in the battery pack while they do it.

“It’s literally the motor just trying to go forwards in the other direction,” explains Casley.

“That’s the easiest way to think about it.

“The crazy thing is an electric motor inside an electric car can do the same torque and the same speed in both directions. There is no limitation. You could do 250km/h in reverse in a Tesla with the same acceleration.

“It’s called four quadrant control. You have forwards driving and reverse driving, but you also have forward torque and reverse torque.

“They are mutually independent. You can be driving forward and apply forward torque which will accelerate you, or you can be driving forward and apply reverse torque which will decelerate you until you come to zero.

So, lift your foot off the accelerator of a modern EV and it will slow as if you’ve hit the brake pedal. At the same time energy is being sucked back into the battery pack to be used the next time you accelerate.

In theory, you could drive around rarely plugging in and recharging. But that’s unlikely, which leads us to…

Something like refuelling a petrol car with a hose from a bowser, you plug an electric cable into your EV to load up on fresh juice.

But while the fuel hose runs at a regulated speed and you know pretty much down to the second how long it will take to fill up your car, it’s a lot more complicated with an EV.

The basic way to recharge is at home via the 240v wall socket. Plug the new Hyundai Kona electric in and it will take 21 hours to recharge fully from empty.

Option the $1950 7.2kW onboard charger and that figure drops to about nine hours.

How long a battery takes to recharge from empty to full depends on the capacity of the charger and the size of the battery.

That 7.2kW charger in the Kona has to recharge a 64kWh battery. You divide the 7.2 into the 64 to figure out how long it will take.

That’s fine if the car is sitting there overnight, but what if you’re in a rush?

DC fast-charging is a bit like the high-flow diesel hose at the servo, only the difference in speed is even more pronounced.

Going DC supply to DC battery pack saves a lot of mucking about and recharging inefficiencies, bypassing the onboard charger of the car. In Australia, 50kW DC chargers are becoming more common, while 100kW chargers are now starting to appear.

Do the maths and a Kona electric can be recharged in a little over an hour by the former and a little under an hour by the latter.

Tritium has 350kW chargers in service overseas and is currently developing even higher power devices, as are other suppliers. When these arrive, recharging rates will match filling the tank of orthodox vehicles and fleets of electric buses and trucks will become more feasible.

The ability of an EV to take charge onboard faster is also improving. The Taycan pioneers an 800-volt fast charging system that could take only 20 minutes to recharge sufficiently for a 400km range. But the Porsche-developed system is not yet widespread.

The new Tesla Model 3 has 650 amp incoming charge capability. But 650 amp fast chargers don’t yet exist in Australia. Max amperage is around 500 amps... for now anyway.

Active thermal management of the battery pack is the key here. Keep it within its operating temperature range via liquid or even air cooling alone and the degradation of the batteries should be minimal, even over an extended period.

“Batteries like to be the same temperature as people,” says Casley.

The problem is when battery cells get too hot and stay that way, durability drops markedly. The original Nissan Leaf had this issue and that’s where a lot of this concern arose.

“A great thing about DC fast charging is you are putting a lot of energy into the battery,” Casley explains. “The downside of it is you have excess heat being generated. If you don’t manage it you are going to keep the battery hot.

“The original Nissan Leaf doesn’t have active thermal management; it doesn’t cool its cells down. If its cells get hot, they stay hot.

“The great thing about cars post the original generation Nissan Leaf … is they actively thermally manage their batteries, they try to keep them in a safe temperature range. They have got liquid flowing through the battery pack all the time cooling it down.”

That deals with most of the problem. Beyond that there’s the indisputable simple fact that batteries degrade in performance as they cycle through thousands of recharges.

“That’s equivalent to hundreds of thousands of kilometres in a petrol-engined car,” Casley points out.

Yes, it does, but less so now that active thermal management is a thing for EV battery packs.

Yet it’s still easier to manage battery pack degradation in hot weather than cold weather, Casley reveals.

“Extreme cold isn’t good for batteries because there is a chemical reaction. Most cars will limit their performance if they have been cold-soaked. But the great thing with active thermal management is you can have a heater element inside the battery pack that actually warms it up.

“Even slight use will warm up the battery pack until it reaches a safe operating temperature. The downside there is you need to put energy into actually warming it up [which diminishes the vehicle’s range].”