Published on October 1st, 2017 |
by Andy Miles
October 1st, 2017 by Andy Miles
Originally published on EV Obsession.
Having owned an electric vehicle for just over 18 months now, I feel able to share my knowledge, which may be of help to those considering an electric vehicle for the first time, or those who have just started out on the road to electric vehicle driving.
One of the most talked about topics is range anxiety, and I can honestly say that range anxiety is very quickly replaced by range awareness, after a very short period of driving an electric vehicle. Range anxiety is where one is constantly worried about whether the car will reach its final destination, or the next charging stop, before running out of battery power. This is just due to inexperience. Range awareness, on the other hand, is where you know precisely how far your car will go on any percentage of the full charge, so that although you are very aware of the range issue, you are always confident about reaching your planned destinations.
Before setting out on any journey where the distance, there and back, is more than the available range for my car, I carefully check the route, check the availability of fast chargers on the route, and plan my journey in precise stages between the charging stations. The only time I do feel the slightest anxiety now is on arriving at a planned charging stop with only 10% of the battery left and thinking how all my best laid plans will lie in ruins if the charger is out of order or in any other way unavailable. So far, I am glad to say, that although I have had a few problems with chargers, I have always either been able to sort them out or find an alternative, and 99% of the time, I have had no trouble with chargers at all.
What Influences Driving Range?
Now that we’re talking about range, a topic that follows on naturally is how to get the best range out of your electric vehicle and what might have an adverse effect on range. One issue, which people need to be aware of but have no real control over, is temperature. The battery works by a process of chemistry to store, and yield, electricity, and chemical reactions work more efficiently at higher temperatures. This is because temperature is a measure of the excitation of molecules, and molecules that are moving about a lot — just like people at a party — will interact with each other more frequently than if they’re all sitting still.
In the UK, where I live, we do not have great extremes of temperature, so seasons really have very little effect, but where temperatures are dropping down to -20°C, batteries are badly affected. Electric motors, on the other hand, work more efficiently in the cold, and in any event, electric vehicles do have systems to regulate the temperature of both batteries and motors, but even so, expect range to be affected in very cold temperatures, and to improve when it’s warm.
Another item affecting range is rolling resistance. When a wheel is rolling, the weight of the vehicle is compressing the rubber in the part of the tyre that is in contact with the road. This causes a constant flexing of the tyre rubber, as each part of the circumference of the tyre is compressed when in contact with the road and then released while in contact only with air. This constant flexing causes heat in the tyre, so that a certain amount of the energy of the car moving forward is converted to this heat, and also lost in the mechanical resistance to movement of the tyre rubber in flexion. Your electric vehicle should be fitted with low-rolling-resistance tyres, which flex less than normal tyres and are inflated to a higher pressure. You can squeeze just a little bit better range out of your car by ensuring that the tyre pressure is always as it should be.
Another important item is acceleration and deceleration. As a vehicle begins to move, it gains what is called kinetic energy. And as its speed increases, so does the kinetic energy. In acceleration, then, the energy from the battery is being used to increase kinetic energy. The formula for kinetic energy is ½ MV², where M is mass and V is velocity, so the kinetic energy increases by the square of the velocity. So, at 60 mph you will have 4 times the kinetic energy that you have at 30 mph, not just double, which is why it takes a lot more braking to slow down from 60 mph to 30 mph than from 30 mph to 0 mph. Similarly, it takes much more than twice as much energy from your battery to accelerate to 60 mph as it would to accelerate to 30 mph.
We can think of the increasing kinetic energy, in an accelerating vehicle, rather like a jug filling up with water, and whether the jug is filled up quickly or slowly, it will take the same amount of water to fill it up. Just like that jug, more or less, it takes the same amount of energy to accelerate to a given speed, whether very fast or very slowly. If that is the case, why are we told that slower acceleration is better for range? It all comes down to inefficiencies and losses in the system, which are greater during fast acceleration than during slower acceleration. Without those losses, it would take exactly the same amount of energy to accelerate to 60 mph in 3 seconds as it would take in 3 hours. To provide that amount of energy in only 3 seconds, however, takes a much more powerful motor, or engine.
Deceleration, is also an important factor. On deceleration, electric vehicles use the spinning of the motor to generate electricity to feed back into the battery. This is called regenerative braking. It feels like applying the brakes, because there is a magnetic resistance against the motor’s spinning while it acts as a generator. In regenerative braking, kinetic energy is being converted into electrical energy. In normal braking, kinetic energy is being converted purely into heat in the discs and brake pads, and is entirely wasted. When you are following behind ICE cars, every time their brake lights come on, it means they are using all the fuel they used to get up to speed as waste heat in their braking systems. As an EV driver, every time you use your brakes, you are doing exactly the same thing, so brakes are to be avoided. I find that, apart from unexpected events or having to come to a dead stop at junctions, traffic queues, and traffic lights, all my speed control is done by regenerative braking. That way I am getting back some of the battery power I used in acceleration, but be aware that it is not all of it — regenerative braking is only about 60% efficient, meaning you only get back about 60% of the electrical energy it took to get up to speed when you slow down using regenerative braking.
The fact that regenerative braking is only 60% efficient in recapturing the energy expended in acceleration should make it clear to anyone that the idea that urban driving gives greater range because of the stopping and starting (regenerative braking) is a myth. In fact, it is the case that in urban driving, every acceleration is taking additional energy from the battery, and every deceleration using regenerative braking is only putting back 60%, at best, of what was expended in that acceleration. Urban driving will give less range than driving at the same average speed, at a constant speed, on a straight and flat road. The only thing that makes urban driving yield a better range is that less battery power is expended in overcoming air resistance, because at the low speeds in urban driving there is very little air resistance.
This leads nicely into the next important factor affecting range, which is air resistance. A vehicle in outer space will continue accelerating while under power, but the unique thing is that if the engine is disengaged, though it will no longer accelerate, it will continue on at the same speed indefinitely. This is because the energy provided by the engine has been converted into kinetic energy of the space vehicle, and as that energy is not reducing in any way, neither does the speed. In a road vehicle, the same principle applies, but the kinetic energy is dissipated by moving the air and the rolling resistance that we’ve already talked about, so speed decreases if the engine is disengaged, and the vehicle requires a constant energy input to keep it moving.
As speed increases, so does the air resistance. Some people call air resistance “wind resistance,” but wind is the air itself moving, whereas we are talking about a car moving through air that is not necessarily moving — we’re talking about the resistance of air flowing over the car. If there is a wind blowing in the direction of the car, this will increase the speed of the car relative to the air, and so is very relevant. When anything moves through the air, the air in front of that object has to flow out of the way, but the faster the object moves, the more difficult it is for the air to move out of the way, and so, pressure builds up in front of the object and that pressure exerts a restraining force. An aeroplane, for example, relies on the pressure buildup under the wings to lift that heavy aircraft off the ground. To take off, the aircraft has to accelerate down the runway until it reaches a critical speed where the air pressure is sufficient to lift it. That speed might be only 60 mph in a light aircraft, or perhaps 120 mph for an airliner. Your electric vehicle is designed not to lift, of course, but to stay firmly on the road while the air flows round it as easily as possible. This is why the Tesla Model S has a very aerodynamic shape, to reduce the airflow resistance to a minimum.
So, one definite downside for electric vehicles is that on long journeys, to conserve range, speed needs to be restricted. If you drive on the motorway (freeway/highway) at a constant 80 mph, your battery capacity will very soon be used up and you will not achieve the range you might hope for. So, one often has to choose a rather more sedate speed of 50 to 55 mph. However, because driving on the motorway can be at a constant speed without the energy losses associated with accelerating and decelerating, motorway driving does offer the best range.
There is a little trick that you can use as well. Large vehicles such as lorries (semi trucks) and buses have the cross-sectional area of a couple of barn doors, moving down the motorway at 60 to 65 mph. The amount of fuel they use in overcoming air resistance at those speeds is quite a high percentage of the total fuel they use. That is bad news for them, but good news for you … if you decide to tuck in behind one of these vehicles. If you do so, not only are you protected from any headwinds, but the vehicle creates a little vacuum behind it, which literally sucks you along the road. The closer you get to the vehicle, the greater the effect, but it is very dangerous to travel too close to the vehicle in front, and in the UK, at least, this is a traffic offence called “tailgating.” However, air resistance is still much reduced when driving behind one of these large vehicles, even at a safe distance. I find that driving behind a big lorry or bus I can increase my range by as much as 20%, and whenever I get onto the motorway, the first thing I look for is a lorry or bus to get behind. Buses on the motorway tend to do a constant 65 mph, which is much faster than I would normally expect to travel, so I get both good range and the added bonus of good speed too.
So, having debunked the myth that urban driving gives the best range, another mythical idea I have come across is that you can increase your range by coasting. By “coasting,” people mean putting the vehicle into neutral and simply rolling down hills. In electric vehicles, although there is a selector, which is similar in appearance to an automatic gearbox, there is no gearbox or clutch of any kind in an electric vehicle. The motor is permanently connected to the driving wheels and the only gear is a permanent reduction gear to provide the required level of torque at the driving wheels. In an ICE car, the engine has to be started, and so, cannot be permanently connected to the driving wheels. The neutral position actually disconnects the engine from the driveshafts, and the “reverse” position actually uses a gear to turn the driveshafts in the opposite direction, while the engine, of course, always spins in the same direction. In an electric vehicle, the “reverse” position operates an electrical switch to reverse the polarity of the motor to make it spin backwards. It is, I have found from my own experience, an alarming feature of electric vehicles that they accelerate just as quickly in reverse as in drive — though, thankfully, the top speed is generally software limited to about 30 mph in reverse.
In an electric vehicle, putting the selector into neutral does not disconnect the motor, but is, again, an electrical device, which isolates the motor from the control system so that the motor just spins freely, with neither drive nor regenerative braking. So, putting your electric vehicle into neutral and coasting does not allow the vehicle to roll more freely, as mechanical losses from the drivetrain remain unaltered. If you are going downhill faster than you would if coasting, then you are using some current from the battery. If you are going at exactly the same speed as you would if you were coasting, then even though your selector is in drive, you will not be using any current. And if you’re going slower than you would be if coasting, then regenerative braking will come into play, and you will be recharging your battery while going downhill. That being the case, there is no advantage in putting your car into neutral, and you may even be missing out on some much-needed recharging. If you actually use the brake to control your speed while coasting, then you are converting your kinetic energy into wasted heat rather than converting it into electrical energy for charging your battery, so this is actually less efficient.
The only time it would be more efficient to select “neutral” rather than “drive” is when stopped with your handbrake on, as even with your foot off the accelerator pedal, a low current is generally fed to the motor. This would be taking power from your battery with no movement, and so reducing your range. The same does not apply to using the brake pedal, as the motor management system cuts out any current to the motor when the brake is applied.
Another issue is, if you keep your vehicle from rolling back while stopped on a hill by using the accelerator pedal rather than holding it with the brake, you are taking energy from the battery, without moving, so are reducing range — and you might also damage your motor, as you are passing current through a single motor winding while the motor is not moving. In an ICE car, holding the car on a hill using the motor might burn out your clutch, but in an EV, it might burn out the motor.
One final thing that affects range is the use of climate control systems. Electric vehicles tend to adapt “off-the-peg” systems from ordinary cars for all their equipment — like indicators, wipers, and lights — but heaters, which on any other car use hot water from the cooling system, and air conditioning, which would be run from an engine drive belt, have required custom-made equipment, which drains power from your battery. For local runs, you can be as cool or warm as you like with no worries, but if out on the highway, hoping to get to that charger just within range, you might need to trade off comfort against conserving energy. The best EVs use heat pumps for heating, which work like a refrigerator in reverse, refrigerating the outside air and dumping the heat into your car interior. Lesser models use what is virtually a fan heater running off your battery and drawing about 3 kW of current. Check before you buy, as heat pumps use about the same current as the air conditioning, which is much less current than a fan heater, and so make keeping warm in winter much less of a sacrifice of range. Most of the time, I get by with using the heated seats, which only use about 12 watts and have no noticeable effect on range at all.
And now into the fun part — local journeys. On any journeys well within the range of the car, which is about 95% of journeys for most EV drivers, you can let all the good advice about conserving range go out of the window, and in quiet and comfort “let your hair down” for the undoubted thrill of driving an EV with smooth but fast acceleration, and a very low centre of gravity that allows exceptional road holding. Enjoy all the freedom of the open road without all the hard work of gear changing, and with your favourite music playing without the unnecessary noise and smell of an internal combustion engine. If you have never driven an EV before, you should try it — you will probably like it.
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