Range Extension: The DIY Guide

Joep Sloot
November 10, 2017

5-minute read

“How much range do I have?” “Will I be able to get to my destination without charging on the way?” “Will driving slower affect how long the battery lasts?” These are questions most first-time electric car users have. We tested our BMW i3’s “Top Gear style” in order to see how driving style affects energy consumption.


If you’re an Amber user, or if you own an electric car (hint: You shouldn’t – you should share one!) you might be familiar with the term range anxiety. Range anxiety is the fear of not being able to make it to your destination because you might run out of juice before you get there. “Juice” in this case can be either electric juice or old-fashioned dinosaur juice like petrol or diesel, but the term “range anxiety” is usually associated with electric cars. In fact, General Motors even filed to trademark the term in the US in 2010, with the purpose of "promoting public awareness of electric vehicle capabilities"[1]. Range anxiety was a real issue back in 2010, and no wonder: Battery technology in the early EV days wasn’t as good as it is today, and the EVs produced at the time only had a range of 80 kilometers, provided that external conditions would allow it. Luckily, the BMW i3s that we use today have a range of 200 kilometers. However, that range can vary by about 40 kilometers.


Let’s talk about that variance. When you were first learning to drive, your driving instructor probably taught you that heavy acceleration and high-speed cruising are bad for your car’s fuel consumption, but you probably never really noticed because you only fill up the tank once or twice a month. However, what we see happening with our cars is that this variance can be up to 20% of a full battery. With the i3, this means that you’re getting anything between 160 and 240 kilometers out of a full battery. Of course, the numbers that are presented in the brochure by the car manufacturer are “best case scenario” (for some reason the EU still allows this, using the much-outdated NEDC drive cycle as a standard…but that’s a discussion for another time). According to the official drive cycle, the i3 has a range of 300 kilometers.


Besides the fact that car makers present the buyer with an unrealistically low number for the car’s energy consumption, they’re also trying to make you believe that in order to reduce energy consumption you need an ultra-clean engine. It naturally follows that in order to get this new engine, you need to buy a new car. But in reality, air is your real enemy if you care about energy consumption. Those of you with a technical background might know the equation for air resistance, and then you’ll also know that it ends in v2: velocity squared. That means that if you double your speed, air resistance increases by a factor 4. What does this mean in practice? Air resistance is by far the most dominant consumer of energy at highway speeds. In fact, driving at 110 km/h instead of 140 km/h can give you up to 50% (!) more range out of the same battery (or the same amount of dinosaur fossils).


So much for the theory – let’s see how our BMW i3 performs in the real world. On a quiet Saturday morning, we went out for a drive and measured the real energy consumption during constant speed driving at various speeds. (Professional driver. Closed course. Do not attempt.) As this is a scientific experiment, we did it properly. All measurements were done with one person in the car, with the air conditioning off, with only a bit of music in the background, on the same road in opposite directions (to cancel out the influence of wind or height differences).


The results speak for themselves: As we see in the first graph, the energy consumption starts to increase really quickly above 80 km/h. This is where air resistance comes into play. If we convert the energy consumption to the practical range of the car, we obtain the results in the second graph. This shows that if you set the cruise control to 30 km/h, you can drive almost 500 kilometers before the battery runs out. It also illustrates that you can easily get 200 kilometers of range if you stick to 100 km/h on the highway, while a constant speed of 130 km/h makes the range drop to below 150 kilometers. Let’s look at a real world example: A trip from Eindhoven to Amsterdam is about 140 kilometers. Cruise at 100 km/h and you’ll use 70% battery charge, and it will take you 90 minutes to get there. Cruise at 130 km/h and you’ll drain the battery, but save 19 minutes in travel time. That’s 30% more energy used for a 20% shorter trip, plus a fine for violating the speed limit of 100 on the A2. Not a great deal.


Don’t get me wrong, we don’t want to oblige you to squeeze yourself in between the trucks in the slow lane just to save us a few kilowatt hours of energy. We just want to raise your awareness about how the interplay among car, battery and driving style determines the range of the car.


As one of the first Amber users, you’re an electric driving pioneer, and we’re here to help you go where you need to go. By understanding how electric driving works and how you control the range of the car, you’ll feel more comfortable driving electric. That’s also why we’re working together with Fastned to ease the process of en-route fast charging (more about that later!). In the meantime, we might file our own European trademark application for the term “range anxiety”, with the exact same purpose: Promoting public awareness of electric vehicle capabilities!