We often talk about fuel economy and how to get more out of your tank of fuel. While they don’t run on fuel, the same principle applies to an Electric Vehicle (EV). Contrary to vehicles with internal combustion engines (ICE), an EV’s battery range or achievable travel distance decreases at a significant rate when you’re on the open road compared to simply driving around town.
According to Tesla’s interactive online distance calculator, a Model S 75D driving at 100km/h could give a range of 432km compared to 481km at 90km/h and 532km at 80km/h. Going by this maths, it suggests for every 10km/h slower you drive, a distance of approximately 50km is gained. If this is true, then it can be assumed that if all your travel is at speeds of 50km/h, you might achieve a distance of around 680km.
This is interesting when comparing with an ICE, where vehicles generally operate most efficiently travelling at 100km/h. This is why EVs are a fantastic choice for the average motorist who travels short distances at urban speeds. This is primarily because the main component that drains the battery in an EV is the electric motor required to drive the vehicle. Under acceleration to reach and maintain motorway speeds, the draw on the battery is far greater than around the city where lower speeds are required.
Other factors that determine vehicle economy/range are individual driving styles (how you drive), where you drive (hills, city, highway), and loads that can draw power from the engine or battery; both physical and electrical.
Loss in EV range can be attributed to four key areas - some that can be changed and some not so much. Manufacturers are constantly making improvements in these key areas with a goal of getting further from a single charge.
1. Aerodynamics - If you look at the front of an EV it usually looks a little strange - dome shaped with no front grille (no radiator to cool). The underbody is all covered up to keep that air passing smoothly under to reduce drag and help the vehicle glide through the air. This is a very important factor if you carry loaded roof racks or bikes, and let’s not forget what happens to the range if you tow something.
2. Tyre and load - This is mainly determined by the weight of the vehicle and the rolling drag of the tyres themselves. The air pressure in tyres impacts rolling resistance, grip and the overall energy loss. Higher pressure gives lower rolling resistance and may reduce tyre wear, but the downside is a harder ride and degraded handling. This is also why EVs have skinnier tyres than fuel powered vehicles. EVs also tend to be heavy due their batteries. Every extra bit of weight can affect how much range a full charge can give. It’s good to make sure that you’re not carrying extra weight (sports equipment, tools etc.) if you are trying to maximise range.
3. Drivetrain - This is something that the driver can’t typically control: the efficiency of the electric motor, controllers, the gearbox, and generally all losses due to converting the electricity from the battery pack into useful torque at the wheels.
4. Ancillaries - These losses are caused by all “other” electrical loads in the vehicle, particularly from the cooling fans and pumps, the radio, internal and external lighting and so forth. These losses are somewhat different than the others because they represent a roughly constant power draw on the vehicle regardless of speed, wind or elevation changes. Because of this, they cause the energy usage per km to start becoming high again at very low speeds. The impact of ancillary losses is relatively small at high speeds because the primary propulsion power is very high and these small power draws make little difference. It’s a bit like the effect of driving a combustion engine at 100kmh with the A/C on which can be more fuel efficient than at 50kmh.