At The Torque Team, we sometimes get questions about electric vehicle range. Most people are puzzled as to why the remaining range varies, even though the battery is fully charged. The answer lays in the complexity of calculating EV range. At face value, this seems like a simple matter, but in practice it’s a highly complex problem.
Briefly, electric cars calculate the remaining range using a combination of factors including the current state of the battery charge, the car’s energy consumption rate, driving habits, weather conditions, and terrain. Here’s a look at each of these factors.
Battery State of Charge (SOC)
The most fundamental factor is the current state of charge of the battery. While measuring SOC sounds simple, in practice it is a highly complex and evolving branch of electrical engineering. This function is performed by the battery management unit (BMS), which monitors several inputs including battery voltage, current draw, charge rates and cell temperature. These inputs are then processed through a sophisticated mathematical model to provide an estimate of SOC. Note that this is an estimate. Error factors of up to 15% are not uncommon.
Researchers and technology companies are racing to improve SOC calculation methods, as these are one factor to improving EV range. Consider an EV with a maximum theoretical range of 500km. If the car maker’s testing shows a 15% SOC error factor, then the maximum range needs to be reduced to 425km to avoid the risk of over-discharging and shortening the life of the battery pack. Now let’s say the car maker can confidently reduce the SOC error factor to 5%. Maximum range can now be confidently set at 475km, an increase of 50km. Same car, same battery pack but now with an extra 50km range.
Energy Consumption Rate
EVs estimate energy consumption based on driving conditions such as speed, acceleration, and terrain. This estimation is often based on historical data and real-time feedback from various sensors in the vehicle.
So let’s say your last few drives used a lot of energy, say from lots of hills and hard acceleration. This historical data will form part of the calculation for your current drive and the remaining range estimate will be reduced accordingly. Conversely, if the last few drives were easy on energy consumption, then you should expect to see a bit extra remaining range.
Weather Conditions
Temperature, wind, and precipitation can all affect energy consumption. Cold weather, for instance, reduces battery efficiency, so the range estimate will probably decrease as the temperature drops. For example, Volvo advise range can decrease by as much as 40% when driving in temperatures of around –10°C
Terrain
Uphill driving requires more energy than driving on flat terrain, so the car considers the route’s topography to estimate remaining range. Some EV makers use data from the car’s GPS system to improve range estimation. So if you have your destination set in the GPS it may give the car a better idea of how much energy is needed to get there, thus improving the range estimate.
Accessory Usage
Energy used by accessories such as heating, air conditioning and entertainment systems is factored into the calculation. So for example, remaining range will be much shorter in hot conditions as the additional energy needed to run the car’s climate control system is factored in.
This is also true for cold conditions. Unlike conventional cars that use waste heat from the engine to warm the cabin, EVs must generate the heat by reverse cycling the air conditioning system, leading to additional energy usage.
Regenerative Braking
Almost all electric vehicles use regenerative braking to recapture energy during deceleration. So when the car is slowing down the electric motor is switched into generator mode. The electrical energy is fed back into the battery pack, providing a small top up during each braking event.
The car may adjust the range estimate based on how much energy it expects to regenerate during the remainder of the trip.
By continuously analysing these factors, the car’s computer can provide a reasonably accurate estimate of the remaining range, allowing drivers to plan their trips accordingly and reduce the risk of running out of power. Keep in mind that these estimates are just that, estimates. Actual range will vary depending on driving conditions and many other factors.
