Why do electric cars consume power so quickly when running on highways?

Electric cars consume power quickly when running on highways, and the driver's intuitive impression is that the SOC drops significantly faster when running on highways than when driving on city roads.

This phenomenon is caused by the superposition of multiple factors, the main influencing factors are:

The vehicle resistance increases rapidly when driving at high speed, so the power consumed to maintain high-speed driving increases rapidly;

When driving at high speed, the vehicle resistance is large, and the corresponding battery discharge power is also large. The chemical characteristics of the battery itself cause the SOC to decrease faster;

There is another important reason. The efficiency of the motor decreases when driving at high speed, further worsening the energy consumption of the vehicle.

The main factor that causes the resistance to increase rapidly when the vehicle speed increases is wind resistance. The air resistance (wind resistance) of a vehicle is proportional to the square of the vehicle speed. In other words, the faster the car travels, the wind resistance of the car increases in the form of a quadratic curve.

The rapid increase in vehicle resistance requires the battery to produce more power to maintain the vehicle's high speed, which means that power consumption increases at high speeds.

The battery of an electric vehicle has the electrical characteristic that the greater the discharge power (discharge current), the faster the battery SOC is consumed.

We know that lithium-ion batteries rely on the reciprocating movement of lithium ions and electrons between the positive and negative electrodes of the battery to achieve charging and discharging. The larger the current, the more lithium ions and electrons move between the positive and negative electrodes per unit time, which makes it easier to cause lithium ion "traffic jams" and cause the battery power to drop rapidly.

Use professional terms to explain why the power drops quickly during high-power discharge:

Under high current discharge, the effective surface area of ​​the electrode active material decreases, and the resistance to the movement of lithium ions and electrons increases relatively. High current requires rapid movement of ions and electrons, so the greater the current, the faster the voltage drops.

During high current discharge, the electrode is controlled by liquid phase diffusion, resulting in severe concentration polarization. The active materials cannot be fully utilized, and the migration speed of electrons in the active materials lags behind the change of the battery's external voltage. The battery is not discharged completely and the voltage drops rapidly.

These two reasons cause users to intuitively feel that the battery is not durable and consumes power quickly when driving at high speeds.

According to the laboratory's experimental report on battery cells, the discharge current gradually increases from 0.2C, 0.4C to 1.2C, and the amount of electricity that can be discharged by the battery cell decreases accordingly, which is reflected in the rapid power consumption of electric vehicles.

Let's return to the efficiency of the drive motor itself.

The figure below is an efficiency diagram of a motor. It can be seen from the figure that when the motor speed is between 5000 and 7000 rpm and the torque is between 40 and 80 Nm, the working efficiency is the highest. Once the motor speed exceeds 8000 rpm, the motor efficiency will drop below 92%.

Electric vehicles have a simple structure and most of them have single-gear transmissions. We assume that the vehicle uses 17-inch tires and a rolling radius of 333mm; the speed ratio of the reducer (to be exact, the total reduction ratio of the reducer gear pair and the main reducer gear pair) is 10. At a high-speed driving speed of 120km/h, the motor speed is about 9600r/min. At this time, the efficiency of the motor is only 91 or even lower. This is also one of the important reasons why electric vehicles consume electricity at high speeds.

Some people ask why electric vehicles don’t use multi-speed transmissions to better adjust the motor’s operating point?

First of all, it is not cost-effective. A multi-speed transmission costs at least several thousand yuan. New energy vehicles are already expensive due to high battery costs. Adding the cost of a multi-speed transmission will make the product even less competitive.

Secondly, domestic electricity prices are relatively low at this stage, and the extra electricity consumption has little impact on the cost of vehicle use.

Finally, in terms of technology, there are many manufacturers of electric vehicles, but the traditional car companies are the most capable of matching multi-speed transmissions. For most new car companies, they avoid multi-speed transmissions if possible. The wide speed range and good torque characteristics of electric vehicle motors lay the foundation for electric vehicles to use only single-speed transmissions.