The major drawback of an electric vehicle (EV) is range, so it’s a good idea to take some time to get a bit more information about the energy consumption of your EV and how to make the most out of your charge.
In general and on level ground there are three major sources of resistance that consume your valuable energy:
- the mass of the car (the inertia)
- the rolling resistance of your tyres
- your internal rotating parts and air drag
While the losses due to the mass of your car can partly be regained during coasting or braking with the recuperation feature of your EV, the energy you have to put in against rolling resistance and air drag is lost forever. These two are the so called irreversible resistances.
The rolling resistance of your tyres and your internal rotating parts (motor, bearings) is more or less independent of your driving style (speed) but – concerning the tyres – a mixture of wheel size, brand and the air pressure you put in. ZOE comes with an optimised standard tyre (see the BMW i3 tyre size for an extremely optimised tyre) so it should be good practice to keep your air pressure high (2.5 – 3 bar) and check it regularly to reduce this resistance.
The air drag is defined by the shape of your car, how the underbody is made and (again) what tyres you use. Renault claims an air drag of 0.75 square meters which is not too hot compared to a similar sized Mercedes B-class with 0.63 square meters. Even more important, air drag rises quadratically with your speed – driving twice as fast creates four times more air resistance – so air drag becomes the highest among the driving resistances around 60 to 80 km/h (40-50 mph).
MyRenaultZoe forum user Nosig posted some data from his dashboard while driving at constant speeds that I used to create a chart (see Figure 1). On the horizontal axis you see the speed in km/h and on the left vertical axis is the Power in kW.
The yellow line shows the power that ZOE consumes at the given speed based on Nosig’s data. The blue line shows the power ZOE needs to compensate air drag.
The orange line belongs to the right horizontal axis and shows the portion of air drag power compared to the total power. Since air drag rises quadratically and the other resistances remain more or less stable you would expect the orange graph to rise and rise with speed. But as you can see clearly it does not.
Up to 100 km/h (62 mph) the graph shows a constant rise but with higher speeds the portion of air drag power remains stable. So there must be an additional loss of energy coming up at around 100 km/h (62 mph).
EV drivers love their electrical motors for having full power and torque from standstill but even these efficient motors (more than 90% efficiency compared to 30-40% for an internal combustion engine) have their limits. From a particular point you have to make some compromises to increase the revs furthers. Due to a limited maximum voltage in the motor you have to reduce the magnetic field to reach a higher revolution (field suppression). This decreases the efficiency of the motor. We can show this in a diagram (see Figure 2).
This diagram describes a slightly different motor to that installed in the ZOE, but we can get important information from it. You see that at 2/3rds of the max RPM the energy efficiency decreases. Let’s take a look at our ZOE to see if we can find similar figures.
ZOE has a max speed of 135 km/h (84 mph). If we assume that this speed is at the max RPM of the motor and with the knowledge that with one revolution of a ZOE tyre the car goes about 1.89 meters (6.2 feet), the wheel has to rotate 1190 times a minute.
From the Renault website we learn that ZOE has a fixed transmission ratio of 9.32 between the motor and the wheel so the motor RPM is 11090 at full speed. On the Renault spec sheet the motor is claimed to have a max power output from 3000 RPM to 11300 RPM (which would be 138 km/h or 86 mph). 2/3rds of this max RPM would be around 7500 which equates to around 92 km/h (57 mph) which fits perfectly to our observations above.
As we all know, ZOE is limited to 96 km/h (60 mph) in Eco mode which is another hint that we are on the right track.
So, to maximise your range you should avoid speeds of more than 60 mph because in addition to the regular resistances that any car faces you’ll encounter an internal inefficiency of your drive train.