“Actual Mileage May Vary.” —Relative Added Weight Impact on EV Range
I’m using metrics and WLTP measurement standards for this piece. If something’s unclear feel free to query in the comments below.
The efficiency of moving vehicles is largely influenced by external conditions and driving patterns. This is true for any type of propulsion method but has gotten more attention with the rise of electric vehicles and it’s range concerns.
In a time when charging stations were rare and electric car ranges short the term “range anxiety” became the sceptic’s buzzword of the new propulsion epoche.
Being in 2019 we reached a time where consumer awareness for electric vehicles is reaching new highs, electric car ranges often exceed a realistic 350km and new fast-charging stations get built every day. So can we stop ignoring factors that influence different cars efficiencies?
For this piece I want to focus on the weight game and what’s to consider related to that before buying an electric car.
Depending on the manufacturer a way to reach high efficiencies, and with that in EVs (Electric Vehicles) electric range, is keeping total vehicle weight low, which is especially important since long range car batteries, that store 50kWh+, can easily weigh 400kg or more. To give you an idea, the most famous electric car, the Tesla Model S weighs up to around 2.3 tonnes and packs a 650kg battery pack. Yes, that vehicle also goes a realistic 600km on a charge. A significant part of achieving this impressive range plays heavy use of lightweight metals such as aluminum in the cars chassis structure.
On the other end of the spectrum is the BMW i3 which only weighs 1.2 tonnes due to heavy use of carbon fiber reinforced plastics in the chassis. Because of the really light weight a lighter battery can be used to propel the car. The new i3’s manage a realistic range of almost 300km with a battery pack that contains 60% less energy and is also much lighter than the Model S one.
Even more impressive considering the BMW’s aero drag coefficient is pretty high at 0.29 compared to the Tesla’s lower 0.23. So clearly, just with weight savings BMW made a lot of efficiency gains.
According to the EPA, the American environmental protection agency, the BMW i3 is about 20% more efficient than the Tesla Model S.
This effect becomes even more obvious when comparing the Tesla Model S with the new Model 3. The Model 3 requires a 75kWh battery to go 600km on the WLTP standard while the Model S with the new more efficient drivetrain takes 100kWh to go 610km WLTP.
The Model 3 Long Range Rear Wheel Drive is about 600kg lighter than the Long Range Dual Motor Model S. It’s fair to assume that a huge chunk of the efficiency gain comes from all the lost weight since the new Model S is now using the super-efficient Model 3 motor as a front drive unit.
Now what about the 5 people in the car with a trailer going uphill? This is adding a new component to the efficiency through weight savings story.
The lighter the vehicle, the higher the penalty of adding absolute weight.
To keep the scenario a bit simpler let’s just assume you go on a weekend ski trip with 3 friends, luggage and a roof rack.
Now let’s add that to the gross vehicle weight of the before mentioned examples.
Tesla Model S: 2’300kg + 400kg = 2’700kg
BMW i3: 1’200kg + 400kg = 1’600kg
In the case of the Tesla Model S the added weight accounts to 17% while with the BMW i3 we’re adding another 33% of the total vehicle weight.
Since getting to the ski area usually requires going uphill the weight will cause an even more significant efficiency penalty. You can assume that the weight-related decrease of range in the BMW will be double that of the one in the Tesla.
When it comes to highway roadtripping the weight difference is less influential and aerodynamics matter more the faster your consistent speed will be. Here’s a good article I found on that
The point I’m trying to make is that if a vehicle gains it’s high range and efficiency numbers out of the lightweight build structure that’s great, but it should play a part in your foreseeable usage pattern.
If you plan to haul a lot of weight a lot of the time the higher efficiency penalty in lighter cars is to be considered.
This will ensure your switch to electric comes with as many redeemed promises as possible.
“Actual Mileage May Vary.” —Relative Added Weight Impact on EV Range
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