Electric Vehicles - Weighing up their mass and the consequence

I’ve noticed the weight of battery electric cars being discussed in the news and social media. The press typically reports that battery cars are on average about 300kg heavier than internal combustion engine equivalents. However, the way the consequences are reported varies wildly. For example, the Daily Mail published an article titled “old car parks could collapse under the weight of heavy electric vehicles” whilst the Guardian covered the topic with a more temperate and considered report of the impacts.

However, I want to advocate a different perspective to both articles. I suggest that the increased weight of battery cars IS significant NOT because cars putting on weight is a new development, it isn’t. Instead, the electrification of cars is now becoming the dominant factor multiplying their weight, adding to a long-standing cumulative trend of previous car developments.

Autocar reported that "between 2016 and 2023 – approximately one model cycle – the mean average kerb weight of new cars rose [by 394kg (25%) from 1553kg to 1947kg."

A PhD thesis by Henrik Molker determined, in relation to car weights, "there is an increase of 13 kg per year from the 1950s to today". Meaning that in the 40 years from 1980 to 2020 cars have put on over half a metric tonne of weight.

Several attributes have driven up the weight of cars over the decades: taller, more spacious, more powerful, more luxurious. Critically, cars now have a much higher level of passive and active safety features.

Now vehicles are migrating from combustion to electric power and this latest development is not just continuing but also accelerating the trend of cars gaining weight.

Just in case you’re wondering why battery electric cars are heavy? The key cause is the 'energy density' (the amount of energy stored per unit of weight) of petrol & diesel (at 46.4 MJ/kg & 44.8 MJ/kg) being about 60 times what is yielded by leading battery car cells (e.g. 0.72 MJ/kg). Despite electric cars converting energy to locomotion at least twice as efficiently as combustion ones they still require substantially larger, heavier fuel storage, whilst also being compromised with comparitely short range. That said, ranges greater than 500km (300 mile) are now common for battery cars.

But how much heavier are battery cars? I found the website 'Carsized' helped find some side-by-side comparisons, on the site even comparing the physical profiles of the models.

For example, the smallest and lightest electric car offered by VW, the ID.3 weighs in at 1772kg which is 468kg (36%) heavier than their similar-sised T-Roc small SUV at 1304kg. Although significantly heavier than its combustion-powered small family car stablemates, the ID.3 is far from the heaviest car on the roads. Larger SUVs have become increasingly popular, take for example the Volvo XC90 which at 2052kg is 280kg (16%) heavier. Yet Volvo have replaced the XC90 with the battery electric EX90 which is 2607kg, 555kg (27%) heavier than the XC90 or 835kg (47%) heavier than the ID.3.

The weight penalty of going electric is not always quite so severe. Looking instead at BMW SUVs the BEV iX weighs in at 2440kg which is only 200kg (9%) heavier than its ever so slightly smaller combustion stablemate, the X5.

The three main areas of R&D research that the automotive industry has invested in over the last 10 years are:

• Battery electric power cells
• Vehicle intelligence, and
• Vehicle lightweighting.

BMW has arguably invested more than other marques in developing and deploying vehicle lightweighting into their production - with a focus on their BEVs. The iX's chassis uses a carbon composite frame and aluminium panels. Being over 150kg lighter than the similar Volvo EX90, with less than a 10% weight penalty over the X5 is evidence that the additional weight of BEVs can be offset by lightweighting innovations.

Yet despite the lightweighting innovations the BMW iX at 2440kg is massive compared to some other electric vehicles. As a final weight comparison, it is nearly twice (91%) as heavy as the BMW i3 at 1280kg. Admittedly these fully electric vehicles fall in different classes with the iX being taller, with more interior space and over double the range. However, either could be chosen and used for exactly the same purpose as a city runaround vehicle. Yet their impact on the environment they're used in will be very different. It shows that even within battery electric vehicles the consumer has extensive choice on how heavy their next car will be.

Having looked at these weight comparisons let's go back to the original question. What are the consequences of cars increasing in weight?

Heavier vehicles consume more energy and require heavier energy supply infrastructure. A 10% reduction in weight improves energy economy, and consequently the operational energy footprint, by 6–8%. So doubling the weight of a car increases the energy footprint by 60-80%. In the big picture, the average size of future BEVs will directly impact the number of vehicles able to be charged within dense areas without expensive and disruptive electrical grid upgrades being required. Lighter vehicles would consume less electrical power, creating a smaller energy footprint and less stress on electrical infrastructure.

A significant portion of a car's lifecycle emissions are from manufacture according to estimates in a ZEMO report 23% for petrol vehicles, rising to 46% for battery electric vehicles. Less massive vehicles result in lower production emissions for example due to using fewer materials. Critically, lighter electric vehicles need smaller batteries for the same range. Battery production can account for about half of an electric vehicle's manufacturing emissions.

Heavier vehicles result in more wear and tear to the physical infrastructure they use. For the benefit of readers who haven't experienced the exciting world of pavement design, it is long established that carriageway wear is proportional to the 4th power of axle load. Following this highly exponential relationship, increasing the weight of a vehicle by 10% increases infrastructure wear by 46%, and a vehicle of double the weight has 16 times more impact on the infrastructure it uses.

Keeping my traffic engineer hat on, I agree with the point in the Guardian article that major roads, built for regular use by 44,000kg lorries, will be little affected by cars increasing in weight. Similarly, infrastructure such as car parks and bridges are not going to suddenly collapse due to the take up of electric vehicles. Significant safety factors are typically used in designs and the infrastructure has already withstood a significant increase in average car weight over the years.

However, we should bear in mind that heavier cars have a bigger impact on lighter-duty local infrastructure, maintained at the expense of local devolved authorities. Heavier vehicles will accelerate the breakup of footways and gully frames, crack utility covers, and knock out kerbs. Local roads will need to be repaired and resurfaced more frequently. Practices for the design, inspection, and maintenance of infrastructure such as local roads, footways, car parks and bridges will need to account for the continuing increase in vehicle weights. All of these generate costs for which society will pay, one way or another. The local devolved authorities who would pick up most of the consequential costs do not have direct control of national policies, they need a coordinated voice on how the cars of the future will impact them and their budgets.

Vehicles being physically larger has impacts too. Returning to the last car comparison, the BMW iX, is 95.4cm (24%) longer than the? BMW i3. Residential parking stresses are a problem for many city dwellers, being able to fit 1 in 4 more vehicles into local streets makes a big difference.

Last, but certainly not least, larger more massive vehicles are riskier to vulnerable road users. A 2021? academic study of road casualty statistics in America yielded that SUVs are 8 times more likely to kill a child in a collision than a passenger car, and several times more lethal to adult pedestrians and cyclists.

Similar to how rainforests are a continual biological competition to be the tallest and capture the sunlight. The urban jungle has felt similar in recent years with the initial adoption of SUVs towering over other car users, spurring many of them to migrate to SUVs to compete and feel safe. Yet it is not the protagonists of this vehicular battle to become massive and tall but the bystanders who are the losers: vulnerable road users, local traffic authorities and the environment are all negatively impacted by the ever-increasing size and weight of cars.

The conclusion I am getting at here is: yes battery electric car adoption is driving up car weights, and yes this is important not just for sustainability but also vulnerable road user safety. However, the consequences do not outweigh the significant benefits of adopting energy-efficient zero-emissions transport. Furthermore, there are plenty of low-hanging fruit opportunities to guide consumers towards lower-mass, lower-impact cars that have nothing to do with electrification.

Existing and prior policies of adjusting vehicle levies based on their emissions, alongside fuel taxation have significantly directed consumer behaviour towards more sustainable paths. However, with zero-emissions electric vehicles having now established a significant foothold perhaps the time is approaching when policymakers need to factor in the weight, size, energy footprint and perhaps level of use of vehicles into future systems of regulation, and levies to influence better outcomes for society.

This is not a completely new idea, for example, Felix Leach and Nick Molden researched and proposed an alternative taxation framework, that account for cars' weight, as reported in this car magazine article.?