Energy Density: Electric Cars are Heavy

This builds off my last post about the total energy in the battery. Despite the Ford Focus Electric having this massive 23 kWh battery it only has the same total energy as 0.7 gallons of gasoline. This means a bunch of things really. The car is really efficient by using its “0.7 gallons of fuel” to go about three times as far as a comparable gas car. It also means that gas cars can go really damn far and electric cars are energy and range limited. Another not-so-obvious outcome is that electric cars are heavy. You probably didn’t know that because a car’s weight isn’t obvious.

I looked up some data so you could compare gas and electric cars’ weights. While the Ford Focus EV has a gasoline counterpart, many other popular electric cars don’t so you’ll have to just think about the size of the car compared to a gasoline one. Edmunds has the curb weight of the “stock” Focuses at about 2,900 lbs. Compare that to the Electric’s 3,625 lbs! It’s nearly a thousand pounds heavier! Here’s the chart if you don’t want to dig through the page to see it.

Weight
Also note the much smaller cargo capacity in the Electric due to the battery taking up part of the hatch.

 

What about other electric cars? The 2013 Nissan Leaf has a weight of about 3,300 lbs while the 2017 Leaf has a weight of 3,500 pounds. The Chevy Bolt pushes upwards of 3,600 lbs while the Volt is slightly lighter at 3,500 pounds. The Honda Civic, a popular compact gasoline car, weights about 2,900 to 3,000 lbs for comparison. Incredibly the Tesla Model S weights near 5,000 lbs; it weighs more due to its longer range and extra batteries. Just for giggles I looked up the weight of a 2018 Chevy Suburban which is one of the bigger SUVs you can find. It weights only 500-800 lbs more than the Tesla Model S!

Curb weight alone isn’t a downside on its own. You never really know how much the vehicle you’re driving weighs and if the thing drives okay who cares? Ideally you’d expect these heavier cars to do better in the snow and other slippery conditions and I’ve seemed to have noticed with our EV Focus. That car will not do an e-brake slide. It sort of tried to slide but just wouldn’t do it. It’s probably due to the extra 500 lbs of batteries over the rear wheels!

The heavier weight should also help keep you safer in an accident. By being heavier the cars have more energy and momentum so that could be a benefit. It won’t help the other vehicle though…

Energy Density?

So why the hell are electric cars so heavy? You’d expect the engine to weigh about the same as a gasoline one, so what else could it be? It probably isn’t any surprise that the other primary difference is the battery. The Focus EV has a big battery, both in physical size and weight. That “tiny” 23 kWh of power doesn’t come in a form that is lightweight or compact. This is the main downfall of batteries in general and is one of the bigger challenges facing electric cars in the near future: they have dismal energy densities.

Whereas in the last post I talked about the energy in a volume of material, you can do the same thing with the weight. Gasoline weighs about 6 pounds per gallon meaning each pound of gasoline holds 5.5 kWh of energy or 12.1 kWh/kg. As you can see adding energy and range to a gas car only involves a tiny increase in weight. How does that compare to an electric car?

This is a bit harder to calculate on your own so I just looked at a table from Wikipedia. Gasoline is listed as having a density of 12.7 kWh/kg (the units are now in kilograms by the way!) which is close to the number I have above. Lithium-ion batteries top out at around 0.25 kWh/kg…that is 2% the density of gasoline! That is terrible. One gallon of gas weighs just 6 pounds whereas a battery with the same energy content would weigh 300 pounds! Just to put that this into perspective Wikipedia has a chart showing various fuels in graphical form. Lithium ion batteries are terrible for storing energy vs. weight.

1024px-Energy_density.svg
If you can’t find it, lithium ion batteries are down near the “zero” of the chart. If only we could use liquid hydrogen as a fuel…

It comes as no surprise that electric cars are very heavy compared to their gasoline counterparts because batteries hold such a tiny amount of energy compared to their weights.

So that’s it: electric cars are much heavier than their gasoline counterparts. This is really noticeable outside of certain conditions as the cars shouldn’t slide as much in snow, ice, or rain. You can attribute this to the fact that lithium-ion batteries have dismal energy densities compared to gasoline and other fuels. This puts an engineering and design limit on the range of these cars and while you could simply add more batteries it comes at a dramatic cost in the vehicle’s weight.

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