Understanding the Energy Capacity of Electric Vehicle 'Fuel Tanks'

Introduction

Individuals interested in electric vehicles often pose two pertinent questions: Why do most electric vehicles have electronically limited top speeds? And, why is the range lower compared to similar internal combustion engine cars?

To address these inquiries, we must examine the energy content of various fuel options and how they compare to the high-voltage battery packs that typically power electric vehicles.

Petrol and Diesel Energy Content

Let’s start by looking at the energy content of the most widely used fuels today. One liter of petrol contains approximately 9 kWh of energy, while one liter of diesel provides nearly 10 kWh. To illustrate, a typical compact car with a 40-liter fuel tank would carry around 360 kWh of energy if it uses petrol, and about 400 kWh if it uses diesel.

High Voltage Battery Energy Content

A modern high-voltage battery in a compact vehicle typically holds between 50-80 kWh of net energy. The term 'net' is crucial here, as not all the energy content in the battery is available for use by the drivetrain. There are essential top and bottom buffers within the battery pack. The top buffer is generally smaller than the bottom buffer because deep discharging can be detrimental to battery life, prompting manufacturers to be more conservative with the upper limit. A larger buffer can extend battery longevity, but it also reduces the net energy available for vehicle operation, necessitating the need for additional modules, which further increases weight and space requirements. As a result, manufacturers recommend maintaining the battery's charge between 20-80% for daily use to enhance both buffers by adjusting charging habits.

Comparing Different Propulsion Methods

The table below compares three propulsion methods within the same vehicle — petrol, diesel, and electric — along with their equivalent energy contents. For our comparison, we used the Peugeot 208, but similar data applies to various models available in multiple fuel configurations. All energy values have been converted to kWh for easier comparison.

Conclusion

Considering current battery technology, a typical fuel tank in a combustion engine vehicle carries almost ten times more energy than a typical battery. Nonetheless, electric vehicles benefit from significantly higher efficiency. As shown in the table, electric vehicles require nearly three times less energy than their internal combustion engine counterparts for the same distance traveled, thanks to electric motors achieving efficiencies of up to 94%, compared to 38-40% for modern petrol and diesel engines. So, how much energy is needed to match the range of a petrol vehicle? Based on the table above, around 128 kWh is necessary to match the petrol variant, and approximately 158 kWh to equal the range of the diesel version. In reality, the required amount will be even higher due to the weight penalty of larger batteries, which increases electricity consumption. Consequently, achieving the same range as an ICE vehicle is virtually impossible with current battery technologies. However, emerging evidence suggests that solid-state batteries may significantly narrow this gap while also addressing related charging issues.

For further context, the table below delineates the charging power comparison for the different fuel types.

The latest electric vehicles can charge at up to 270 kW. In contrast, a 50-liter tank of petrol can 'charge' at 13,500 kW and a diesel tank at 15,000 kW, with refueling times taking about 2 minutes for internal combustion engine vehicles. The disparity in 'charging' speeds enormously favors traditional fuel vehicles.

This difference can be somewhat alleviated by charging electric vehicles overnight when not in use—something not possible with ICE vehicles. Moreover, the relatively small size of the electric vehicle 'tank' means that the actual disparity in charging times is less significant than the differences in charging rates might suggest. Nonetheless, a faster charging infrastructure is crucial, and fortunately, advancements in this area are on the horizon.