BEVs generally have a small driving range: the Nissan Leaf’s range is approximately 150 km, depending on how you drive it. However, the top-of-the-range Tesla Model S has a claimed driving range of up to 500 km.
Among FCEVs the small Toyota Mirai and the medium-sized Hyundai ix35s both have a claimed range around 500 km on a single 5 kg tank of hydrogen.
What’s going on here? Why can’t all BEVs have a 500 km range. How does a small FCEV manage this trick? To understand what’s going on, we should first remember that power (measured in kilowatts) is the rate of production or use of energy (measured in kilowatt-hours).
The key point about the FCEV is that the energy-containing hydrogen fuel is retrieved from the fuel tank at the rate needed by the fuel cell to provide the required electric power to the traction motors. The maximum continuous power of the vehicle is set by the fuel cell and the motors. The range is set by the volume of the fuel tank, just as in an ICEV.
In other words, power production in a FCEV is decoupled from its energy storage capacity. As long as the volume of the fuel tank can be accommodated within the vehicle’s envelope, the range can be extended to match an ICEV with very little need for increased fuel-cell and motor power, because the hydrogen tank constitutes a small proportion of the vehicle’s weight.
Now let’s think about the BEV in the same way. Vehicle batteries are very heavy compared with a hydrogen tank for the same stored electrical or electric-equivalent energy. The BEV battery pack is modular, so to make a bigger battery that stores more electrical energy we just add more modules. This sounds simple and obvious, but the consequences are far-reaching for our BEV’s design.
The maximum power of the vehicle is set by the traction motors, so the battery pack should supply this power, but greater power cannot be used unless the traction motors are upgraded. That’s the point: the power rating and energy storage capacity of a modular battery pack are proportional, so adding more batteries to increase the driving range also adds more power rating, whether or not it is needed. It also adds more weight to the vehicle.
To get a big driving range requires a big, heavy battery pack, and this big, heavy battery pack can deliver awesome power. This big, heavy, awesomely powerful battery pack is also very expensive, so we end up with a big, heavy, awesomely powerful and expensive luxury vehicle as the logical outcome of wanting a 500 km driving range.
If we opt for a small battery pack with modest power output, we end up with a small, relatively cheap BEV with a restricted driving range. The driving range problem of current BEVs comes down to the fact that the power production and energy storage are naturally coupled in modular battery packs.
