Fast charging batteries: the limitations of physics

By Peter German.

We don’t count anymore Success announcements Announcing that the new XZZ Plus (or other) will finally solve the battery capacity and rapid recharging problem, making the electric car just as efficient as a conventional thermal vehicle, with less than 5 minutes of recharging time and capacity mileage exceeding the 800 km range of most diesel vehicles today.

Our specialty journalists, in the modern age of electric cars that need to be charged 24 hours, their charging time seems to increase rapidly to 12 hours and then to 6 hours, decreasing rapidly over time. Themselves: 5 minutes coming soon!

No, the physicist answers. There is a barrier, invisible but present.

Battery research

All-round research on batteries may be the material that has already amassed the most extensive research resources in the world for decades. And that Without really conclusive results : The decisive technological leap never occurred (one might be surprised to see this relentlessness, but it is out of the question today).

New batteries are also said to have lasted a very long time (150 years?) During its initial development. The reason is that physics stubbornly opposes the invention of electric batteries with performance comparable to ordinary fossil fuels.

The challenge is simple, but difficult to achieve. This includes building a battery with the following characteristics:

1. It is capable of storing 60 liters of fuel or 48 kg of energy in the tank of a conventional diesel vehicle.
2. Can be refilled within 5 minutes (average full time).
3. These first two performances will not diminish the life of the vehicle.
4. The vehicle will remain intact (solidity) for the rest of its life.

Despite a great deal of research, current batteries (2020) are still far from these performances. Moreover, physics clearly limits the possibilities for innovation in this field.

The main problem, which is not clearly stated by the manufacturers, comes from attribute number 2. To understand that, you need to examine what happens in the pump pipe when you refuel.

I would like to talk about the flow of energy, i.e. the amount of energy that must pass through the charging cable in the form of a pipe, fuel, or electricity when refueling or charging. .

To meet condition number 2, it is necessary to go through the charging cable of the electric vehicle, which has an energy equal to the energy that goes through the hose, and this is where the shoe pinches.

One of the goals is unattainable

Here is why:

Conventional diesel fuel has a power output of 44 megajoules per kilogram or 12.2 kWh of combustion (Note)

The total (60 liters, or 48 kg) of a diesel tank tank therefore has a total discharge capacity:

12.2 x 48 = 585.6kWh

The capacity of the batteries fitted in current electric cars is about 50 kWh, which is about 10 times less than the value to be achieved above and the Tesla Model 3 may have been fitted with a 100 kWh battery. 5 times less than this value.

However, one must take into account Yield Activities. According to Wikipedia, the overall performance of a thermal vehicle on the highway is limited 20% Fuel for the wheels. So, the energy that can actually be fully utilized is:

585.6 x 0.2 = 117.1kWh

The performance of an electric vehicle on the highway is even better, according to Wikipedia: it will be 74% From the battery to the wheels, a performance that needs to be further amplified by the ability to recharge the battery 85%.

For a reasonable comparison with an electric vehicle, we need to divide the above 117.1 kWh by EV efficiency (motor and charging), plus power:

117.1 / (0.74 x 0.85) = 186.2kWh

The energy calculated above must be transferred to the tank by the pump In 5 minutes. The Fake– Power related to transport at this time of the same amount of energy in an imaginary fast charging battery (5 minutes or 1/12)^Th Hours) So 186.2 x 12 = 2,234 kW or approximately 2.2 MW

This value is close to the power of a Medium power transformer Providing several hundred homes (about 12 kW for a large apartment) rather than a domestic installation.

Since changing the amount of electrical energy from one generator to one battery is a question, it is very similar to the result of dividing the amount of energy at a given time, in this case, Recharging generator electric power.

This is the amount of electrical energy Important Should be replaced in a relatively short period of time Short. To fix the ideas, under 500 volts DC voltage, the connection cable between the station and the battery should support the intensity of 4400 amps, which seems unreliable.

In fact, the battery is adapted to charge at 500 volts DC and 4400 amperes and the required power (over 2 MW) will remain at this terminal, assuming that a charging station can be installed to provide these characteristics. It will be installed on certain specifications and only on certain platforms and there will be no question of installing two terminals in one place, which is equivalent to 4.4 MW of power.

According to the data in Chart p 14, the cable capable of supporting the required 4400 amps (Note) Should be a 225 x 20 mm copper strip that can withstand currents with temperatures up to 30 செல் C above ambient temperature. This type of device can cause almost insoluble problems with the actual connection (quality of contacts) and the precise positioning of the vehicle in relation to the power bar.

We recognize that these restrictions eliminate both the possibility of charging stations continuously distributed on the roads, but also eliminate both the configuration of the batteries and the connection systems capable of withstanding these obstacles.

Batteries: Possible Solutions

Replace copper with silver

Since silver is the most conductive of all metals, it can be hoped that the dimensional control of the conductor can be reduced by replacing copper with silver. Unfortunately, the differences in resistance between the two metals are small (copper: 1.72 ohm.centimeter, silver: 1.59 µΩ.cm. (Note: CRC Chemistry and Physics 46th Edition).

This replacement would change the dimensions of the conductors a few percent better, without any fundamental improvement.

Use superconductivity

Liquid nitrogen = -195 செல் C can carry a current of 4400 amperes in a superconducting material maintained at a temperature lower than its core temperature. Since the resistance of such a conductor is zero, its dimensions may make the conductor flexible.

The main drawback of the system is the duty to maintain the conductor at its operating temperature, which imposes a heavy refrigeration at very low temperatures.

Moreover, this solution does not easily extend to the internal conductors of the vehicle, which limits the advantage of superconductivity.

Simply approaching, failing to achieve important objectives

• Achieving full charge of the battery is more difficult than 75% or 50% partial charging. In fact, in these cases, the value of the energy to be transmitted is multiplied by 0.75 or 0.50, which helps to reduce the intensity at the same rate: we go to 3300 amps (75%) or 2200 amps (50%). .
• Instead of five minutes, we can settle for a 10 minute cooldown. The current then increases to 1650 A at 75% load and 1100 A at 50% load.
• Divide the battery capacity by two (accept 292.8 instead of 585.6 kWh, which is still three times the battery capacity of the Tesla 3. Then we come to 550 amps, which becomes realistic with the current methods.
• If it is agreed to increase the charging voltage to 1000 volts, this value can be divided by two.

Manufacturers will turn to this third solution, forgetting the initial intentions and accepting reduced actual autonomy (300 or 400 km?) And 10 minutes of recharging time, which becomes a reality if the number of charging stations is the same. Large, and they are found everywhere, which is possible by lowering the barriers.

Conclusion

Calculations in this small corner show that electric car batteries are far from the performance of a tank of diesel fuel.

Also, we have to resign ourselves to the fact that the batteries can not reach them not for related reasons but for power supply reasons. We must reduce our ambitions. And for everyone the electric vehicle is not for tomorrow or even the day after tomorrow.

The article was first published on August 14, 2020.