Fast charging batteries: the limits of physics

Published April 19, 2022



HAS
+

By Peter German.

We no longer count on the triumphant announcements proclaiming that the new XZZ Plus battery (or another) will finally solve the problem of capacity and fast charging that will make the electric car as efficient as the conventional thermal vehicle with a charging time of less than 5 minutes. and a mileage equal to or greater than the 800 km range of most current diesel vehicles.

And our specialized journalists, seeing that the electric cars that needed 24 hours to charge in the early modern era, saw how their possible charging time increased rapidly to 12 hours, then to 6 hours, and it seems which are shrinking rapidly over time, for example, confident in themselves: the 5 minutes will come soon!

Well no, the physicist answers. There is a barrier, invisible but very present.

battery research

Comprehensive research on batteries is most likely the topic that has mobilized the most research resources in the world over several decades. And this without really convincing results: the decisive technological leap never took place (this relentlessness may come as a surprise, but that is off topic today).

The new batteries are also said to be the project whose initial development period lasted the longest (150 years?). The reason is that physics is stubbornly opposed to the discovery of electric batteries with performance comparable to that of a modest fossil fuel.

The challenge is simple to state, but difficult to achieve. This involves creating a battery with the following features:

  1. Capable of storing as much energy as the tank of a conventional diesel vehicle, ie 60 liters of fuel, or 48 kg.
  2. Rechargeable in less than 5 minutes (full time average).
  3. These first two benefits do not diminish over the life of the vehicle.
  4. Keep intact (solidity) throughout the life of the vehicle.

Despite the large amount of research, the current batteries (2020) are still far from these performances. In addition, physics clearly limits the possibilities for innovation in this field.

The main problem, never clearly mentioned by the manufacturers, comes from feature number 2. To understand this, you need to examine what happens to the pipe of a pump when it is filled with fuel.

I want to talk about the flow of energy, that is, the amount of energy that has to go through, during the time of refueling or charging, either in the pipe, in the form of fuel, or in the charging cable in the form of electricity. .

To meet condition number 2, it is necessary to be able to pass through the charging cable of the electric vehicle, an amount of energy equivalent to that which passes through the pipe, and this is where the shoe is pinched.

One of the goals is impossible to achieve

Here’s why:

Conventional diesel contains an energy that can be released by combustion of 44 megajoules or 12.2 kWh per kilo (reference).

Therefore, the full tank (60 liters or 48 kilos) of a diesel vehicle contains a total release energy of:

12.2 x 48 = 585.6 kWh

It should be noted that the capacity of the batteries installed in today’s electric cars is about 50 kWh, which is about 10 times less than the value previously achieved and that the Tesla model 3 could be equipped with a battery of 100 kWh, or approximately. 5 times less than this value.

However, the performance operations. According to Wikipedia, the overall efficiency of a thermal vehicle on the highway would be only 20% of the fuel on the wheels. Therefore, the energy that can be fully utilized is only:

585.6 x 0.2 = 117.1 kWh

The performance of an electric vehicle on the highway, again according to Wikipedia, is much better: it would be 74% of the battery on wheels, a performance that must be multiplied by the recharging performance of the battery that would be of 85%. .

For a fair comparison with an electric vehicle, we have to divide the previous 117.1 kWh by the product of the efficiency of the electric vehicle (motor and load) and the energy becomes:

117.1 / (0.74 x 0.85) = 186.2 kWh

The energy calculated above must be transferred by the pump to the tank in 5 minutes. He pseudo– power corresponding to the transport in this time of the same amount of energy in a hypothetical battery of fast charge (5 minutes, or 1/12th hour) will therefore be 186.2 x 12 = 2,234 kilowatts, or approximately 2.2 MW

This value is closer to the power of a medium power transformer that feeds several hundred homes than that of a domestic installation (about 12 kW for a large home).

Note that since it is a question of transferring an amount of electrical energy from a generator to a battery, and this in a given time, the result of dividing the amount of energy by time corresponds well, in this case, to the recharging electrical power from the generator.

It is an amount of electrical energy important which has to be moved in a relatively short time short. To fix ideas, under a voltage of 500 volts DC, the connection cable between the station and the battery should withstand an intensity of 4400 amps, which seems unrealistic.

In fact, even assuming that the battery is modified to be able to receive a charge at 500 volts DC and 4400 amps and that a charging station with these characteristics can be installed, the required power (more than 2 megawatts) is such that this terminal could only be installed in certain and few locations and that it would not be a question of installing two terminals in one place, which would correspond to a power of 4.4 MW.

The cable capable of withstanding the required 4400 amps should be, according to the data in graph p. 14 (reference), a 225 x 20 mm copper bar to withstand the intensity with a temperature rise limited to 30 ° C at room temperature above. Such devices would pose almost insoluble problems in terms of the actual connection (quality of contacts) as well as the precise positioning of the vehicle in relation to the power bar.

We recognize that these limitations are such as to eliminate both the possible existence of regularly distributed charging stations on the roads, and that of a configuration of batteries and connection systems capable of withstanding these limitations.

Batteries: possible solutions

Replace copper with silver

Since silver is the most conductive of all metals, it can be expected to reduce the conductor size restriction by replacing copper with silver. Unfortunately, the resistivity differences between the two metals are small (copper: 1.72 µohm.centimeter, silver: 1.59 µΩ.cm. (Reference: CRC Handbook of Chemistry and Physics 46th edition).

This replacement can change the dimensions of the drivers by a few percent at best, without any fundamental improvement.

Use superconductivity

It is possible to transport a current of 4400 amperes in a superconducting material maintained at a temperature below its critical temperature by a circulation of liquid nitrogen = -195 ° C. Because the resistance of this conductor is zero, its dimensions can be such that the conductor is flexible.

The big drawback of the system is the obligation to keep the driver at its operating temperature, which imposes a heavy cooling station at very low temperatures.

In addition, this solution cannot be easily extended to the internal drivers of the vehicle, which restricts the advantage of superconductivity.

Just getting closer, not reaching critical goals

  • A full battery charge is much harder to achieve than a 75% or even 50% partial charge. Indeed, in these cases, the value of the energy to be transported is multiplied by 0.75 or 0.50, which reduces the intensity in the same proportions: we go to 3300 amperes (75%) or 2200 amps (50% ). .
  • We can settle for 10 minutes of cooling, instead of five. Then the current increases to 1650 A for 75% load and 1100 A for 50% load.
  • We can accept a battery capacity divided by two (292.8 instead of 585.6 kWh, which still corresponds to three times the battery capacity of the Tesla 3. Then we reach 550 amps, a value that becomes realistic with the current media.
  • This value can still be halved to finally reach 275 amps, if we agree to increase the charging voltage to 1000 volts.

It is likely that this is the third solution that manufacturers will turn to, forgetting the initial goals and accepting a reduced real range (300 or 400 km?) And a recharge time of 10 minutes that becomes realistic if the number of points of Recharging is great, and they are everywhere, which is possible by reducing the limitations.

Conclusion

These small table calculations show that electric car batteries are quite far from the performance of a single diesel tank.

In addition, we must resign ourselves to the fact that they simply will not be able to reach them, not for reasons related to the batteries themselves, but for reasons of energy distribution. We must reduce our ambitions. And the electric vehicle for everyone is probably not for tomorrow, not even for tomorrow.

Article originally published on August 14, 2020.

Leave a Comment