Soul Painter CEO shows you the design secrets of Lucid batteries

March and April are indeed a good time for recruitment. Not long after we posted the recruitment article, we welcomed a new colleague in operations.

After the new colleague who is extremely health-conscious found out that no one has the habit of taking a nap at noon, "too curly" became her catchphrase.

Please, our company doesn’t start work until 10 o’clock, so there is no way I can sleep at noon.

If we really want to talk about "volume", it has to be the bosses.

Nowadays, it is not uncommon for the bosses of major technology companies to show up in public to endorse their own products.

However, recently Lucid Motors CEO Peter Rawlinson raised this standard much higher.

As the former chief engineer of Tesla Model S, Rawlinson is undoubtedly a technical manager.

Through a battery technology communication meeting, he fully demonstrated to everyone what it means that "a soul painter who does not understand technology is not a good CEO."

As the mass-produced car with the longest EPA range in the world,   what  are  the  little secrets of the Lucid Air  's battery pack?

High voltage = high efficiency

The Lucid Air Grand Touring is equipped with a 112 kWh battery pack. How much energy does 112 kWh have?

Rawlinson gives us a very vivid example.

"We pick up a 100-gram apple from the ground and place it on a table 1 meter above the ground. This process consumes 1 joule of energy. The 112 kWh battery pack is equivalent to about 400 million joules, which allows us to pick up the apple 400 million times."

How is the parameter 112 kWh determined?

Rawlinson first explained the relationship between energy and power and introduced us to a very clear and easy to understand calculation process.

About 10 years ago, electric vehicles consumed about 330 Wh of electricity per mile, but with the advancement of technology, today, the amount of electricity consumed by electric vehicles per mile has dropped to 250 Wh.

If you want an electric car to have a range of 400 miles, you need a 100 kWh battery pack.

We continue to calculate that the 2170 battery used by Lucid Air has a capacity of about 17-18 Wh per cell, and it takes about 6,000 2170 cells to make a 100 kWh battery pack.

This is the design basis of the Lucid Air battery pack, 6600 2170 cells.

But everyone knows how to stack batteries. Of course, there are some different techniques for an electric car with the longest EPA range in the world.

For example, at present, the battery pack voltage of electric vehicles is generally around 400V, while Lucid uses a high voltage of 924V.

As for why this is done, Rawlinson used several very basic physical formulas to explain to us the theoretical support behind choosing a high-voltage battery.

Ohm's Law

Voltage U = Current I × Resistance R ------------ ①

Electric power calculation formula

Power P = Voltage U × Current I ------------ ②

Substituting formula ① into formula ②, we can get

P = I^2 × R

This is also the calculation formula for the system resistance heating.

But this heat is useless to us, we hope it is as small as possible.

According to P = I^2 × R, the smaller the current I, the smaller the heat generation power P.

The question is, how to reduce the current while keeping the total system power unchanged?

From P = U × I, we can see that when the power P remains unchanged and we increase the voltage U, the current I will decrease.

Taking Lucid Air as an example, if Lucid doubles the battery pack voltage, the current will become half of the original, and the heat generation power will be reduced to one quarter of the original.

Therefore, high voltage means higher driving efficiency, which is one of the main reasons why Lucid chose a high-voltage battery pack.

How to build 924V voltage

The fully charged voltage of a 2170 battery cell is 4.2V, so how to boost 4.2V to 924V?

First, let's introduce a knowledge point about battery series and parallel connection. Batteries connected in series can increase the output voltage; batteries connected in parallel can increase the output current.

Rawlinson used three flashlights of different lengths and voltage levels as an example.

These flashlights all use 1.5V No. 1 dry cell batteries as their energy source. If you put three dry cells in series into a flashlight, you will get a 4.5V flashlight; if you put four dry cells in series, you will get 6V; if you put six dry cells in series, you will get 9V.

Similarly, if you want to boost the voltage of a 4.2V 2170 battery cell to 924V, you need to connect 220 2170 batteries in series.

But if you connect the batteries in series like a flashlight, 220 2170 cells connected end to end, the length would be about 15 meters, which is obviously not suitable for a car.

In fact, in order to take into account both voltage level and current output capacity, Lucid divided 6,600 2170 battery cells into 22 groups, with each 300 battery cells forming a battery module.

In a battery module, 300 2170 cells are evenly divided into 10 groups. Every 30 cells are connected in parallel to form a "Group".

When 10 groups in a module are connected in series, a 42V output voltage is obtained.

In order to arrange 300 battery cells "vertically and parallel" to form a battery module, Lucid made two efforts.

First, make sure the positive and negative poles of the 2170 battery cell are at the same end of the battery cell.

We have actually mentioned this design in our previous article on Tesla's patents. Interested parties can click here to jump and view it. The benefits of doing this are also obvious. All battery cells only need to be placed vertically upwards, and the series and parallel connection between battery cells can be completed through the positive and negative busbar connectors.

Second, battery connection technology.

When talking about these connectors, Peter mentioned a very fashionable word, "one-piece die casting". Yes, these connectors are made by one-piece die casting technology.

At the same time, Rawlinson also explained in great detail why aluminum was chosen as the material for the connector. The main function of the connector is to conduct electricity. The lower the resistance of the connector, the lower the heat generation and the higher the efficiency. From this perspective, we should choose the metal with the lowest resistance - silver. But silver is too expensive. Considering the cost and conductivity, copper becomes the first choice. But copper is relatively heavy.

Considering conductivity, cost and weight, aluminum becomes the best choice for connector material.

Lucid also has its own little ideas when soldering the battery cells to the busbar connector.

To ensure safety, the industry usually welds the positive and negative poles of the battery cell to the busbar connector with fuses. If a single battery cell short-circuits, the fuse will instantly blow, disconnecting the battery cell from the battery module, thus ensuring the safety of the battery module.

But Lucid found that it is not necessary to use two fuses for a single battery cell. First, if one of the positive and negative terminals of the battery is disconnected, the battery cell will be disconnected from the battery module. Second, compared with ordinary connection materials, fuses have greater resistance and lower efficiency.

Therefore, Lucid chose to use fuse welding on the positive electrode of the battery cell and thicker ordinary conductor welding on the negative electrode.

After talking about a single battery module, if we connect 22 battery modules in series, we can get an output voltage of 924V.

To sum up, Lucid uses 220 series and 30 parallel connections to form a 112 kWh, 924V battery pack with 6,600 2170 cells.

Different cooling solutions

Rawlinson mentioned that the commonly used battery module cooling solution in the industry is to set up cooling pipes between each battery cell, but Lucid believes that this cooling method is extremely inefficient for the following two reasons.

1. Battery cell heat problem

When working, the battery cell will emit heat in all directions, but the heat conduction efficiency is different in different directions. Lucid found that the heat conduction efficiency of the battery cell in the longitudinal direction is much greater than that in the lateral direction.

The result is that the heat generated by the battery cell will accumulate at the bottom of the battery cell.

2. Efficiency Issues

The cooling pipes arranged between the battery cells, firstly, the contact area between the cooling pipes and the battery cells is not as large as in an ideal state. In fact, the contact area between them is not continuous but intermittent.

Secondly, the cooling pipes will occupy the space of the battery module. The most direct consequence is that the number of battery cells that can be accommodated in a battery module of the same volume will be reduced, thereby reducing the energy density of the battery module.

Based on the above two points, Lucid adopted a cooling solution that is completely different from Tesla. Lucid eliminated the cooling pipes between the battery cells and instead set up a cooling plate at the bottom of the battery module.

There are three advantages to this design.

1. Efficient heat dissipation

As mentioned earlier, the heat of the battery cell will eventually accumulate at the bottom of the battery cell, and it is undoubtedly the most efficient to remove the heat directly from the bottom of the battery cell. Secondly, the bottom of the battery cell is a plane, and the contact surface of the heat sink is also a plane. In this way, the two planes can easily and closely contact each other, which further improves the efficiency of heat dissipation.

2. Improve energy density

Without the heat dissipation pipes between the cells, the cells can be arranged at a closer distance, which means that the energy density of the battery module will be higher.

3. Easy to manufacture

The cooling plate is made of two pieces of stamped aluminum. Stamping is a production process that is very conducive to large-scale mass production. This means that this type of cooling plate has lower cost, lower production difficulty, better consistency and reliability.