Battery patent: The square round cell from BYD

While Tesla has always relied on round cells, German manufacturers in particular still rely on prismatic cells or pouch batteries. The Chinese battery and car manufacturer BYD is taking a different approach and is working on the hexagonal round cell.

As is so often the case, the engineers are inspired by nature when developing batteries. This is probably one of the reasons why the round cell was developed. Because only a spherical battery cell would enable even more efficient use of volume and better stability - in production, however, it would be much more complicated than the rolled-up perennial favorite of the battery world. However, this only applies if you look at the cell individually. If you put many of these round cells together to form a battery pack, gaps are created that cause inefficiency in cooling, the use of space and thus the power density.

Hexagon battery cell with six corners

BYD has applied for a patent that also solves this problem - and is also inspired by nature. The idea: An angular shape of the round cell, which is then arranged in a honeycomb shape and can thus form the inside of the so-called blade battery , for example. Strictly speaking, the hexagon cell is actually a hexagonal prismatic cell.

Especially in the view from above, it quickly becomes clear what the advantage of this arrangement is. Because the gaps to the round cell counterpart then no longer exist. Instead, the gaps can be used completely so that, for example, more electrolyte can be introduced, which increases the service life of the cell. In addition, the hexagonal shape allows a more stable battery pack to be formed that has better crash properties.

In terms of size, the BYD patent speaks of a height of between 60 and 150 millimeters and a diameter of 15 to 60 millimeters. But it also becomes more specific. Elsewhere there is talk of a height of 90 millimeters and a diameter of 40 millimeters. According to common logic, the hexagonal cell from BYD would therefore be referred to as 9040.

9040 hexagon battery with LFP technology

Even if the format differs somewhat from Tesla's large 4680 cell , the American system is thicker and smaller, but it's still worth comparing. Because both cells are many times larger compared to the usual round cell formats on the market. The hexagon cell from BYD uses LFP technology (lithium iron phosphate). The Tesla cell, on the other hand, uses classic NMC technology (nickel, manganese, cobalt). The 4680 cell from Tesla has an energy density of around 244 Wh/kg. The data of the BYD counterpart has not yet been published, but theoretically up to 290 Wh/kg would be possible.

Another special feature of the 9040 hexagon cell are the plus and minus poles of the BYD battery.Unlike many other batteries, the connections are not at the top and bottom, but both at the top. That alone would not be a unique selling proposition. The hexagonal shape of the cell makes it special. The negative electrode is placed in the middle - as far as usual - the positive electrode is divided into three and placed on the edge. In this way, the cells in the battery pack could be accommodated more easily and with less risk of short circuits.

Hexagonal cell format for more stability

Speaking of the battery pack: In the patent, BYD speaks of an aluminum housing from which the honeycomb structure is made. BYD also uses this structure for the blade battery. Due to the tight packing, BYD states a cell proportion of 80 to 90 percent of the entire battery. Other battery packs sometimes only have around 60 percent cells in the battery pack. The blade battery is 96 centimeters long, nine centimeters wide and 1.35 centimeters high. According to the dimensions of the 9040, the battery pack made of hexagon cells would be at least nine centimeters high, plus space for the housing and cooling. Overall, the battery pack would still be quite slim, since the battery packs on Mercedes EQ models usually measure more than 20 centimeters.

Another advantage: The honeycomb structure also offers an extraordinarily high stability and thus enables a Cell-to-Pack (CTP) or Cell-to-Chassis (CTC) construction. This means that the individual cells are not first combined into modules that then form the battery pack of the electric car, but are placed directly from the cell into a battery housing. This is exactly how Tesla uses its 4680 cells and BMW is also planning a cell-to-open-body concept for the new battery generation, which is announced for the new class from 2025 .

At the top of the gallery you will find pictures of BYD's big electric sedan called Han.


With more stability, longer service life and better efficiency, the concept of the square round cell of the BYD battery promises many problem solutions. However, it remains questionable whether battery packs with the honeycomb structure and the large cells can also be manufactured in large numbers on an industrial scale. Because that was exactly the unbeatable advantage of the classic round cell, for which most manufacturers have accepted their disadvantages.


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