For manufacturers of electric cars and those of their batteries, autonomy has been a workhorse with which they still fight, dedicating important investments in its R&D programs. While there are numerous design factors that can affect autonomy of an electric vehicle, the most important variable is the drums. The industry is investing heavily in research into different battery technologies, chemistries and architectures in hopes of improving performance, cost and capacity. One of them, the most basic, is the format and dimensions of the cells that form them.
Earlier this month, BMW confirmed that it had decided to replace the prismatic cells with which it currently packs its batteries and which are used by all its electric cars with new ones, in cylindrical format. The dimensions of these cells could correspond to those presented by Tesla in its Battery Day, that is, in 4680 format (46 mm in diameter and 80 mm in height), although BMW is considering maintaining the diameter and having different heights depending on the specifications required by each of its electric cars.
Why has BMW decided to make this radical change? The differences between cylindrical and prismatic cell geometry are important. BMW wants to take advantage of the benefits of the former to advance in the specifications of its electric cars with which it ensures that it will achieve increase autonomy and charging speed.
Cylindrical cells vs. prismatic cells
When it comes to lithium-ion batteries, two of the most common cell geometries are prismatic and cylindrical cells. Cells in bag format are also widely used, which, for example, General Motos uses in its Ultium batteries, although their size increases the probability of thermal leaks. The arrival of safer LFP chemistry is giving this format a second life.
The prismatic cells are a battery geometry that is made up of a series of positive and negative electrodes interspersed in a rectangular shape. The chemistry of a prismatic cell is encased in a rigid shell, mostly aluminum or steel, to ensure a high level of stability. Due to this unique architecture, prismatic cells are often light and thin. For these reasons, prismatic cells tend to be used in applications that require a high energy densityas is the case with energy storage systems and electric vehicles.
On the other hand, cylindrical cells, on the other they are configured with a series of electrodes that are enclosed in a circular geometry. The cells are extremely tightly wound and are confined within a special metal casing which reduces the chances of battery swelling and electrode damage. Cylindrical cells tend to be small and round, allowing them to stack on top of each other. Generally, cylindrical cells are used in medical devices, electric bicycles and satellites.
Advantages of cylindrical cells
Although prismatic cells have been the most used by the electric vehicle industry, cylindrical cells offer a series of advantages that are contributing to a clear change in this trend.
One of the main advantages of a cylindrical geometry in lithium-ion batteries is the fact that their construction lends itself better to different types of automation and ease of manufacturing. Because of this, round batteries can be produced much faster and at a lower cost. lower cost per kilowatt hour than prismatic cells.
Furthermore, another great advantage of cylindrical cells is that they are extremely safe In most cases. One of the reasons for this is the geometry of the cell which increases the stability of the battery. Beyond this, round battery cells are often designed in such a way that if their internal pressure gets too high, they rupture, meaning they are unlikely to catch fire or explode.
Finally, although cylindrical cells often have a lower capacity than prismatic cells, have advantage in terms of power output. Today, some of the major electric vehicle brands, such as Tesla, Rivian and Lucid, use them in their electric cars and some of the traditional manufacturers that had not opted for them are beginning to change their criteria.
The case of BMW
In the quest for greater autonomy, BMW has announced a transition from its current prismatic cells to the new cylindrical cells. These will come to the new range New Class from BMW, initially with two different dimensions: 46 mm and 95 or 120 mm in height. Inside, the cathode of its cells will maintain the ternary chemistry of lithium ions NCM (nickel, cobalt, manganese) increasing the nickel content and reducing the cobalt content. At the other electrode, the anode, will increase the silicon content replacing graphite. In addition, the electrical voltage will be increased to 800 voltswith current intensities of 500 amps.
According to BMW, compared to the fifth-generation prismatic cells, the cylindrical ones will raise the volumetric density by a twenty %will increase autonomy by 30%and they will achieve that the recharge speed is a 30% elderly. In addition, the production cost of such a battery pack will be fifty % with a reduction in CO emissionstwo of 60%.
With this change, the first information speculates that BMW will be able to offer in its new Neue Klasse line a greater autonomy than the 800 kilometers per charge.