Can the electric vehicle battery management system really monitor the status of each battery cell?

The power battery is the most important component in electric vehicles, and it is also the part that makes the biggest difference between electric vehicles and traditional fuel vehicles. According to market data, the battery pack of an electric vehicle basically accounts for 40% of the cost of the entire vehicle. Therefore, many car companies also use how much battery technology they have mastered as a gimmick to brag about themselves. We all know that the power battery pack of an electric vehicle is composed of hundreds or thousands of single cells. These single cells increase the voltage by connecting in series and increase the capacity by connecting in parallel to meet the voltage and capacity requirements of the electric vehicle. Why don't we just make a large battery instead of connecting in series and parallel for the power battery?

Take Tesla Model S as an example. Its power battery pack weighs 900 kg. The 85kwh battery version has a total of 7,200 18650 cylindrical cells. The principle that "new and old batteries in remote controls cannot be mixed" is well known to all children. This is also true for the power battery packs of electric vehicles. In fact, it is even more stringent. Power Battery Talk - Why is the consistency of power batteries so important?

How do you ensure the consistency of cells in electric vehicle battery packs? Is it really as advertised by car manufacturers or battery manufacturers: their super-powerful battery management system (BMS) can identify the voltage and temperature of each cell?

Obviously, this is impossible. Let's take the Model S as an example. If you want to measure the voltage of more than 7,000 cells in the battery pack, based on two wires, one positive and one negative, for each cell, a total of more than 14,000 wires are required, which is obviously unreasonable. Even if some of the cells are connected in series and the cells are connected end to end, some wires can be saved, but in total, no less than 7,000 wires are required, which is obviously unreasonable in the manufacture of power batteries. In addition, the same is true if you want to measure the temperature of each cell, which requires a large number of wire harnesses. More importantly, even if these wire harnesses are really connected in this way, it is absolutely a catastrophic problem for the battery management system.

So what is actually happening? How does the battery management system measure the voltage and temperature of each cell inside the power battery to ensure consistency? As we mentioned before, the power battery pack is composed of cells connected in series and in parallel. Therefore, in fact, the voltages of the parallel cells are the same because they are connected head to head and tail to tail. When the power battery management system collects voltage, it directly collects the voltage of the integrated cell, and the voltage obtained is the voltage of each cell in the cell.

Most power batteries are first connected in parallel to form a battery pack, and then the battery packs are connected in series to increase the voltage to form a power battery. Each string of power batteries is a large battery pack. When the battery management system measures, it measures the voltage of one string to obtain the voltage of the whole battery pack, and to obtain the voltage of each battery cell in the battery pack. When our battery management system determines the consistency of the battery pack, it also uses the voltage of each string for comparison.

Tesla's Model S battery pack has a rated voltage of about 345V and uses ternary cells (3.6V per string), which means that the total number of strings is 345/3.6≈96 (strings). In other words, the battery pack collects the voltage of 96 cells in total, not more than 7,000 cells. The same is true for temperature collection. The battery pack does not collect the temperature of each cell, but arranges sensors at certain locations inside.

Therefore, whether it is a battery factory or a car company, when they promote their power battery management system as being able to collect the parameters of each battery cell and allow each battery cell to be connected to the Internet, what they express is different from what we understand. However, in theory, the battery management system does collect the voltage of each battery cell, but logically, such a description is suspected of being deliberately misleading. The management system only collects the voltage of each string of batteries (but the voltage of all batteries in each string is equal), not every battery. As for the battery cell temperature, it is obviously unreasonable and unnecessary to collect the temperature of each battery cell. Usually, a certain number of collection points are allocated according to a certain number of batteries.