CR2032 button cell battery

What is the core technology of CR2032 button cell battery management system?

The biggest difference between new energy vehicles and traditional vehicles is that they use batteries as power drive, so the technology of power batteries is the core of new energy vehicles.

What is the core technology of BMS?

Recently, I saw a billboard of a domestic company, claiming that it "fully masters BMS software and hardware technology", "reaches the world's advanced level", and "adopts multiple balancing control capabilities" because of the use of underlying software such as AUTOSAR software architecture. It is very eye-catching. Are these things the core technology of BMS?

Usually, BMS systems usually include detection modules and operation control modules.

Detection refers to measuring the voltage, current and temperature of the battery cell and the voltage of the battery pack, and then transmitting these signals to the operation module for processing and issuing instructions. Therefore, the operation control module is the brain of BMS. The control module generally includes hardware, basic software, runtime environment (RTE) and application software. The most core part of them is application software. For the environment developed with Simulink, it is generally divided into two parts: battery state estimation algorithm and fault diagnosis and protection. State estimation includes SOC (State Of Charge), SOP (State Of Power), SOH (State Of Health) as well as balancing and thermal management.

Battery state estimation usually estimates SOC, SOP, and SOH. SOC (state of charge) simply means how much power is left in the battery; SOC is the most important parameter in BMS, because everything else is based on SOC, so its accuracy and robustness (also called error correction capability) are extremely important. Without accurate SOC, no matter how many protection functions are added, the BMS cannot work properly, because the battery will often be in a protected state, and the battery life cannot be extended.

In addition, the accuracy of SOC estimation is also very important. The higher the accuracy, the higher the range for the same capacity battery. Therefore, high-precision SOC estimation can effectively reduce the required battery cost. For example, Chrysler's Fiat 500eBEV can always discharge SOC=5%. It became the electric car with the longest range at that time.

The figure below is an example of algorithm robustness. The battery is a lithium iron phosphate battery. Its SOCvsOCV curve only changes by about 2-3mV in the SOC range from 70% to 95%. The measurement error of the voltage sensor is 3-4mV. In this case, we intentionally let the initial SOC have a 20% error to see if the algorithm can correct this 20% error. If there is no error correction function, the SOC will follow the SOCI curve. The SOC output by the algorithm is the CombinedSOC, which is the blue solid line in the figure. CalculatedSOC is the real SOC deduced from the final verification result.

SOP is the maximum discharge and charging power that the battery can provide at the next moment, such as the next 2 seconds, 10 seconds, 30 seconds, and when the current is continuous. Of course, the impact of continuous high current on the fuse should also be considered.

Accurate estimation of SOP can maximize the utilization efficiency of the battery. For example, when braking, you can absorb as much feedback energy as possible without damaging the battery. When accelerating, you can provide greater power to obtain greater acceleration without damaging the battery. At the same time, it can also ensure that the car will not lose power due to undervoltage or overcurrent protection during driving, even when the SOC is very low. In this way, the so-called primary protection and secondary protection are fleeting in front of accurate SOP. It is not that protection is not important. Protection is always needed. But it cannot be the core technology of BMS. Accurate SOP estimation is particularly important for low temperature, old batteries and very low SOC. For example, for a group of well-balanced battery packs, at a relatively high SOC, the SOC difference between each other may be very small, such as 1-2%. But when the SOC is very low, the voltage of a certain battery cell will drop rapidly. The voltage of this battery cell may even be more than 1V lower than the voltage of other batteries. To ensure that the voltage of each battery cell is always not lower than the minimum voltage given by the battery supplier, SOP must accurately estimate the maximum output power of the battery cell with a rapidly dropping voltage at the next moment to limit the use of the battery and protect the battery. The core of estimating SOP is to estimate each equivalent impedance of the battery online in real time.

SOH refers to the health status of the battery. It includes two parts: ampere-hour capacity and power change. It is generally believed that when the ampere-hour capacity decays by 20% or the output power decays by 25%, the battery life is over. However, this does not mean that the car cannot be driven. For pure electric vehicles EV, the estimation of ampere-hour capacity is more important because it is directly related to the cruising range, while power limitation is only important at low SOC. For HEV or PHEV, the change of power is more important because the battery has a relatively small ampere-hour capacity and can only provide limited power, especially at low temperatures. The requirements for SOH are both high precision and robustness. And SOH without robustness is meaningless. If the accuracy is less than 20%, it is meaningless. The estimation of SOH is also based on the estimation of SOC. Therefore, the SOC algorithm is the core of the algorithm. The battery state estimation algorithm is the core of BMS. Everything else serves this algorithm. So when someone claims to have broken through or mastered the core technology of BMS, you should ask him what he has done in BMS? Is it the algorithm or active balancing or just the hardware and underlying software of BMS? Or just propose a structural method of BMS?

Some people say that the reason why Tesla is so powerful is that its BMS can manage 7104 batteries. Is this its advantage? Does it really manage 7104 batteries? Tesla modelS does use 7104 batteries, but only 96 batteries are connected in series, and the parallel ones can only be counted as one battery no matter how many batteries you connect in parallel. Why? Because other companies' battery packs also only count the number of batteries in series, not the number of batteries in parallel. Why should Tesla be special? In fact, if you understand Tesla's algorithm, you will know that Tesla's algorithm not only requires a large amount of operating data for calibration, but also cannot guarantee the estimation accuracy under any circumstances, especially after battery aging. Of course, Tesla's algorithm is much better than almost all domestic BMS algorithms. Domestic BMS algorithms are almost all current integral plus open circuit voltage methods, using open circuit voltage to calculate the initial SOC, and then using current integral to calculate the change in SOC. The problem is that if the starting point voltage is wrong, or the ampere-hour capacity is inaccurate, won't it be wrong until it is fully charged again to correct it? Will the starting point voltage be wrong? Experience tells us that it will, although the probability is very low. If you want to ensure that there is no risk, you can't just rely on the precise starting point voltage to ensure the correct starting SOC.

What is the problem with China's new energy vehicle balancing?

Last year, a certain active balancing technology selected by experts won a certain lithium battery Golden Globe Award. The reason is that its core technology-active balancing technology can extend battery life by 30% and mileage by 20%. This is unreliable at first glance. Because it is impossible to quantify. Who can you compare with to extend the life by 30%? Is it meaningful to compare with yourself? Is it meaningful to compare with those who have no balance? Then your level is far behind. Comparing with others should be compared with the best. The world's BMS, if not the best, at least has no balancing problem. How can you extend the life by 30%? The same principle applies to extending the range. For example, Chrysler's Fiat500e, its SOC tolerance is always 5%. How can you extend the range by 20%? Going further, is active balancing difficult? Hardware In 2008, TI promoted its active balancing IC to the company I was working for at the time. The algorithm is nothing more than balancing the batteries in the same module and balancing the batteries between different modules. General Motors completed simulation verification 6-7 years ago. There are even articles. From the algorithm point of view, there is no difficulty at all. Moreover, active balancing is not what is said on the Internet that "active balancing function has always been the killer feature of foreign products." Why do foreign countries basically not use active balancing? Mainly considering the cost issue. If passive balancing can be done, why do we need active balancing? Why do people in China strongly advocate active balancing? I think it is mainly because passive balancing cannot be done. When it comes to passive balancing, most people told me that it is because the quality and consistency of domestic batteries are too poor. But through conversation, I found that the root cause lies in unclear concepts and incorrect methods. Otherwise, how could the balancing get worse and worse when driving? The effect of balancing can be calculated. The so-called multiple balancing technology clearly does not have a single method to achieve balancing. Some people say that passive balancing wastes a lot of electricity. So it is not good. Taking a battery pack with 96 cells in series as an example, we can calculate how much electricity passive balancing wastes in the worst case. If the balancing current is 0.1A, one battery will waste about 0.4W when being balanced. The worst case is that 95 batteries need to be discharged, so the worst case is 0.4X95=38W. It is not as energy-consuming as a car headlight (about 45 watts). If it is not the worst case, maybe only a dozen watts or even a few watts are enough. So, although passive balancing wastes a little electricity, if it can greatly extend the life of the battery, why not do it? Some people also say that 0.1A current is too small for batteries with a relatively large ampere-hour capacity. If the imbalance can be eliminated in the bud, there will be no helplessness. If the battery itself can no longer work properly, neither active balancing nor passive balancing can do anything. Therefore, we cannot blame the poor consistency of the battery. We also need to find the reasons from ourselves. The car I have worked on has two PHEV cars. After driving for only a few months, the SOC difference in the battery pack is as high as 45%. And because of the problems of SOC and SOP, the car often breaks down on the road. The company unanimously believed that it was a battery quality problem and unanimously agreed to change the battery supplier. But I only changed the algorithm and solved the problem of balancing. And I did it when the company clearly stipulated that charging was not allowed. Because a car had already had an accident due to battery problems. The difference in SOC of the battery cells in the battery pack has dropped from 45% to 3%. Now the car has traveled more than 100,000 kilometers. The problem of breaking down has never happened again.

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