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

What is the core technology of BMS?

Recently, I saw a propaganda brand of a domestic company, claiming to "comprehensively master BMS software and hardware technology", "reach the world's advanced level", and "adopt multiple balanced control capabilities" because of the use of the underlying software such as AUTOSAR's software architecture. It is very eye-catching. Are these things the core technology of BMS?

Usually BMS system usually includes detection module and operation control module.

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 arithmetic module for processing and issuing instructions. So the computing control module is the brain of BMS. The control module generally includes hardware, basic software, runtime environment (RTE) and application software. One of the most core parts-application software. The environment developed with Simulink is generally divided into two parts: the estimation algorithm of the battery state and the fault diagnosis and protection. State estimation includes SOC (State Of Charge), SOP (State Of Power), SOH (State of Health), balance and thermal management.

The battery state estimation usually estimates SOC, SOP and SOH. SOC (State of Charge) is simply how much electricity 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) Ability) is extremely important. If there is no accurate SOC, no amount of protection functions can make the BMS work normally, because the battery will often be in a protected state, and it will not be able to extend the life of the battery.

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

The battery is a lithium iron phosphate battery. Its SOCvsOCV curve only changes about 2-3mV in the range of SOC from 70% to 95%. The measurement error of the voltage sensor is 3-4mV. In this case, we deliberately let the initial SOC have an error of 20%, and see if the algorithm can correct the 20% error. If there is no error correction function, the SOC will follow the SOCI curve. The SOC output by the algorithm is CombinedSOC, which is the solid blue line in the figure. CalculatedSOC is the real SOC that is reversed based on 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 continuous high current. Of course, the effect of continuous high current on the fuse should also be considered.

The accurate estimation of SOP can maximize the utilization efficiency of the battery. For example, when braking, it can absorb as much energy as possible without harming the battery. When accelerating, it can provide more power to obtain greater acceleration without harming the battery. At the same time, it can also ensure that the car will not lose power due to undervoltage or overcurrent protection even when the SOC is very low. In this way, the so-called first-level protection and second-level protection are nothing short of a glimpse before accurate SOPs. Not that protection is not important. Protection is always needed. But it cannot be the core technology of BMS. For low temperature, old batteries and very low SOC, accurate SOP estimation is especially important. For example, for a group of well-balanced battery packs, when the SOC is relatively high, 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 cell will drop rapidly. The voltage of this battery cell is even more than 1V lower than the voltage of other batteries. To ensure that the voltage of each cell is not lower than the minimum voltage given by the battery supplier, SOP must accurately estimate the maximum output power of the cell whose voltage drops rapidly 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 state of health of the battery. It includes two parts: Amp-hour capacity and power changes. It is generally believed that when the ampere-hour capacity decreases by 20% or the output power decreases by 25%, the battery life is reached. However, this is not to say that the car cannot be driven. For pure electric vehicle EV, the estimation of the ampere-hour capacity is more important because it is directly related to the cruising range and the power limit is only important when the SOC is low. For HEV or PHEV, the power change is more important. This is because the ampere-hour capacity of the battery is relatively small, and the power that can be provided is limited, 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. So 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, one should ask him what exactly did BMS do? Is it an algorithm or an active balance or only the hardware and underlying software of the BMS? Or just propose a BMS structure?

Some people say that Tesla is good because its BMS can manage 7,104 batteries. Is this its best place? Does it really manage 7104 batteries? Tesla modelS does use 7104 batteries, but only 96 batteries are connected in series. Parallel connection can only be counted as one battery. No matter how much you connect in parallel Festival. Why? Because the battery packs of other companies only count the number of batteries in series instead of the number in parallel. Why is Tesla special? In fact, if you understand Tesla’s algorithm, you will know that Tesla’s algorithm not only requires a large amount of working condition data calibration, but also cannot guarantee that in any case, especially Estimated accuracy after battery aging. Of course, Tesla's algorithm is still much better than almost all domestic BMS algorithms. Almost all domestic BMS algorithms use current integration and open circuit voltage to calculate the initial SOC, and then use current integration to calculate the SOC change. The problem is that if the voltage at the starting point is wrong, or the ampere-hour capacity is incorrect, shouldn't it be corrected until it is fully charged again? Will the voltage at the starting point go wrong? Experience tells us that it will, although the probability is very low. If you want to be foolproof, you can't just rely on the accurate voltage of the starting point to ensure that the starting SOC is correct.