Helping the endurance: analyzing battery management BMS and SOC accuracy
First, energy management
First, let's consider the energy distribution. In the SOC from the whole 100% to zero or the entire battery energy management process can be decomposed into: the unavailable energy left in the upper and lower sections (the pure electric vehicle stays more and more left), the available energy, SOC error, energy required to regulate battery temperature, compensation for capacity reduction, CS buffer for plug-in hybrid vehicles (no pure electric). 　
What we call energy management is to strictly use the above energy divisions to give consumers a more uniform experience. The following figure is an example. It is a full separation, and the SOC directly corresponds to one mileage. The cruising range is closely related to the SOC, and the SOC is not equal to the actual cruising range. The SOC from 100% to 50% must be longer than the SOC running from 50% to 0. Because the less the SOC, the faster the battery energy is consumed.
The cruising range is actually a very interesting thing, because it is a complicated function. If you use P-diagram to express it, it is as follows:
The controllable part is mainly the SOC, power limit and temperature control algorithm; the disturbance part is mainly the vehicle parameters, the road environment (working conditions), the driver's driving habits and the estimation error of the SOC.
There are still some that are not written: such as the systematic differences between summer and winter, including the use of HVAC power for the entire energy.
The following data is a measured example and shows a change.
The difference in temperature is too great
The estimation accuracy of SOC in BMS has a great influence on the cruising range of vehicles. Especially for pure electric vehicles, the greater the difference in estimation accuracy, the easier it is for users to feel mileage anxiety; if there is a systematic calculation error, the user has no electric cart. Going home or calling a towing trailer is a big deal.
Second, SOC accuracy
The article "Deep Analysis of SOC Accuracy Verification Method" written by Dr. Lin Jian, a special expert of the National Thousand Talents Program, wrote that DVP needs to compare BMS from SOC100% to unavailability (5%), under working conditions, After the actual battery conditions are collected, the test bench is used for comparison, model error correction (accounting Ah and power limit).
Speaking of personal opinion, all things are accurate under certain working conditions, temperature and usage conditions. The higher the system requirements, the more control content for the extension of the periphery, so the more the BMS algorithm needs to be directly Contact the VCU for access to control the HVAC system, control the drive system, and load system. Dr. Lin Jian emphasized that the relationship between OCV and SOC is not very useful under the LFP system. The actual test is probably a precision table, which is somewhat difficult.
In simple terms, the accuracy of the BMS is higher, and more windows can be opened, but the difference is 5%. The difference in cruising range due to external reasons is larger than this one, so the accuracy of SOC is a reasonable value, which is worth considering. It is very important to manage customer expectations. We can only report less and not report more. Therefore, the experience of pure electric vehicles for small batteries at this stage is very bad. It is also the main reason why everyone is desperately going to 50 degrees or more.
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