Electric vehicle terminology: BMS and SOC analysis

At present, the electric vehicles people use are all equipped with small-capacity batteries, and battery life has become a decisive factor hindering the development of pure electric vehicles. The issue of user experience has been raised to a stage that requires a new review. Next, let's first discuss two professional terms related to electric vehicles: battery energy management (BMS) and the accuracy of the state of charge (SOC), so that everyone can understand this. Before introducing the main text, let's take a look at the latest trends of some manufacturers on new energy and the policies issued by the state related to new energy.

Nowadays, the development of electric vehicles is in full swing. The emergence of new energy vehicles replacing traditional fuel vehicles not only has an impact on environmental ecological improvement, but also brings many conveniences to people's daily life and travel. The country responds to the continued enthusiasm of new energy vehicle manufacturers for electric vehicle manufacturing. At the recent Global New Energy Vehicle Conference, BAIC launched four high-performance self-driving electric vehicles to meet the main multi-faceted uses [Learn to read]; Qingdao accelerates the construction of charging facilities and plans to build 49,000 road traffic charging piles in the next four years [Learn to read], Shenzhen has become the second new energy vehicle city after Beijing [Learn to read], the country has issued 7 policies on new energy electric vehicles [Learn to read], and the country has built a total of 81,800 charging facilities... [Learn to read] Countless examples tell us that the era of smart electric and fuel-free new energy driving is approaching in a big way.

At present, many users of pure electric vehicles with smaller battery capacity have reported a poor user experience. So let's discuss two related topics: battery energy management (BMS) and the accuracy of the state of charge (SOC), and see how these two things can help improve battery life.

1. Energy Management

First, let's consider energy distribution. When the SOC goes from 100% to zero, or the entire battery energy management process, it can be decomposed into: unavailable energy retained in the upper and lower sections (pure electric vehicles retain more energy in the upper and lower sections), available energy, SOC error, energy required to adjust the battery temperature, compensation for capacity reduction, and the CS buffer zone of plug-in hybrid vehicles (pure electric vehicles do not have this section).

What we call energy management is to strictly use the above energy partitions to give consumers a more uniform experience. Take the following figure as an example, which is a full partition, directly corresponding to a mileage SOC. The cruising range is closely related to SOC, and SOC is not equal to the actual cruising range. The distance from SOC 100% to 50% must be longer than the distance from SOC 50% to 0. Because the less SOC, the faster the battery energy is consumed.

The driving range is actually a very interesting thing, because it is a complex function. If expressed with a P-diagram, it is roughly as follows:

The controllable part mainly includes SOC, power limit and temperature control algorithms; the disturbance part mainly includes vehicle parameters, road environment (working conditions), driver's driving habits and SOC estimation errors.

There is another part that is not written: for example, the systematic differences between summer and winter, including the power of HVAC and the overall energy usage.

The following data is a measured example, showing a changing situation.

The temperature difference is huge.

The estimation accuracy of SOC in BMS has a great impact on the vehicle's range, especially for pure electric vehicles. The greater the difference in estimation accuracy, the more likely it is for users to feel range anxiety. If there is a systematic calculation error that causes the user to have no power to push the cart home or call a tow truck, it will be a big deal.

2. SOC Accuracy

In the article "In-depth Analysis of SOC Accuracy Verification Methods" written by Dr. Lin Jian, a special expert of the National Thousand Talents Program, it is written that DVP needs to compare the BMS from SOC100% to the unusable range (5%). Under working conditions, after collecting the actual battery working conditions, comparison is carried out through the test bench, model error correction (calculation of Ah and power limits), etc.

In my opinion, everything is accurate under certain working conditions, temperatures and usage conditions. The higher the requirements of the system, the more control content will be extended to the periphery, so the BMS algorithm needs to directly contact the VCU to obtain the authority to control the HVAC system, the drive system and the load system. The relationship between OCV and SOC emphasized by Dr. Lin Jian is not very useful under the LFP system. The actual test results are probably an accuracy table, which is somewhat difficult to say in general.

Simply put, a higher BMS accuracy can open more windows, but the difference is only 5%. The difference in mileage caused by external factors is greater than this, so the accuracy of SOC is a reasonable value and worth considering. It is important to manage customer expectations. You can only report less, not more. Therefore, the current user experience of pure electric vehicles with small batteries is very bad, which is also the main reason why everyone is desperately trying to get above 50 degrees.