Battery Technical Info

BMS Top 3 Core Functions: Cell Monitor, SOC estimation, single-cell balance


The Battery Management System( BMS) is an important link between onboard power battery and electric vehicle.

BMS collects, processes and stores important information during the work of the battery, and exchanges information with the peripheral equipment, such as the vehicle controller, to solve the key problems of safety, availability, ease of use and service life of the lithium battery system.

Take the BMS of SmartPropel 72V 100Ah Electric Vehicle Lithium Battery for example, the main function is to be able to improve the battery utilization rate, prevent the battery from overcharging and over-discharging, and extend the service life of the battery, monitor the battery status. In layman’s terms, it’s a system for managing, controlling, and using battery packs.

The three core functions of BMS are Cell Monitoring, state of charge (SOC) estimation, and cell equalization.


1.Cell Monitoring Technology

  1. Single Cell Voltage Acquisition
  2. Single Cell Temperature Acquisition
  3. Battery pack current detection

Accurate measurement of the temperature is also important for the working state of the battery pack, including the temperature measurement of the individual battery and the temperature monitoring of the battery pack cooling liquid. It is necessary to set the proper position and temperature sensors number to cooperate well with the BMS control module.

The monitoring of the battery liquid temperature is focused on the liquid temperature at the inlet and outlet, and the monitoring accuracy is similar to that of the single battery.

2.SOC Technology

Single-cell SOC is the key and difficult parameter in BMS, SOC is the most important parameter in BMS, because everything else is based on SOC, so its accuracy and Shandong Bar (also known as error-correcting ability) are extremely important. Without an accurate SOC, even plenty of protections can not keep the BMS from working properly, as the battery will often be in a protected state and its life will not be extended. The higher accuracy of SOC estimation, the higher the range that EV can be for the same capacity battery. High precision SOC estimation can maximize the performance of the battery pack.

At present, the most commonly used calculation methods are the ampere-hour integral method and the open-circuit voltage calibration method. By establishing the battery model and collecting a large amount of data, the actual data is compared with the calculated data, which is also the technical secret of each family, it takes a lot of data to accumulate over a long period, and it’s one of the most technical parts of SmartPropel. SmartPropel and our partners have applied for core patents in areas related to BMS, such as battery cooling, safety, and charge balancing.

3.The Equalization Technique

Passive equalization usually uses resistance to release the “extra charge” of high capacity battery, so as to achieve the goal of equalization. The circuit is simple and reliable, the cost is low, but the efficiency of the battery is also low.

Active equalization is transferring the surplus power to the high-capacity cell during the charging process and transferring surplus power to the low-capacity cell during discharging, and the cost is higher, the circuit is complex and the reliability is low. In the future, as the consistency of the cell increases, the demand for passive equalization may decrease.

Almost all major automobile manufacturers have passive equalization technology, most of them have active equalization technology reserve. The passive equilibrium BMS has a large number of machines and occupies a high share of the new energy vehicle market. The fundamental factors for this phenomenon is cost, active equalization BMS cost is far higher than the passive equalization BMS.

Considering the consumption habits of the global market, many new energy vehicles are mainly marketed as mid-and low-end brands. To strictly control costs, the requirement for spare parts of the main engine factory is based on the principle of “satisfying basic functions and lower costs”, the cost of active equalization is much higher than that of passive equalization. When the passive equalization satisfies the basic functions, the EV factory prefers to choose the passive equilibrium BMS.

4.Security Technology

Battery Management System design should be based on the battery voltage, temperature, and the application environment to determine the battery charge and discharge power, information feedback back to the vehicle so that the battery used in a more comfortable green area.

The battery is an electrochemical carrier when charging, there will be a variety of reactions, in the case of many external unsafe factors, how to ensure the safety of battery system is the core issue for battery management technology.

SmartPropel Original Work Writer:Nancy