At the moment when new energy vehicles are rapidly popularizing, the safety and performance of batteries, as the core power source, have attracted much attention. Inconsistencies in cell performance can lead to dangers such as overcharging, over-discharging, and local overheating, affecting battery life and safety. At this time, BMS (battery management system) came into being, with battery status monitoring, balanced management, life optimization, safety protection and data communication and other functions to ensure the safe and stable operation of the battery, which has become the key to the development of new energy vehicles.
At a time when new energy vehicles are rapidly popularizing, batteries, as the core power source, have attracted much attention for their safety and performance.
And there are multiple single cells produced in the same batch, and their performance cannot be completely the same due to production process errors, differences in use environment, etc. Moreover, this inconsistency will gradually expand during use, resulting in the risk of overcharging, over-discharging, and local overheating, which will affect the service life and safety of the battery pack in severe cases.
This requires BMS (battery management system) to show its skills.
The specific functions of the battery management system include: protecting the battery; Prevent overcharge, over-discharge, and voltage balancing functions; Prevents overheating; Calculate the remaining power; calculate battery life; Fault diagnosis. Its core implementation path is: monitor the voltage, current, temperature, internal resistance and other working state parameters of the internal battery module, judge the current battery state, allow charging and discharging power, etc., and send them to the vehicle controller.
01 Related key concepts
SOC (State of Charge) refers to the state of charge of the battery pack, which is the remaining charge of the battery, usually expressed as a percentage. Just like the scale on the fuel gauge of a fuel vehicle, it allows the owner to clearly know how far the vehicle can still be traveled, so that it can plan the trip and charging plan reasonably.
For example, when the displayed SOC is 30%, it means that the battery charge is 30% left, and the owner needs to consider finding a charging pile to avoid the vehicle breaking down due to running out of power.
There are many methods for calculating SOC, including the ampere-hour integration method, the open-circuit voltage method, and the Kalman filter method. The ampere-hour integration method calculates the amount of power flowing out or inflowing from the battery by recording the current and time during the charging and discharging process of the battery, and then calculates the remaining power. However, this method will be affected by factors such as current measurement error and battery self-discharge, resulting in deviations in the calculation results. The open-circuit voltage rule is estimated based on the correspondence between the battery’s open-circuit voltage and the SOC. However, the open circuit voltage of the battery will be disturbed by factors such as temperature and resting time, so there are certain limitations. The Kalman filtering method is relatively more complex and accurate, but it can comprehensively consider various parameters and operating states of the battery, dynamically estimate the SOC, and effectively reduce errors, and is more and more widely used in modern BMS systems.
SOC directly affects the driving experience and charging strategy of new energy vehicles. When the SOC is low, the vehicle may limit power output to save power, resulting in weaker acceleration and lower top speed. At the same time, car owners need to plan the charging time and place reasonably. If the SOC is too low for a long time and is frequently over-discharged, it will accelerate battery aging and shorten battery life. During the charging process, the BMS will adjust the charging current and voltage according to the SOC, such as using a larger current for fast charging when the battery is low, and reducing the current for trickle charging when it is almost full, ensuring safe and efficient charging.
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SOH (State of Health) represents the health status of the battery, reflecting the ratio of the battery’s current performance to its performance in a new state, and is used to measure the degree of aging and remaining service life of the battery. The SOH of the new battery is 100%, and with the increase in the number of charging and discharging, temperature changes and other factors during use, the battery performance will gradually decline, and the SOH value will also decrease.
For example, when the SOH of a new energy vehicle battery drops to 80%, it means that the battery’s capacity, charging and discharging efficiency and other performance indicators are only 80% of that of when it was new, and the cruising range may be shortened accordingly.
Metrics for evaluating SOH include battery capacity, internal resistance, charge-discharge efficiency, etc. Battery capacity is the most intuitive indicator, as the battery ages, its actual storage power decreases; The internal resistance of the battery will gradually increase with use, and excessive internal resistance will not only affect the charging and discharging performance of the battery, but also generate too much heat, which is a potential safety hazard; The charging and discharging efficiency reflects the battery’s ability to convert energy during the charging and discharging process, and the reduced efficiency means that the battery loses more energy during the charging and discharging process. The BMS combines these metrics to estimate SOH through a specific algorithm.
SOH is an important basis for determining whether a battery needs maintenance or replacement. When SOH drops to a certain level, such as below 70% – 80%, the vehicle’s cruising range will be significantly shortened, and the charging speed will also be slower. At the same time, understanding SOH helps car owners use their vehicles rationally, such as avoiding strenuous driving when the battery SOH is low, frequent fast charging, etc., to slow down the rate of battery aging. For car companies and maintenance personnel, SOH data can help them analyze battery performance trends and optimize battery management strategies and after-sales service solutions.
02 BMS core functions
Accurate monitoring of battery status.
Continuously collect the voltage, current, temperature and other parameters of each battery in the battery pack in real time to ensure that the battery is working normally.
For example, during driving, the BMS can monitor battery temperature changes in time, and when the temperature is too high, it will issue an alarm and take corresponding measures.
At the same time, it accurately calculates the remaining power (SOC) and health status (SOH) of the battery to provide accurate range information for the driver. Through precise monitoring, BMS can detect subtle changes in the battery in a timely manner, providing accurate data support for subsequent decision-making.
Battery equalization management.
The battery pack is composed of multiple cells connected in series or parallel, due to the inevitable performance differences in the battery cell during the production process, in the process of charging and discharging the battery pack, it is easy to have an unbalanced phenomenon. The BMS can be used to make the voltage and power of each battery consistent through passive or active balancing to avoid the overall performance degradation caused by battery imbalance
For example, in a battery pack composed of multiple battery cells, some battery cells may be fully charged first due to better performance, while others are not yet fully charged. At this point, the BMS will discharge the already full battery cells with a small current, making them wait for other cells until all cells in the entire battery pack reach a more balanced state of charge.
Battery equalization management can effectively improve the overall performance of the battery pack and extend the service life of the battery pack.
Battery life optimization.
BMS takes care of the battery’s lifespan by optimizing charge-discharge strategies and temperature management. It will formulate intelligent charging and discharging algorithms based on the actual usage of the battery.
For example, when the battery is low, the BMS will control the charging current and use a smaller current for charging to reduce battery losses.
At the same time, the BMS will also work in tandem with the vehicle’s cooling system, when the battery temperature is too high, start the cooling system to cool the battery; When the battery temperature is too low, the heating system is activated to increase the battery temperature to ensure that the battery is always operating within the appropriate temperature range. Through these measures, BMS can reduce battery aging during charging and discharging, significantly improving battery cycle life and energy density.
All-round battery pack protection.
The BMS has protection functions such as overcharge, over-discharge, over-current, over-temperature, and short circuit to prevent the battery from being damaged due to abnormal working conditions and even causing safety accidents.
For example, when a new energy vehicle is charged at night, if there is no BMS protection, once the charging equipment fails, the battery may be overcharged, resulting in serious consequences such as battery bulging or even fire. When the BMS detects that the battery charging voltage has reached the upper limit, it will quickly cut off the charging circuit to ensure battery safety.
Data communication and information management.
The BMS is also an “information hub” with powerful data communication capabilities. During the driving process of the vehicle, the BMS will transmit the collected battery status information to the vehicle control system in real time, so that the vehicle’s vehicle controller can reasonably adjust the vehicle’s power output, energy recovery and other strategies according to the actual situation of the battery. At the same time, the BMS can also transmit battery information to the owner’s mobile phone APP, and the owner can know the health status of the vehicle battery and the remaining power at any time through the mobile phone. Additionally, the BMS records historical data from the battery, which is crucial for analyzing battery performance, diagnosing faults, and more.
The core functions of BMS are: battery monitoring, management optimization, and safety protection.
03 BMS architecture
The centralized BMS centralizes the monitoring and control functions of all battery cells in a main controller, and the vehicle controller directly controls the relay control box, and the information of all battery cells is summarized to the main controller for unified processing. The advantage of this design is that the system architecture is relatively simple and the cost is low, but the sampling harness is long and the design is complex, making it suitable for low-voltage hybrid electric vehicles.
The distributed BMS equips each battery cell with a dedicated “little butler”, the battery monitoring module. These “little housekeepers” are responsible for monitoring and controlling their respective battery cells, and transmitting battery cell information to the main controller through communication protocols. The advantage of this architecture is that the wiring harness is short and uniform, even if there is a problem with a battery cell, its corresponding “little housekeeper” can deal with it in time, which will not affect the normal operation of the entire battery pack, greatly improving the reliability and stability of the system. It can support large battery packs and is suitable for battery packs of different sizes.
Modular BMS divides the battery cell into several modules, each with an independent monitoring and control system, just like dividing a large team into multiple small teams, each with its own leader. This design combines some of the advantages of distributed and centralized, retaining a certain degree of independence and facilitating modular management and maintenance.
BMS battery management system plays a crucial role in new energy vehicles, ensuring the safe and stable operation of batteries with powerful functions and intelligent management modes, and promoting the development of the new energy vehicle industry. With the continuous advancement of technology, future BMS will be more intelligent and efficient, bringing better performance and user bodies to new energy vehicles
