Exploring The BMS Technology Of Lithium Battery For Two-wheelers

Exploring the BMS technology of lithium battery for two-wheelers

The partial replacement of lead-acid batteries by lithium batteries is a trend, and a consensus has gradually formed. Especially in the field of electric bicycles, as the new national standard for electric bicycles made technical decisions, lithium batteries began to accelerate their entry. The market demand for electric bicycles has risen strongly. This kind of policy resonance with the market has brought a huge new market space for lithium batteries.

The replacement of lead-acid batteries by lithium batteries will cause major changes in the existing market supply and demand pattern, not only on the product and technology side, but also on the entire supply chain system, business model, and operating model.

The following is the sharing of the topic "Discussion on BMS Technology of Two-wheeled Vehicle Lithium Battery" made by Dr. Yang, general manager of FIRSTEK.

  FIRSTEK is an enterprise specializing in R&D, production and innovation of battery management system platform technology and battery big data technology. The products are mainly used in civil industry and power plant energy storage power supply, pure electric two or three wheels, auxiliary robots and military power supply fields. At present, some products have been exported to Europe, America and other countries. As early as the beginning of 2018, FIRSTEK began to customize and develop smart protection boards for the two-wheeler shared battery pack market, and gradually batches were followed. More than 100,000 sets of products have been used on the market terminals.

 The first aspect is the current industry situation. At present, two-wheeler batteries mainly include two directions: first, the lead-acid change to the lithium battery market; second, the lithium battery market. In the lead-acid change to lithium battery, the original product-shaped interface on the car is used. The BMS product is based on a pure hardware protection board solution. It is difficult to achieve communication functions. At the same time, it is easy to ignite during use, and it takes a long time. Cause damage to the connector. In addition, because it does not have the communication function, the controller cannot communicate with the battery pack, and the vehicle cannot achieve limited power operation. In terms of lithium batteries, most of the BMS interfaces have communication functions and can be used to communicate with controllers and meters. Generally speaking, not only the current, voltage and fault information can be displayed on the meter. At the same time, through the information interaction between the BMS and the controller, output power adjustment, data interaction, etc. can be achieved, which greatly improves the overall performance of the vehicle. This type of vehicle usually uses intelligent protection board products.

In the second aspect, we will introduce the wake-up technology of the smart protection board. Two-wheeled electric vehicles seem simple, but the actual application scenarios are a bit more complicated than cars. Next, I will introduce the principles and application scenarios of several wake-up methods:

    1. Switch to wake up. Through the auxiliary interface on the interface, the switch status of the two nodes is used to let the intelligent protection board recognize that the battery pack is on the car or the charger and during transportation. The most obvious advantage is that the battery pack can be placed on the ground or during transportation to ensure that the main line interface of the battery pack is not charged, which is of great benefit to battery safety. If the BMS does not have the recognition function, the P positive and P negative of the battery pack are likely to cause safety hazards when the battery pack is always charged. Through the simplest switch wake-up function, it can easily solve the problem of interface charging. At the same time, it can also solve the power-on pre-charging function, avoiding the ignition of the battery pack due to the charging process.

    2. Load wake up. This application is related to the back-end load. Generally, P positive and P negative are used to detect whether the back-end has a load to determine whether it is in the state of the car to wake up the management system. This function is simple to do, but there are more considerations in practical applications. It is not a simple load detection, just after waking up, because there is no other signal input, so as a BMS, it can detect when it is awakened, but It is impossible to detect the load removal information of the car. If you want to know this information, you need to have other wake-up methods combined with this wake-up method, otherwise the load wake-up function alone cannot achieve low-power sleep. .

    3. Wake up after discharge. This refers to the wake-up by the discharge current. The load wake-up mentioned earlier is used to detect whether there is a load. Discharge wake-up refers to wake-up by detecting the magnitude of the discharge current. Generally speaking, the battery is placed in the car. As far as the electric motorcycle is concerned, although the user has no use for a week or two, the battery is always plugged in the car. In this state, the power consumption of the BMS itself will cause When the battery is fully charged, it lasts for about 40 days at most. In order to be able to extend the use time, we will do some sleep work, for example, how long does the car go to sleep if it is not used, and how to wake it up with BMS after entering the sleep state? At this time, the current mode can be used to wake up.

    4. Wake up when charging. The BMS is awakened by the voltage output by the charger. However, it should be noted that the charger for charging and wake-up cannot be the kind of passenger car that needs to exchange data before outputting the charging voltage. The charging wake-up requires that the charger's working method is to provide a charging voltage to wake up the BMS, and then transfer to the normal charging process after data exchange. The biggest advantage of this wake-up function is: insufficient battery power leads to undervoltage, and the BMS cannot work automatically. After waking up by charging, the BMS can work normally. This method is very useful for undervoltage protection. But in order to charge more reasonably, we generally recommend that when customers do it in this place, first let the charger go through a small current limit charging, and then switch to normal current charging after interacting with the charger data.

5. Communication wake up. Generally refers to waking up the BMS through data communication. In the two-wheeler electric motorcycle project we contacted, from the low-cost 485 communication to the current common CAN communication, it is also common to wake up the battery management system (BMS) through these communication methods.

    6. Vibration wakes up. It is a way to wake up by adding a vibration sensor to the BMS. Generally speaking, BMS is easy to do sleep. In order to save power on the electric motorcycle, the BMS will automatically enter the sleep mode according to a certain strategy, but under what circumstances will it wake up? If a high-current wake-up method is used, the cost of the design is actually relatively high, and the technical indicators are also relatively difficult. A simple method can also be achieved through vibration wake-up.

    7. Open the cover to wake up. Mainly refers to the packaged battery pack is used to record abnormal events when it is abnormally opened. This feature is usually found on small battery packs. The electronic locks of Mobike and OFO bicycles are equipped with this function, mainly to prevent users from misusing the product or opening the product cover without permission. The realization of waking up when the cover is opened is generally realized by using a light sensor. Usually, the BMS is installed inside the battery pack without light. The BMS can realize the waking up function when the cover is opened by detecting changes in light.

    8. Remote wake up. This function means that the user realizes the wake-up function of the BMS by adding a remote data module. Usually used for two-wheeler leasing. During the leasing process, the user does not pay on time and on schedule. The operator can lock the battery pack remotely, and the BMS will also enter the dormant state. In this case, the BMS can use remote wake-up to achieve the purpose of reuse. On the other hand, when the battery has not been used for a long time, such as being placed in a corner by the customer, in this case, the BMS can be remotely awakened to find the battery pack and the status of the battery pack can be remotely monitored, and the current status can be transmitted to the server To avoid waste of battery pack resources and over-discharge of the battery caused by long-term storage.

The third part is the calculation of SOC for two-wheeled vehicles. In fact, this aspect is a relatively hot topic in passenger cars, and it is more difficult in terms of two-wheelers than in passenger cars, because the abuse situation is more complicated. The calculation of SOC generally includes the following methods: first, ampere-hour integration method; second, reset to full calibration strategy; third, OCV calibration; fourth, dynamic compensation and calibration.

The following is a list of common factors affecting SOC calculation in the use of two-wheelers.

In the application of two-wheeled vehicles, the problem is highlighted because of the SOC error introduced by the use of shallow charging and shallow discharge. Most users use the battery pack after it is fully charged. However, when two-wheelers are used, they often recharge when they are out of power, and almost ride away when they are charged. Generally, the battery pack cannot be fully charged, especially in shared battery swap applications. For example, when express riders use shared battery packs, in order to ensure convenient transportation, they will change to a battery pack with more capacity when they see the battery cabinet, which will cause the battery to always be in a state of shallow charge and shallow discharge. The influence of the error of the SOC of the two-wheeled vehicle is relatively large.

Second, the influence of ambient temperature and discharge rate on the battery's own capacity. Electric motorcycles have high temperature and low temperature conditions when they are driving. These conditions have a greater impact on the battery itself. As a BMS, the original data we can monitor is voltage, current, temperature and other information, but there is no way to control the battery. Its own capacity does not decay, so the external environment and the usage habits of different riders have a great influence on the battery's own capacity.

Third, the battery cycle life. As the cost of using batteries for two-wheeled vehicles is lower than that for passenger cars, the cycle life of batteries for two-wheeled vehicles is generally shorter than that of passenger cars. Therefore, different manufacturers need to pay attention to the cycle life of batteries according to different models and different customer groups.

    Fourth, the inconsistency of the batteries. Since the capacity of the two-wheeled vehicle battery pack is generally not very large, but the charging and discharging power is not very small, the consistency of the battery core is relatively easy to appear. Especially after half a year and a year, there will be a big difference in battery cell voltage, which will seriously affect the estimation of SOC.

Fifth, the impact of BMS current and voltage acquisition accuracy on SOC estimation. BMS needs to obtain some raw battery pack data for SOC estimation. However, in the two-wheeled vehicle BMS, in order to better meet the customer's low-cost requirements for BMS, some accuracy must be given up sometimes. But how much accuracy should be reduced? This also needs to consider the degree of influence on SOC.

On the other hand, the power consumption of the BMS itself also has a greater impact on SOC estimation. For BMS applications in the automotive field, the BMS can achieve zero power consumption after the key is turned off. Once the low-voltage power is turned off, the BMS will shut down without power consumption. But in low-power products, BMS is not easy to achieve zero power consumption.

BMS sleep is generally divided into deep sleep and shallow sleep. When entering deep sleep, it can be below 20 mA. If you calculate according to the power consumption current of 10 mA, you will find that the battery power is about 40- after a long time. About 50 days, the battery pack is basically consumed. So when we calculate the SOC, we need to include the power consumption of the BMS itself.

The fourth aspect is the new infrastructure for two-wheelers. The service platform of the two-wheeled vehicle is the remote data monitoring platform. At present, more data collection and collection work is done. It is further necessary to estimate the SOH of the battery cell and the PACK package, which can provide early warning to the user, avoid the battery, and There are adverse effects on the user's use.

In fact, we found a problem in the project we contacted before, and we need to put forward different requirements for the remote data transmission function according to different usage scenarios. For example, in terms of passenger cars, the state later unified the proposal to upload data to the big data platform for unified supervision, but for the application of two-wheeled electric motorcycles, is the remote data transmission function really necessary? We know that the remote data transmission function will increase the cost. The current 2G card telecom operators will no longer operate in the near future. In addition to the high power consumption of a 4G module, the cost is also relatively high, compared to the cost of a small-capacity battery pack. In other words, the cost of installing a remote data transmission module is very high. Some customers increase the purpose of remote data transmission to prevent the loss of battery packs. However, after one or two years of statistics, it is found that even if the value of the lost battery pack is directly paid, it is still less than the cost of adding a remote module to each battery pack. Therefore, adding remote data transmission functions in the field of two-wheelers is not so meaningful at present.

    thank you all!