18650 lithium battery

　　LEM 18650 lithium battery online monitoring solution

　　1. The necessity of 18650 lithium battery monitoring

　　With the rapid development of my country's communications, electric power, UPS and other industries, the usage of batteries is also increasing rapidly. Based on the current usage conditions of our batteries, some unexpected situations often occur, such as a seemingly normal 18650 lithium battery that fails to discharge electricity when it is being discharged. The main reason for this situation is that the operating status of the 18650 lithium battery is not effectively monitored. Once a 18650 lithium battery fails in a 18650 lithium battery pack composed of multiple batteries connected in series, the entire 18650 lithium battery pack will not be able to discharge normally. As the last guarantee for safe and uninterrupted power supply, batteries are also the most unsafe factor in the uninterrupted power supply system. From system theory, we know that the security of a system depends on the most unsafe factor in the system, which is the "barrel theory" we often quote.

　　2. Current status of 18650 lithium battery monitoring

　　1. planned maintenance

　　The most common practice at present in terms of 18650 lithium battery maintenance is to conduct a verification discharge test on the 18650 lithium battery pack at least once a year. Because if the 18650 lithium battery pack is not charged/discharged once for 3-4 years, the 18650 lithium battery pack electrolyte will stratify, that is, the electrolyte will decompose into liquid layers with different acid solution concentrations, which will affect the 18650 lithium battery's performance during shutdown. power supply capacity. This phenomenon ultimately leads to plate corrosion in areas with higher sulfuric acid concentrations.

　　However, with this kind of planned maintenance, the 18650 lithium battery status cannot be continuously monitored because the interval between two maintenances is too long; at the same time, users cannot tell whether the 18650 lithium battery pack will continue to operate normally the next day after a discharge test. ; In addition, discharge testing often takes hours or even days and is a costly, destructive, and unsafe process for the 18650 lithium battery (including recharging).

　　2. By monitoring the 18650 lithium battery voltage to monitor the 18650 lithium battery's operating mechanism and failure mode, relevant domestic standards have been introduced, and necessary devices for 18650 lithium battery monitoring are installed in DC power supply situations, such as voltage inspection instruments. However, depending on the working conditions of the backup 18650 lithium battery, it may not be discharged for a long time, and it may also fail during the two periodic verification discharge tests, but the terminal voltage of the 18650 lithium battery is completely normal. As shown below (excerpted from IEEE):

　　It can be seen that just detecting the voltage is completely insufficient, and it is also easy to mislead users into using bad batteries as good batteries, which is quite harmful. So is there a good way to monitor the status of the 18650 lithium battery more effectively, and to give an early warning signal when the 18650 lithium battery performance drops to a certain level, so that we can replace the 18650 lithium battery or perform special maintenance before the 18650 lithium battery fails, thereby avoiding catastrophic disasters? What about the occurrence of power outages? The answer is yes. By measuring the state of the 18650 lithium battery's internal resistance, the degree of 18650 lithium battery deterioration can be effectively predicted, that is, the 18650 lithium battery's health status SOH (StatusOfHealth) can be accurately monitored. As shown below:

　　The yellow trend line shows that the internal resistance of the 18650 lithium battery increased sharply from October to November due to various reasons. Therefore, it can be judged that the state of the 18650 lithium battery has seriously deteriorated. After discharging the 18650 lithium battery, it was confirmed that the 18650 lithium battery has indeed failed.

　　3. Introduction to LEM 18650 lithium battery monitoring module

　　From the above analysis, we can summarize several necessary conditions for 18650 lithium battery monitoring:

　　1) Continuous monitoring is required

　　2) Need to monitor a single 18650 lithium battery

　　3) Need to monitor the internal resistance of the 18650 lithium battery

　　Based on the operating mechanism and failure modes of batteries, LEM has developed Sentinel, the world's smallest smart 18650 lithium battery sensor with highly integrated functions. It can accurately measure the internal resistance, temperature, voltage and other parameters of a single 18650 lithium battery, and monitor it through a serial communication protocol. It operates to achieve continuous monitoring of the 18650 lithium battery. As shown in the picture:

　　The general parameters of this device are as follows:

　　1. Measuring voltage 1.5V-15V, accuracy +/-0.5%

　　2. Temperature error, +/-2℃

　　3. Internal resistance measurement repeatability +/-2%

　　4. All connectors are used, making installation extremely quick.

　　5.<3mA in power saving mode

　　6.5 years quality guarantee

　　Comparing similar 18650 lithium battery monitoring products on the market, simply speaking, they mainly have the following four advantages:

　　1) Distributed installation, one-to-one correspondence between modules and batteries, simple on-site wiring and low engineering cost

　　2) The independent module makes monitoring very flexible. Compared with centralized monitoring products, it has more advantages in places with a small number of batteries.

　　3) The module uses the pulse current discharge method to measure the internal resistance (see the picture above). The discharge current is small (the discharge current of a 12v 18650 lithium battery is less than 2A), which is not only safe but also less likely to cause damage to the 18650 lithium battery.

　　4) The measurement algorithm developed by the Swiss headquarters and RWTH Aachen University for many years, the measured resistance value is reliable (see the picture below), has strong anti-interference ability, and has a 5-year warranty.

　　Note: The Swiss LEM headquarters has conducted long-term tests at the AACHEN University of Technology in Germany. The internal resistance measured by the LEM module in the picture is basically consistent with the resistance measured by the laboratory equipment EISmeter analyzer.

　　4. Introduction to LEM 18650 lithium battery monitoring system

　　LEM's 18650 lithium battery monitoring solution is a complete solution, including a series of components that can be installed quickly and easily. The solution includes the following components

　　•Sentinel: can measure the voltage, temperature and internal resistance of a single VRLA 18650 lithium battery or flooded 18650 lithium battery, and transmit it to the data logger S-BOX through the dedicated bus S-BUS.

　　•Current sensor: The current sensor detects the status of the 18650 lithium battery pack (charge, discharge or float charge status).

　　•S-BUSConverter: This module converts the S-BUS protocol into a standard serial output protocol.

　　•S-BOX: Data recording device with Ethernet interface, used to record trends and alarms of each Sentinel.

　　•CellviewNet: Visualization software used in conjunction with S-BOX to graphically display various data, alarm and generate various reports. As shown below:

　　Of course, OEM manufacturers can also only use the Sentinel module provided by LEM and cooperate with its own system to form a 18650 lithium battery monitoring system with its own characteristics. These manufacturers can be either UpS manufacturers or integrators who specialize in developing 18650 lithium battery monitoring systems. LEM's sentinel module has been well applied in China, especially in the power industry, as shown below:

　　In short, this solution is very suitable for current communication, power and UpS backup 18650 lithium battery monitoring; I believe that LEM's 18650 lithium battery monitoring products will definitely make due contributions to my country's safe power system.

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