The newly implemented technology can help you identify whether the battery's self-discharge performance is good in just a few minutes, instead of spending several weeks as before, so it can help you save costs and speed up the time to market.
How to shorten the self-discharge test time of lithium batteries?
The newly implemented technology can help you identify whether the battery's self-discharge performance is good in just a few minutes, instead of spending several weeks as before, so it can help you save costs and speed up the time to market.
With the popularity of electric vehicles, the lithium battery market is experiencing explosive growth. This trend requires higher battery capacity and better performance. Optimizing the time and cost of battery evaluation becomes critical. Battery self-discharge performance evaluation has an important impact on battery cost and time to market, but the evaluation process is very time-consuming. Now a new test method has emerged that can greatly save evaluation time and speed up the time to market.
What is self-discharge of lithium battery? What is the significance of it?
Even if no load is connected, the storage capacity of the lithium battery will gradually decrease. This process is called self-discharge. Figure 1 shows the self-discharge model. The self-discharge current ISD flows through the parallel resistor RSD. When no load is connected, the battery is discharged through a high value RSD. After several weeks or months, this self-discharge path will consume a considerable part of the energy stored in the battery, resulting in a drop in VCELL.
Figure 1. Self-discharge battery model.
A certain amount of self-discharge is a natural result of chemical reactions occurring in the battery. The gradual loss of stored energy will cause the battery's usable capacity to be lower than expected. When multiple batteries are assembled into a battery pack, the difference in the battery self-discharge rate will cause the cells in the battery pack to appear unbalanced.
If there is a leakage current path inside the battery, it can also cause self-discharge. Particulate contaminants and dendrite growth will create "micro short circuits" inside the battery, thereby forming this leakage current path. These are not normal phenomena, and they may cause catastrophic failure of the battery. A battery with excessive self-discharge indicates a possible malfunction.
Therefore, it is very important to measure and evaluate self-discharge during battery design and manufacturing. In the battery design process, the first task is to eliminate factors that may cause excessive self-discharge. During the manufacturing process, any batteries that exhibit excessive self-discharge must be screened out as early as possible.
Challenges and Defects of Open Circuit Voltage Method
Usually, testers evaluate the self-discharge performance by measuring the characteristics of the battery's open circuit voltage (OCV) drop over time. It is quite easy to measure with a voltmeter or digital multimeter. The challenge is not how complicated the measurement is, but that it takes too long to evaluate the battery's self-discharge performance based on changes in OCV.
Compared with other types of rechargeable chemical batteries, the degree of self-discharge of lithium batteries is relatively light. Usually only about 0.5% to 1% of electricity is consumed every month. Because ISD is very small, it generally consumes a few microamperes to hundreds of microamperes (depending on the size of the battery), and the battery voltage drops very slowly. Since the OCV change of lithium batteries is very small during discharge, it takes several weeks to several months to detect the battery's state of charge (SOC) with a large loss, so as to distinguish qualified batteries from unqualified batteries with excessive self-discharge Open.
It is a challenge for designers, users and manufacturers to use the OCV method to quickly measure the self-discharge characteristics of their batteries. The time spent on single cell measurement is not very long, but if a series of such measurements are performed continuously in weeks or even months, it will have a great impact on the design cycle. In this evaluation process, the designer must store the battery and track its state under temperature-controllable conditions, because the battery voltage will change at different temperatures. This not only limits the design cycle, but also limits the time to market. If a design iteration requires multiple test cycles, the delay time will increase exponentially with the number of test cycles. Delays in the first delivery of a new design will not only lead to the loss of market opportunities, but may even give up market share to competitors.
In the manufacturing industry, self-discharge characterization has greatly increased the number of products in process (WIP). At the same time, the complexity and risk of production have greatly increased due to the long-term storage of a large number of batteries. For larger capacity batteries, this problem is even more serious. This type of battery is a higher-value inventory, and its stabilization time is longer than that of a small-capacity battery, which brings more risks to the inventory.
Potentiostat
To measure the self-discharge performance of the battery, you are most likely to directly measure the self-discharge current of the battery. Under static conditions, if this current can be measured, it will tell you whether the battery is qualified faster than waiting for the OCV to send a change. The OCV method measures the change in battery open circuit voltage over time. This is an indirect method and cannot accurately indicate the battery's self-discharge rate.
The potentiostatic method is shown in Figure 2. This method evaluates the self-discharge degree of the battery by measuring the self-discharge current ISD. The self-discharge rate is directly expressed in coulombs per second. In other words, it measures the amount of charge lost over time. Compared with the OCV method, this method can evaluate the battery in a shorter time. It only takes a few hours or less to determine the self-discharge current. It takes less time to select batteries with excessive self-discharge current, generally less than one hour.
Using the potentiostat method, a low-noise, very stable DC power supply can be set to match the OCV of the battery. Then connect the DC power supply to the battery through a microammeter to measure the current between the DC power supply and the battery. When the battery continues to self-discharge, the DC power supply will intervene to provide enough current to keep the battery at a constant voltage and SOC. When the DC power supply and the battery reach an equilibrium state, the ISD is converted from the battery internal to the DC power supply completely externally provided. Then, ISD can be measured directly using a microammeter.
Figure 2. The potentiostat method measures the self-discharge of the battery by measuring ISD.
New potentiostatic solution
Keysight has worked with battery designers and manufacturers to cope with the challenges of self-discharge measurement and created two new solutions: BT2191A self-discharge measurement system and BT2152A self-discharge analyzer. Both solutions use potentiostatic technology to provide microvolt stability and resolution, as well as multiple functions to quickly and accurately measure the battery's self-discharge current.
The design of BT2191A takes into account the needs of designers and can significantly shorten the time required to measure battery self-discharge current. It directly measures the self-discharge current, which can be completed in 1-2 hours, eliminating the need to spend weeks or months to monitor the open circuit voltage of the battery. In addition to self-discharge current, it can also measure battery voltage and temperature. Engineers can greatly shorten the design cycle, help optimize the battery's self-discharge performance and characteristics, and shorten the time to market.
For lithium battery manufacturers, BT2152A provides a new type of analyzer that can measure the self-discharge current of up to 32 batteries at the same time. In addition to the significant time savings using new technologies, test throughput has also been greatly improved. More importantly, it is usually less than 30 minutes to clearly distinguish the self-discharge current of a normal battery from a bad battery. This helps manufacturers significantly reduce WIP inventory, save working capital and facility costs. Figure 3 shows the self-discharge current of eight 18650 batteries recorded at the same time. You can find the battery with the greater self-discharge in this group of batteries within a few minutes.
Figure 3. Use BT2152A to distinguish between normal batteries and batteries with excessive self-discharge.
in conclusion
It is very important to test the self-discharge characteristics of lithium batteries. Excessive self-discharge indicates a possible malfunction. Designers and manufacturers must identify these batteries and isolate them from normal batteries to prevent catastrophic failures, and identify the root causes of problems and correct them during the battery design or manufacturing process. Keysight's new constant potential solution enables designers and manufacturers to significantly shorten the time and cost of battery self-discharge measurement, thereby solving the challenges brought by battery self-discharge measurement, and ultimately speeding the market to market.
