Battery Test Recipes (4) Rest & OCV & Pattern


Editor : Mr.Overain
2017/11/22

 

 In this article we will discuss monitoring a cell while at rest. We will also discuss how the users can create their own input current patterns to examine the response of the cell.



(Figure 1) Recipe icons for Open Circuit Voltage, Rest and Pattern


 The simplest recipe to monitor a resting cell is the Rest recipe. The name is quite self-explanatory since during Rest there is no input signal to the cell. However it is still possible to monitor the potential difference across the terminals.?


 Monitoring the open circuit voltage (OCV) across a cell is a popular method of examining the cell status. Measuring the change of the OCV has the potential to reveal many properties of the cell including the state of charge, amount of leakage and many more. A limiting factor is that the OCV typically changes in a very small-scale such that a highly precise measurement device is required. To overcome this obstacle McScience’s battery parameter tester provides ‘dc-cancelling’ technology which allows a precise measurement in the range of tens of microseconds to be possible.?


 Unlike the two recipes above Pattern recipe exists for the testers to customize their own input signals instead of preset current DC steps and AC signals. Users can create their own pattern of input current by setting the current value and the width of each step. Composed pattern can be saved as a file to be loaded for later use.


(Figure 2) OCV recipe measurement properties


 Metadata for the Rest, OCV and Pattern recipes consist of a common part and a unique part. The common part records the experiment background information including the sample information and the test log, which were elaborated in the previous article “Test Recipe (3) Metadata”.?


 The unique part is where the raw result data for the OCV recipe are converted into metadata files. A group of chosen raw data are directly extracted to be a part of the ‘Extracted Data’ section of metadata. These include the voltage values at certain times. The ‘Characteristic Parameters’ aim to fully represent the result by recording initial and final voltage values as well as the predicted OCV value after total relaxation. Such prediction can be achieved by curve fitting with a model function to calculate the asymptote of the voltage plot. Naturally the time constant of such a function is extracted. The time constant can act as a simple parameter to compare various OCV profiles between different samples.?
Due to their unique nature, Rest and Patter recipes do not have characteristic metadata.?


(Figure 3) An example of the Extracted Data section for the OCV recipe



(Figure 4) An example of the Characteristic Parameter section for the OCV recipe


 The EigenPlots for Rest, OCV and Pattern recipes are simply the plot of monitored voltage against time of measurement. In the case of Pattern the input current is also shown.?


(Figure 5) Examples of the EigenPlots for the OCV and Pattern recipes respectively.


 Thank you for reading this article and we will discuss the structure of MetroSequence in the next article.?


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