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What is The Purpose of Lithium Battery Aging Test?

What is The Purpose of the Lithium Battery Aging Test?

If you keep an emergency stock of unused batteries at home, you may have found, in your hour of need, that some of the batteries need to be fixed. In the world of Lithium ion batteries, old is not gold! Like human beings, batteries lose their capacity with age. Just as the life expectancy of human beings depends on their health, the aging of Lithium-ion batteries depends upon their State of Health (SoH). Aging tests are used to assess the battery’s SoH and predict its lifespan.

Before delving deeper into the subject of aging tests, let us understand a bit more about battery aging. Figure 1 illustrates typical aging pattern observed in Lithium ion batteries.

The aging pattern displays three phases. The first phase, termed “initial settling” shows a slight capacity rise, even though the battery is under use. This may be the contribution of extra Lithium ions parked in the Anode structure, beyond to active electrode area, known as Lithium overhang. The next phase of aging shows a linear rate of capacity decrease. The battery starts running-out more rapidly than when it was fresh, but it is gradual. The final stage of capacity loss occurs at the “knee point” which may occur at 50-85 percent of initial capacity. At this stage the capacity decline rate is very steep and the battery’s useful life is over.

The two main forms of aging are Calendric aging and Cyclic aging. Calendric aging happens even when the battery isn’t being used. It depends upon the temperature and the State of Charge (SoC). At higher temperatures, the battery ages faster. Similarly, a battery kept constantly at a high SoC will have a shorter calendar life than a battery normally operated at a lower SoC.

Cyclic aging occurs due to battery usage. This is due to irreversible chemical and physical deterioration of the battery’s internal structures during every charge and discharge cycle. The depth of discharge (DoD)during each cycle is a factor. Deep discharge during each cycle will cause rapid aging. Hence DoD must be kept within moderate limits. This illustrated in Figure 2 .

Battery age is something that users would like to track. However, during usage, it is not practical to keep track of cell aging variables like number of cycles, SoC, DoD and operating temperature. The best time to perform aging tests is when the battery is new and ready for packing at the factory. Battery aging test are therefore, mainly performed at the factory, for quality assurance. Once the batteries have been sold, then some customers may choose to do specific tests based on their requirements, such as for electric vehicles or storage batteries. In the following sections, you will learn about battery aging tests that should be performed at the factory, when the batteries are new.

What Is The Aging Test

The aging test is a limited duration test to gauge the performance and aging characteristics of a battery under controlled conditions.Different operating conditions are imposed upon the test samples, and expected lifetime is estimated. Test practices vary in different countries and among manufacturers. The general methodology is to employ accelerated aging techniques, together with software models that can translate the test data to characterize aging parameters. The use of software models is necessitated as the aging test must be able to predict the life of the battery under various scenarios.

But how to accelerate battery aging for test purposes? This is done by placing the battery under increased stress for the test duration.

For example, elevated temperature will accelerate calendar aging. Cyclic aging can be speeded up by manipulating charge and discharge cycles, for different DoD, charge and discharge rates (C-rates).

Purpose of The Aging Test

The battery aging test is a quality control step. It provides a scientific basis to specify the expected calendar life and cycle life of a battery.

Importance of The Battery Aging Test

Consumers need to have confidence that new batteries will perform satisfactorily over their expected lifetimes. Nobody wants to purchase a battery that is packaged as new but turns out to be at the end of its life. This is a common occurrence with cheaper quality Lithium ion batteries. Hence battery aging tests are important as they provide the necessary quality assurance on life expectancy.

What Are The Ways Of Battery Aging Test

Battery aging tests for batch quality control at the factory must be fast and accurate. Representative samples from every production batch have to be tested. While the required test measurements can be easily obtained, predicting battery lifespan based on the data is complex. Sophisticated software models are used for this purpose.

In this section, we look at testing of Lithium ion batteries for the general purpose consumer market. The tests required for automotive and large industrial battery users will be more stringent and take several weeks and even months. Such specialized aging tests are not discussed here.

A new, unused, battery gradually loses capacity due to calendar aging, depending on temperature and SoC. Once the battery enters active service, aging is greatly influenced by variable dynamic factors which cannot be reliably reproduced in a factory setting. Hence for normal consumer applications, the battery aging test is designed to measure capacity fade. The aging test has three stages as described below:

  • The first stage involves baseline measurement of battery capacity. These measurements are done at temperatures representing possible storage conditions, typically 25 oC and 45 o
  • In the second stage, the batteries are subjected to accelerated aging.
  • Capacity fade is induced by repeated constant current and constant voltage charging and discharge cycles. The C-rate is also changed from high to low. For example, the cycling sequence for a test batch of 18650 Lithium ion batteries could be as follows:
    • Charge at a constant current (CC) of C/3 to a cut-off voltage of 4.2 V, followed by a constant voltage (CV) step at 4.2 V until the charge current drops to 0.1 A.
    • Give a Rest period of 1 minute
    • Perform the discharge at a constant current typically 1C for normal discharge and 4C when doing high discharge tests. The constant current should be maintained till the voltage drops to 2.5V.
    • Give a rest period of 1 minute
    • Repeat the above sequence 30 times at 1C rate and 40 times at 4C rate.
  • The third stage involves repeating the capacity measurements for each battery. There repeat measurements are called Reference Performance Test (RPT). The change in capacity indicates how much the battery has deteriorated due to aging.
  • Thereafter, software models are used to analyse the data and make predictions regarding future life of the battery.

The aging test bench consists of climate-controlled testing machines with several battery testing channels, one for each cell. Everything is hooked up to computers which process the data the aging

Figure 3 shows a typical Lithium ion battery age testing machine for normal factory use.

Why Is It Necessary To Do The Aging Test After The Battery Has Been Categorized And Divided?

The aging test is done after manufacturing is complete, because the consumer is going to buy only the finished product. Hence the test samples must be already segregated into the respective sales categories.

How To Quickly Determine Aging Degree Of Lithium Battery Pack?

The capacity tests that have been mentioned above are the fastest way to obtain relevant measurement. At the same time, appropriate age testing software must be used, which can analyse hundreds of aging scenarios, for new as well as old batteries.

 

What Other Tests are Needed Besides Aging Tests

Routine quality control checks  for all Lithium ion batteries are as below:

  • Visual Check:  There should not be any visible defects such as deep scratch, crack, rust, discoloration or leakage.
  • Dimension Check: The battery dimensions should match the specified format.
  • Battery Power test: This is done by charge and discharge tests. This test is not done on every battery, a representative sample size of each production batch can be selected. The standard charge involves charging at constant current of 0.5C and also by charging at constant voltage of 4.2V with tapering current, finally stopping at 50 mA. The standard discharge is involves discharging at a constant current of 0.2C, till a voltage of 2.50V is reached.
  • High Drain Charge/Discharge test: This test is also done on a representative set of batteries from each production batch. The charging rate in this case is at constant current of 0.5C till voltage of 4.20V, and end current of 50mA. Cells are then discharged at constant current of 0.5C till a low voltage of 2.50V. Cells are rested for 10 minutes after charge and 20 minutes after discharge.
  • Cycle Life: Representative test cells are charged and discharged as per the high drain test, for 500 cycles. A cycle is defined as one charge and one discharge. The 501st discharge power should be greater than 70 percent of the initial battery power.
  • Storage test: Sample cells are charged at standard rate and stored in a temperature-controlled environment at 23ºC ± 2ºC for 30 days. After storage, cells are discharged at the standard rate. The residual power should be greater than 90 percent of initial power.
  • High Temperature test: Sample cells are charged at standard rate and stored in a temperature-controlled environment at 60ºC for 1 week. After storage, the cells are discharged at standard rate and then cycled for 3 cycles to obtain recovered power. The power recovery should be greater than 80 percent.
  • Drop test: Sample cells are charged at standard rate and are dropped onto a wooden floor from 1.0 meter height. A total of 3 drops are performed, comprising 2 drops from each cell terminal and 1 drop from the side of cell casing. There should not be any leakage or temperature rise.
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