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Lithium-ion Battery Testing Process

Testing process for lithium-ion batteries play a crucial role in evaluating battery performance and reliability.

Whether used in portable electronic devices or electric vehicles, the performance of lithium-ion batteries must undergo comprehensive testing under various conditions.

Here, we will introduce a series of steps for testing lithium-ion batteries to ensure their outstanding performance in real-world applications.

Lithium-ion Battery Testing Process

Step 1 : Extraction

Initiating the lithium-ion cell salvaging process involves the initial removal of the battery pack from its original casing.

Typically, this extraction is performed with tools such as pliers or wire cutters, meticulously disassembling the battery pack to retrieve the cells.

After extraction, a visual inspection of the cells is conducted to assess their condition.

Lithium-ion Battery Testing Process-3

Step 2: Visual Examination

Following the removal of cells from the battery pack, a meticulous visual examination is conducted to ascertain their physical integrity and the absence of any leakage indicators.

This step is of paramount importance in ensuring the safety and reliability of the cells.

A comprehensive visual inspection is often adequate to identify and segregate cells showing signs of significant deterioration. It is essential to scrutinize the cells for common indicators of degradation, including dents, which are infrequent in cylindrical cells, and evidence of dried substances.

Any cells exhibiting damage indications, such as cracks, dents, or leakage, must be promptly discarded.

It is important to note that not all marks and discolorations signify damage, as some can be remedied with cleaning.

While a minor scratch on the cell’s wrapping may not raise significant concerns, a cell that is swollen, leaking, or discolored should be promptly discarded.

In the case of leakage, it may emit a somewhat sweet odor.

Lithium-ion Battery Testing Process-2

Step 3: Cleaning

The subsequent phase of this procedure involves the meticulous cleaning of the cells to eliminate any residual glue, paper, or tape that was previously utilized to secure the battery pack.

This cleaning operation serves to mitigate any additional contact resistance and prevent potential contamination that could compromise cell performance.

The cleaning process can be executed using an appropriate solvent or specialized cleaning solution, tailored to the nature of the residue.

An abrasive metal scrub brush can prove invaluable in this process. In cases where adhesive substances like tape and hot glue present difficulties during removal, the application of controlled heat via a heat gun may facilitate detachment.
It is important to exercise caution when using heat, as prolonged exposure can be detrimental to battery cells. Generally, a brief application of surface heat is adequate for loosening adhesives and glues.

Once the cells have been meticulously cleaned, they are prepared for the charging phase.

Step 4: Charging

Subsequent to the thorough cleaning of the cells, proceed to charge them to full capacity while carefully monitoring their temperatures.

This step plays a crucial role in evaluating the cell’s overall health and verifying its ability to retain a charge.

If any cells exhibit abnormal warmth during the charging process or fail to retain their charge, it is imperative to promptly dispose of such cells.

These observations can serve as indications of cell damage or manufacturing defects, rendering them unsafe for further use.

Once the cell reaches full charge, you can assess its self-discharge behavior.

Step 5: Self-Discharge Test

Allow the fully charged cells to remain stationary for approximately one week.

This waiting period serves to assess the cells’ capacity to retain their charge over time. Cells displaying a voltage drop exceeding 0.1 volts after one week of storage should be promptly disposed of.

After confirming that the cells exhibit no issues with self-discharge, you may proceed with capacity testing.

Step 6: Capacity Testing

The subsequent phase in the process involves conducting a capacity test on the cells and labeling them to confirm their ability to store the specified amount of energy.

This step is of paramount importance in assessing the viability of the cells.

Cells that fall below 90% of the original manufacturer’s capacity may not have the potential to supply adequate energy for practical applications.

After fully charging the cells, they are left undisturbed for an entire week.

If any cell’s voltage decreases by more than 0.1 volts during this time, it is necessary to dispose of the affected cells.

- Significance of Capacity Testing

Capacity testing holds immense importance due to its direct relevance to the operation of Battery Management Systems (BMS).

Failure to ensure uniform capacity across cell groups may result in one group reaching a full charge or becoming depleted earlier than the others.

To maintain safety, the BMS will disengage the battery if any individual cell group approaches the over-discharge or over-charge thresholds.

- Factors Behind Voltage Decline in Faulty Batteries Over Time

One of the primary factors contributing to the gradual voltage drop in deteriorating batteries is internal discharge.

As a cell experiences wear and tear, its electrodes slowly shift towards each other. While the cell incorporates mechanisms to prevent direct contact, this phenomenon eventually occurs.

It leads to a gradual self-discharge within the cell, which intensifies over time.

- Do High-Quality 18650 Cells Also Undergo Voltage Reduction?

Indeed, they do, although to a minimal extent.

Various factors contribute to the gradual voltage reduction in batteries over time.

All cells manifest a degree of self-discharge, but in the case of robust cells, this process transpires at a notably sluggish pace.

Anticipate a voltage decrease of approximately 0.5% to 2% per month for a well-maintained cell.

Step 7: Internal Resistance (IR) Testing

Internal Series Resistance (ISR) stands as the paramount indicator of a battery cell’s State of Health (SoH). This step serves to evaluate the degree of variance from the manufacturer’s specified parameters in a crucial aspect.

Cells that deviate significantly from the manufacturer’s specifications should be eliminated due to the potential for excessive heat generation.

Lower resistance equates to reduced voltage drop when the cell operates under load. This, in turn, mitigates heat accumulation, ultimately extending the cell’s overall lifespan and, consequently, the battery pack’s longevity.

For these reasons, each remaining cell undergoes IR testing, and the results are cross-referenced with the manufacturer’s specification sheet. Any cell displaying substantial deviation from the manufacturer’s stipulated values should be promptly excluded.

Lithium-ion Battery Testing Process

- Significance of Uniform IR Among All Cells

The Internal Series Resistance (ISR) of a cell dictates both its operating temperature and the extent of voltage drop it experiences under a given load. Therefore, if one of the P groups contains a cell with significantly higher ISR than the other cells, it will exhibit a more pronounced voltage drop.

Consequently, this results in energy transfer from the well-performing cells to the underperforming ones, further exacerbating the issue. It is essential to bear in mind that an increase in a cell’s temperature leads to an increase in its resistance. This sets the stage for a potentially hazardous positive feedback loop, ultimately accelerating the failure of your battery pack.

Moreover, if a parallel group contains a cell with an exceptionally high ISR, it may prompt the Battery Management System (BMS) to shut down the battery pack well in advance of the depletion of the other parallel groups.

You might be interested in learning more about the factors that can cause battery pack failures. If so, please check out the article: Why Is The 18650 Battery Pack Not Working Properly?

How To Check If 18650 Battery Is Bad

(1) Presence of visible damage such as swelling, dents, or indications of dried or leaking fluids.

If the battery exhibits any physical damage or signs of leakage, it is probable that the battery is compromised and should be avoided.

A swollen battery cell is particularly hazardous as the swelling indicates an accumulation of heat and gas within the cell.

Excessive heat and gas buildup may lead to venting with flames or, in extreme cases, even an explosion.

(2) If you notice a sweet odor: The presence of any perforation or hole in a cell is a highly concerning issue.

Holes can result from physical impacts or internal damage. They compromise the integrity of the electrolyte used to separate the negative and positive sides of lithium-ion batteries.

Therefore, if you notice an unusual, mildly sweet odor, it indicates that the cell’s electrolyte has leaked or is leaking and should be safely disposed of.

If a cell’s voltage falls below 2.6 volts or exceeds 4.2 volts, it indicates that the battery cell has undergone some form of stress or damage. Any battery that remains outside of this voltage range for any period is likely to develop issues.

NOTE: If a cell’s voltage surpasses 4.2 volts, it poses a safety hazard and should be immediately discarded.

(3) If conducting a load test results in a substantial voltage drop or if the voltage drop exhibits significant variation between tests, it could be indicative of a weak battery or one with elevated internal resistance.

(4) When the cell’s capacity falls below 10% of its rated capacity, it serves as a strong indicator of a subpar battery that will have a shortened lifespan and may overheat.

(5) If the cell’s resistance exceeds 50 milliohms by a significant margin, it essentially signifies that the battery is compromised and will not perform up to its expected standards.

How To Check If 18650 Battery Is Good

(1) Absence of any visual damage, including dents, swelling, or indications of leaked substances: If the battery appears undamaged and there are no signs of leaks, it is probable that the battery remains in good condition and can be utilized safely.

(2) Voltage falling within the range of 2.6 to 4.2 volts: A battery voltage within this range, from 2.6 to 4.2 volts, generally signifies that the cell has not experienced significant stress or damage.

(3) A load test that does not yield a substantial or erratic voltage drop: When the battery voltage remains stable under load testing, it typically indicates that the cell is performing adequately and can meet the expected standards.

(4) The cell’s capacity falls within a 10% range of its rated capacity: When the battery’s capacity remains within 10% of its specified capacity, it suggests that the battery has not undergone an excessive number of charge cycles. This typically indicates that the cell has several more years of operational life remaining.

(5) The cell’s resistance falls within the 30 to 50 milliohms range: If the battery’s resistance is situated within the 30-50 milliohms range, it can be an indicator that the battery is still in a satisfactory condition and is capable of performing effectively.


When dealing with 18650 cells, it is imperative to acquire the skills necessary for meticulously examining cells for visual impairment, evaluating cells for voltage fluctuations, and conducting thorough charge and discharge assessments for each individual cell.

It is essential to bear in mind that an assemblage of lithium-ion battery cells, featuring disparities in capacity and internal resistance, may lead to suboptimal or potentially hazardous outcomes.

We hope that this article can provide you with valuable insights into the process of independently testing 18650 cells.

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