Typically, after lithium batteries undergo formation and aging processes, they require inspection and analysis. What tests are generally performed? Today, we will briefly discuss battery testing. In general, for the lithium-ion batteries or other systems under analysis, researchers in electrochemistry conduct basic inspections and electrochemical performance tests to further identify and assess any issues present in the battery or system.

I. Visual Inspection

Pouch cells: We must verify that the battery is intact and undamaged—checking for swelling, leakage, surface corrosion, and so on. These surface observations allow for a preliminary assessment of whether the battery is usable or what conditions it may have been subjected to.

Prismatic aluminum-cased cells: Similarly, checks are performed for leakage, deformation or dents in the aluminum casing, and irregularities such as asymmetrical terminal posts.

Regarding battery swelling: Consider whether the swelling occurred at high temperatures or at room temperature, and evaluate consistency—is it an isolated case or does it affect the entire batch? If the whole batch is affected, the issue likely stems from the chemical system or the overall manufacturing process; for instance, swelling is notably common in lithium titanate (LTO) systems, and can also occur across a batch due to poor assembly design or processes. If only a single cell is affected, consider factors such as severe degradation or gas generation.

Regarding battery leakage: First, identify the location of the leak—such as at the tab or the electrolyte filling port. Finally, determine whether the cause is a design flaw in the casing or an operational error, such as improper sealing of the aluminum-plastic film or a faulty weld at the filling port.

If surface corrosion is detected, consider whether it was caused by electrolyte spillage during filling, internal leakage, or casing corrosion resulting from short circuits, physical impact, or electrochemical reactions. Any defective units identified should be promptly removed from the batch.

II. Basic Electrochemical Performance Testing

 1. Measurement of OCV, IR, and Volume (Thickness)

Upon receiving a batch of batteries, the first step is to measure parameters such as Open-Circuit Voltage (OCV), Internal Resistance (IR), and volume (or thickness). Measuring OCV helps identify anomalies in the battery's initial state, while measuring IR allows for the timely identification of problematic cells—enabling the monitoring of potential future issues like degradation or micro-short circuits. Volume or thickness measurements are straightforward; batteries tend to swell during use (see the link at the end of this article for the causes of battery swelling).

 2. Battery Capacity Verification

Capacity verification typically involves 2 to 3 test cycles. Why not just one? A single test may lack accuracy, yet excessive testing is unnecessary.

Capacity testing usually employs a current of 0.5C or 1C; current national standards specify a 1C charge and 1C discharge to determine capacity. Naturally, capacity measured at a 0.5C discharge rate will exceed that at 1C, so it is best to adhere to the national standard.

Additionally, following standard capacity verification, low-current charge/discharge tests are conducted—typically at 0.1C or lower—to eliminate the influence of polarization impedance. At such low currents, polarization is negligible, allowing for an accurate assessment of the battery's intrinsic capacity, median voltage, cycling curves, and other fundamental data.

For more detailed analysis—and if conditions permit—external voltage data loggers can be used to precisely measure the voltage-capacity curve. Applying differential analysis to this data reveals the battery's internal state; changes in peak shapes and positions help determine the extent and type of degradation.

 3. Three-Electrode System

When current flows through a battery system, it generates an ohmic voltage drop (solution resistance) and polarization at the counter electrode, making it difficult to accurately measure the working electrode's potential. Consequently, a reference electrode is introduced. The reference electrode maintains a highly stable potential; since no current flows through it, no polarization occurs. This allows the working electrode's potential to be determined relative to the reference electrode, while the current flows through the working electrode-counter electrode circuit. Three-electrode configurations are standard practice in electrochemical testing, including research on lithium-ion batteries. Using a three-electrode setup allows for the clear differentiation of positive and negative electrode potentials, making it possible to pinpoint exactly which side is experiencing issues or potential deviations.

In this configuration, the placement of the reference electrode—typically a lithium metal electrode—is critical; it must be positioned between the positive and negative electrodes, rather than merely somewhere inside the cell.

The construction of the three-electrode setup must ensure minimal impact on the battery's intrinsic state, avoiding short circuits or interference caused by contact.

 4. EIS Testing

EIS (Electrochemical Impedance Spectroscopy) is a technique where the quality of impedance data fitting and analysis directly determines the accuracy of the battery assessment.

According to the law of conservation of energy, all processes involve energy consumption; impedance provides a precise measurement of how specific portions of that energy are dissipated.

Standard impedance plots—such as Nyquist and Bode (frequency) plots—offer an intuitive way to evaluate battery reactions, identify the various interfaces involved, quantify their respective impedance values, and characterize subsequent diffusion processes.

Typically, impedance plots are analyzed comparatively—juxtaposing known and unknown samples—to clearly distinguish their relative merits, strengths, and weaknesses. This approach helps identify directions for future improvement or pinpoint potential problem areas.

Combining a three-electrode setup with EIS yields richer data; it allows for the decomposition of total battery impedance into the individual contributions of the positive and negative electrodes, providing a clearer picture of the internal impedance distribution. This enables the acquisition of detailed battery information without destructively dismantling the cell.

III. Lithium Battery Performance Testing

After undergoing basic electrochemical performance testing, lithium batteries can be subjected to in-depth performance analysis; cells demonstrating good consistency and low impedance are selected for comprehensive performance testing.

 1. Cycle Life

The cycle count of a lithium battery reflects the number of times it can undergo repeated charge and discharge cycles. Depending on the operating environment, cycle life testing evaluates the battery's longevity under low, ambient, and high temperatures. The criteria for deeming a battery "spent" (end-of-life) are determined by its intended application: for power batteries (used in electric vehicles or forklifts), the standard threshold is typically a discharge capacity retention rate of 80%, whereas for energy storage applications, this threshold may be relaxed to 60%. For the batteries we commonly encounter, if the discharge capacity drops below 60% of the initial capacity, they are effectively no longer viable, as they would fail to sustain operation for even half a day.

 2. C-Rate (Charge/Discharge Rate)

Nowadays, lithium batteries are used not only in 3C electronics but increasingly in power applications. Electric vehicles require varying currents depending on driving conditions. Furthermore, in today's fast-paced world, the shortage of charging stations has driven a growing demand for rapid charging capabilities. Consequently, testing the C-rate performance of lithium batteries is essential; such testing can be conducted in accordance with national standards for power batteries. Manufacturers worldwide are producing specialized high-rate batteries to meet market demand. The design of these batteries involves optimizing factors such as active material types, electrode areal density, compaction density, tab selection, welding techniques, and assembly processes—topics that interested readers may wish to explore further.

 3. Safety Testing

Safety is arguably the primary concern for battery users; incidents such as mobile phone battery explosions or electric vehicle fires are deeply alarming. Safety verification is a mandatory requirement for lithium batteries. Tests cover scenarios including overcharge, over-discharge, short-circuiting, dropping, heating, vibration, crushing, and nail penetration. However, from the perspective of "Lithium Battery Experts" (Li-Dian-Pai), these are essentially passive safety tests—meaning they involve subjecting the battery to external forces or objects designed to compromise its integrity in order to verify its safety. When submitting batteries and modules for certification, specific design measures are required to pass safety tests; however, real-world scenarios—such as an electric vehicle losing control and colliding with another vehicle or object—involve irregular impacts and more complex conditions. Given the high cost of testing, it is necessary to select the most reliable and representative test protocols.

Currently, the market is dominated by ternary batteries and lithium iron phosphate (LFP) batteries. Ternary batteries are significantly less safe than LFP batteries because their material structure can collapse and become unstable at high temperatures; however, they offer higher energy density, which is why both technologies continue to coexist and develop.

In addition, performance characteristics such as discharge capabilities at both low and high temperatures also require testing.