How to effectively test battery cells: a comprehensive review
- Mauro Malizia
- 4 days ago
- 6 min read
As the need for decarbonisation becomes ever more apparent, there has been an increasing focus on the development of the ‘better battery’.
In recent years, battery cell testing has strongly driven the development of cells for many applications. The testing aims to thoroughly evaluate the cells while operating in various scenarios, both in terms of driving performance and environmental conditions. The cells also need to be tested to withstand vibration, shock and mechanical stress, so they are safer to operate on moving vehicles.
By accurately and thoroughly testing, then analysing battery cells, changes to the cell chemistry, electrodes, and overall design can be made, improving the performance and the safety of the cells iteratively as they are developed. This in turn drives down the final cost of the EV, and improves performance and longevity.
Cell testing can be divided into three main categories: life testing, performance testing and safety testing. Below is an outline on effectively testing battery cells.
Life tests
Life tests aim at determining the change in the performance of the cell during a time frame of several weeks, months, or even years under repeated use. Life testing is a fundamental step in understanding how the cells perform over time, which is essential for the day to day use of an EV, as it will improve the performance, range, reliability, and safety. The most common tests performed are:
Cycling
This test charges/discharges the cell hundred times to induce the degradation of capacity and internal resistance. The main parameters that can be chosen for this test are the C-rates for both charge and discharge (i.e. the magnitude of the current that flows into the cell), the voltage cycling window, and the operating temperature. Often, short characterisation tests can be run at periodic intervals during the cycling test to assess the degradation of the cell every nth number of cycles. This is what is often referred to as Reference Performance Test (RPT). The cycling test is usually stopped when the cell reaches a target State of Health (SOH), which represents the capacity measured at the end of life vs. the capacity measured at the beginning of life.
Drive cycle
Similarly to the Cycling test, the Drive cycle test performs repeated charge/discharge on the cell until a target SOH is reached. In this case, the charge or discharge (or both) current is not constant, but varies significantly for the duration of the half-cycle to mimic real-life conditions such as a battery cell being used in automotive applications which is frequently charged/discharged for short periods of time.
Storage ageing
The battery cells are stored in a thermal chamber at a fixed temperature for a predetermined period of time (up to several months) to estimate how they degrade when the cells are not actually in use. In addition, various States of Charge (SOC) of the cells can be chosen to obtain different results. Periodically, the cells undergo a brief test (RPT) to collect relevant information regarding their capacity retention and other parameters during the storage period.
Performance tests
Performance tests measure the ability of the cell to deliver and store energy efficiently under specific conditions. These tests help engineers identify for instance which battery cells are more suitable for fast charge applications, can deliver higher power, or perform better in extreme conditions.
Fast charge
This test can help understand the maximum C-rate that a cell can withstand during fast charge without incurring unwanted side effects such as excessive overheating or significantly accelerated degradation. The C-rate during the charge can be chosen to be constant or variable (as a function of SOC/voltage, temperature, or other parameters).
Rate capability
This test performs the charge and discharge of the cell at different C-rates, to determine the actual capacity when it is not charged/discharged at nominal C-rates. In general, a cell charged/discharged at low C-rates would show higher capacities compared to the very same cell operated at higher C-rates. Different temperatures also give different results, with the capacity typically dropping with decreasing temperatures, due to the fact that the ionic conductivity of the electrolyte is badly affected by very low temperatures.
HPPC
The Hybrid Pulse Power Characterization test is often used to evaluate the performance of power battery cells. In particular, the cells are discharged and charged with short current pulses at different SOCs. The duration of the pulse is often 30 s or less, whereas the C-rates can typically be chosen to be as high as 3C. The C-rate also depends on the SOC probed and the temperature at which the test is performed. The HPPC test gives information on: the internal resistance as a function of SOC, pulse power capability, and usable energy/power of a cell.
GITT
The Galvanostatic Intermittent Titration Technique consists of a series of current pulses followed by relaxation of the cell, that is used to retrieve useful thermodynamic and kinetic parameters of the cells, together with the chemical diffusion coefficient.
DCIR and EIS
These are techniques that are utilised to measure the internal resistance of the cell: Direct Current Internal Resistance (DCIR) and Electrochemical Impedance Spectroscopy (EIS). DCIR can be estimated by applying a short discharge pulse to the cell, followed by a suitable relaxation time. This can be performed at several different SOCs on the very same cell. EIS often needs to be performed by specific equipment, and works by applying an AC perturbation to the cell at various frequencies. The resulting data is usually shown in a Nyquist plot, and gives the information regarding the impedance of the cell. Also in this case, the EIS technique can be used to probe the cell at various SOCs.
Safety tests
Safety testing is performed to verify how the cell operates under normal and abnormal conditions to mitigate hazards like overheating, fires, or explosions.
Overcharge/over-discharge
The cell is either charged above its maximum safety limit, or discharged down to 0 V under close monitoring.
Thermal runaway
Cells are exposed to temperatures above 100 °C to induce thermal runaway due to the degradation of components that lead to the formation of internal short-circuits, triggering exo-thermic reactions that cause the venting of the cell, rupture, fire and explosion.
Short circuit
The electrodes of the cell are shorted with a very low resistance, causing a high current to flow across them that leads to overheating, fire or explosion.
Nail penetration
This test simulates the perforation of the cell by an external object such as a nail, leading to cell rupture, fire and explosion.
Mechanical abuse
This test aims at simulating physical stresses such as falls from height, crushing or vibration, mimicking applications where the battery cells face harsh physical conditions, such as in vehicles or portable electronics.
Equipment usage
Along with careful consideration of which tests are most relevant to complete, the equipment utilised for cell testing must also be carefully chosen to ensure that the tests are performed in an accurate and reproducible way. Testing should mimic the eventual real-life application of the cells.
The most fundamental equipment used within the test is the cell cyclers. These enable the Engineer to program and execute specific test protocols. The choice of the cycler depends on several factors, including the number of channels required (number of cells to be tested) and maximum current they can deliver. Moreover, it is always important to make sure that the cycler is periodically calibrated and maintained to ensure that tests are performed in an accurate and reproducible manner.
Further to the cell cyclers, the environment the cell is tested in, is extremely important. It is fundamental to mimic real-world conditions, enabling results to portray real-world results as far as possible. The most common method for controlling the cell environment is through the use of thermal chambers for the duration of the test, ensuring the environment is at a consistent temperature. Whilst thermal chambers are fairly standard environments used within cell testing, it must be considered that more accurate and real-world conditions could be used to improve data output quality. These include water cooling, immersion in an oil bath and base/surface cooling of the cells to improve the heat transfer. The final choice depends on the expected real-life application of the cells under test, so in the case of an EV, this would be the design of the cooling solution in the battery pack.
Additional to this, it must be considered how the cell is connected to the cycler via a holder providing the electrical connection. The main goal is to reduce the contact resistance between the cell and the current cables of the cycler. With high contact resistance, there is increased localised heat at the electrodes that could interfere with proper cell testing and even cause damage to the cell. A good quality holder is therefore necessary.
Effective battery cell testing conclusions
In conclusion, performing accurate and reproducible testing of battery cells in a well-controlled environment can bring a series of benefits, including:
Overall better cell performance validation
Better understanding and deeper insights on how to improve single battery cell components and reduce costs
Improved safety and reduced risk when the cells are operating under both standard and extreme conditions
Better understanding of how the cell would perform in a battery module/pack
Better data for parameterising the models behind the vehicle’s BMS
This would eventually positively affect the quality and performance of the final product, whether the battery cells are used in an electric vehicle, portable electronics, aerospace industry or energy storage systems.
To learn more about the cell testing offered by Cognition Energy: