Capacity testing of rechargeable Lithium-ion cells

Guest author Mr Neeraj Kumar Singal elaborates the process of Lithium-ion Cell testing for the estimation of capacity performance.

Please refer to part 1 to read the preceding article on acceptance parameters of a purchased lot of Li-ion cells and sorting process to group the cells with similar performance characteristics together.

This article covers:

• Characteristics of good cell testing equipment
• Considerations while selecting test equipment
• Key factors that affect the capacity data during the lithium cell test process
• Performing Cell Charge Discharge (Capacity) Test
• Interpretation of test data

After the sorting of cells, now we have a batch of cells which is expected to have comparable electrical properties. The next step is to get these ready for a detailed performance test.

Before testing, cells need to be brought to the same voltage. On the cell charge-discharge-testing machine, they are fully charged to the upper cut-off value and then discharged to the Depth of Discharge (DoD). Thereafter, the State of Charge (SoC) of 50% is attained. Now, the cells are taken to the cell testing machines.   

Characteristics of good cell testing equipment

The cell testing equipment performs the basic function of cell charge & discharge. Good testing equipment also provides:

• Provision for thermal measurement
• Has better parameter recording periodicity (i.e. the number of captured readings per unit of time is more)
• Least count of Voltage, Current and Time up to three or more decimal places
• Such equipment is self calculating, with the facility of saving and retaining the test data in its internal soft memory.
• Can analyse the long term performance of cells by extrapolating the test data of a few charge-discharge cycles.
• Some test equipment can test only a limited size of cells while others have interchangeable adapters which make them more versatile.
• Provisions for resuming the test from the point it got interrupted (say due to a power supply failure) and also un-monitored continuation of long duration tests which may continue beyond the normal shift hours.
• In the market, there is testing equipment that can do a single cell testing and then there are others that can test multiple cells (say up to 1064 cells) at a time.
• Many equipment manufacturers also provide remote monitoring as well as software up-gradation support to assess the performance.

Considerations while selecting test equipment

For selecting the equipment for the test, one should be sure of what tests are required to be conducted by the organisation. The test equipment for the routine tests, DC Internal resistance, End of Life & cell grading is generally different and also each equipment has restrictions on the Voltage & Current of the individual cell as well as the total electric load handling capability. The restrictions are also on the rate of charge/discharge at constant current, voltage or power. Additional restrictions may be with regard to total testing time and variable rates of charge/discharge during a single test cycle.  

Considering the above factors, the suitable Test Equipment may be selected & procured. Most of the test equipment are imported and therefore, the local support office of the agent should be available for guidance, spares supply and periodic maintenance. 

Key factors that affect the capacity data during the lithium cell test process

1. Cell performance

Generally, for new cells or cells with good performance, the capacity fluctuation range is generally small, while the test capacity data of older cells have a large fluctuation range. Cell performance determines the stability of the test data.

2. Ambient temperature

The ambient temperature has a certain impact on the test data. Try to maintain a constant ambient temperature (20±5℃). If the temperature fluctuation range is small, and the test data error range is also small.

3. Number of cycles

Under the same test conditions and the same test parameters, the capacity data of the lithium-ion cell will fluctuate and it will gradually stabilize in the later stage.

Before the test officially starts, it is necessary to perform a pre-treatment cycle at room temperature to ensure that the cell performance is in an activated and stable state. The pre-treatment generally requires multiple charge-rest-discharge-rest cycles. During the pre-treatment process, it is necessary to ensure that the change for two consecutive times does not exceed 3% of the rated capacity.

4. Discharge rate

Using different test currents, the capacity data obtained by the test may have a big deviation. Generally, the 0.5C current test is used but would depend upon the requirement & the Cell Data Sheet.

5. Tester accuracy

Professional quality test equipment must be used. The quality of electronic components used in the test instrument and the connectors will be slightly different from equipment to equipment and the test data will have certain errors due to the higher variability of internal temperature of the chassis which may adversely impact the accurate reporting of test results.

6. Operating voltage of the cell

Working voltage, also known as terminal voltage, refers to the potential difference between the positive and negative terminals of the cell when current flows through the circuit. The terminal voltage of the cell lower than the electromotive force of the cell when it is discharged and the terminal voltage is higher than the electromotive force when the cell is charged.

Performing Cell Charge Discharge (Capacity) Test

Set up the test equipment, place the desired number of cells and start the required test cycle under the modes like – CC, CV,  CCCV, CD, CPD, Rest  Cycle etc. Thereafter, set parameters as per cell datasheet of the cell manufacturer (e.g. Charging current, upper cut off voltage lower cut off voltage). Key in testing time – a minimum of 10 minutes (or as per your own requirement), Cell data recording interval and Cell protection parameters (temperature, rate of charging, humidity limit) as per the datasheet. Similarly, conduct the cell discharge test.

Details of tests and Interpretation of test data

The charging characteristics of lithium-ion cells are completely different from those of cadmium nickel and nickel hydrogen. Lithium-ion cells can be charged at any point in its discharge cycle, and can maintain its charge very effectively and the retention time is more than twice that of nickel-hydrogen cells. 

When the lithium-ion cell starts to charge, the voltage rises slowly and the charging current gradually decreases. When the cell voltage reaches about 4.2V, the cell voltage remains constant and the charging current continues to drop.

Charging of Li-ion cell

The charging process of lithium-ion cells can be divided into four stages: trickle charging (low-voltage pre-charge), constant current charging, constant voltage charging, and charging termination.

Phase 1: Trickle charge – Trickle charge is used to pre-charge the fully discharged cell (recovery charge). When the cell voltage is lower than 3V, trickle charging is used. The trickle charging current is one tenth of the constant current charging current, which is 0.1C e.g. for a constant charging current of 1A, the trickle charging current is 100mA).

(C is a way of expressing the nominal capacity of the cell against the current. If the cell has a capacity of 1000mAh, 1C is the charging current of 1000mA.)

Phase 2: Constant current charging – When the cell voltage rises above the trickle charge threshold, increase the charging current for constant current charging. The current for constant current charging is between 0.2C and 1.0C. The cell voltage gradually increases with the constant current charging process. Generally, the voltage set for a single cell is 3.0-4.2V.

Phase 3: Constant voltage charging – When the cell voltage rises to 4.2V, the constant current charging ends and the constant voltage charging phase begins. According to the saturation of the cell, the charging current gradually decreases as the charging process continues. When it decreases to 0.01C, the charging is considered to be terminated. 

Phase 4: Charge termination -There are two typical charging termination methods: use the minimum charging current to determine or use a timer (or a combination of the two). The minimum current method monitors the charging current in the constant voltage charging stage, and terminates the charging when the charging current decreases to the range of 0.02C to 0.07C. The second method starts timing at the beginning of the constant voltage charging phase, and terminates the charging process after two hours of continuous charging.

The above four-stage charging method takes about 2.5 to 3 hours to complete the fully discharged cell. Advanced chargers also adopt more safety measures. e.g. if the cell temperature exceeds the specified window (usually 0°C to 45°C), charging gets suspended. 

Basic principle of discharge test

The discharge curve basically reflects the state of the electrode, which is the superposition of the state changes of the positive and negative electrodes.

During the entire discharge process, the voltage curve of a lithium-ion cell can be divided into 3 stages:

1. The terminal voltage of the cell drops rapidly in the initial stage – greater the discharge rate, faster the voltage drop

2. The cell voltage enters a stage of slow change. This period is called the cell platform area. Smaller the discharge rate, longer the platform area lasts. Higher the platform voltage, slower the voltage drop.

3. When the cell is nearly discharged, the cell load voltage begins to drop sharply until it reaches the discharge cut-off voltage.

Discharge test mode 

Li-ion cell discharge test modes mainly include constant current discharge, constant resistance discharge, and constant power discharge. In each discharge mode, continuous discharge and interval discharge can also be divided. According to the length of time, interval discharge can be divided into intermittent discharge and pulse discharge. 

During the discharge test, the cell discharges according to the set mode, and stops discharging when the set conditions are reached. The discharge cut-off conditions include set voltage cut-off, set time cut-off, set capacity cut-off, set negative voltage gradient cut-off, and so on. The change of the cell discharge voltage is related to the discharge system, that is: discharge current, discharge temperature, discharge termination voltage; intermittent or continuous discharge. The greater the discharge current, the faster the working voltage drop; with the increase of the discharge temperature, the discharge curve changes more smoothly.

Constant current discharge

Constant current discharge is the most commonly used discharge method in lithium ion cell testing.

Set the current value and collect the change of the cell terminal voltage to detect the discharge characteristics of the cell. In constant current discharge, the discharge current does not change but the cell voltage continues to drop, so the power continues to drop. 

Constant power discharge

Set value of constant power (P) and collect the output voltage (V) of the cell. In this mode, P is required to be constant but V is constantly changing, so the current (I) of the numerically controlled constant current source needs to be continuously adjusted according to the formula I = P / V to achieve the purpose of constant power discharge. Keeping the discharge power constant, the cell voltage continues to drop and the current continues to rise. 

Constant resistance discharge

First set a constant resistance value R and collect the output voltage V of the cell. During the discharging process, R is required to be constant but V is constantly changing, so it needs to be continuously changed according to the formula I=V/R. The voltage of the cell drops during the discharge process and the resistance remains unchanged, so the discharge current I also decreases.

Continuous discharge, intermittent discharge and pulse discharge

While the cell is discharged under three modes of constant current, constant power and constant resistance, the timing function is used to realize the control of continuous discharge, intermittent discharge and pulse discharge. 

Information contained in the discharge curve 

The discharge curve refers to the curve of the voltage, current, and capacity of the cell over time during the discharge process. The information contained in the charge and discharge curve is very rich including the capacity, energy, working voltage and voltage platform, the relationship between electrode potential and state of charge, etc. The main data recorded during the discharge test is the time evolution of current and voltage. The following details the parameters that can be obtained by the discharge curve:

Voltage

In the lithium-ion cell discharge test, the voltage parameters mainly include voltage platform, median voltage, average voltage, cut-off voltage etc. Platform voltage refers to the voltage value corresponding to the minimum voltage change and the large capacity change, which can be obtained by the peak value of dQ/dV. The median voltage is the voltage value corresponding to half of the cell capacity. For materials with obvious platforms, such as lithium iron phosphate and lithium titanate, the median voltage is the platform voltage. The average voltage is the effective area of ​​the voltage-capacity curve (ie cell discharge energy) divided by the capacity. The cut-off voltage refers to the lowest voltage allowed when the cell is discharged. If the voltage is lower than the discharge cut-off voltage and the discharge continues, the voltage at both ends of the cell will drop rapidly, resulting in over-discharge. Over-discharge may cause damage to the electrode active material, loss of reactivity and shorten the cell life because the voltage of the cell is related to the state of charge of the positive and negative materials and the electrode potential.

Capacity and specific capacity 

Cell capacity refers to the amount of electricity discharged by the cell under a certain discharge system (under a certain discharge current I, discharge temperature T and discharge cut-off voltage V). It represents the ability of the cell to store energy, in Ah or C. The capacity is affected by many factors, such as: discharge current, discharge temperature, etc. The capacity is determined by the number of active materials in the positive and negative electrodes. Theoretical capacity: the capacity given by all active substances participating in the reaction. Actual capacity: The capacity actually discharged under a certain discharge system. Rated capacity: refers to the guaranteed minimum power of the cell under the designed discharge conditions. 

In the discharge test, the capacity is calculated by integrating the current over time. In order to compare different cells, the concept of specific capacity is introduced. Specific capacity refers to the capacity given by the electrode active material per unit mass or unit volume, which is called mass specific capacity or volume specific capacity. The usual calculation method is: specific capacity = cell first discharge capacity / (active material mass * active material utilization rate)

Factors affecting cell capacity

– The discharge current of the cell – the larger the current, the lower the output capacity

– The discharge temperature of the cell –  the output capacity decreases with the decreasing temperature

– The discharge cut-off voltage of the cell – it is generally set at 3.0V or 2.75V during discharge, which is set by the limits of the electrode material and the electrode reaction itself

– Cell charge and discharge time – After the cell has been charged and discharged many times, the discharge capacity of the cell will be reduced due to the failure of the electrode material

– The charging conditions of the cell – the charging rate, temperature, cut-off voltage, etc. affect the capacity of the charged cell, thereby determining the discharge capacity.

Each time the cell is charged and discharged, the data will have a certain deviation, which is related to the cell performance, testing conditions and the accuracy of the machine itself.

About the Author

This write-up is authored by Neeraj Kumar Singal, Founder and CEO of Semco Group.

He is passionate about Clean Energy and working on various projects to build robust Lithium-Ion ecosystem. One of his ventures, SEMCO Infratech is a solution provider of  Lithium-ion cell manufacturing and pack assembly equipments. He can be reached at nks@semcoindia.com