Battery Cyclers: Testing Capacity, Coulombic Efficiency and Cycle Life of Lithium Batteries

Every new battery material, whether a cathode, anode, electrolyte or additive, is eventually judged by the same questions: how much capacity, how efficient, and how many cycles does it last? The instrument that answers them is the battery cycler, also called a battery testing system or charge-discharge tester. This article covers how it works, what it measures, common test protocols, and how to choose one that fits the cells you study.

What a battery cycler is

A battery cycler is a multichannel instrument. Each channel works independently as a programmable current or voltage source: it charges a cell, discharges it, rests, and repeats this hundreds or thousands of times, while logging voltage, current, time, capacity and energy. Because a single cycle-life test can run for weeks, the number of channels decides how many cells and variations you can test at once.

A potentiostat is flexible across many electrochemical techniques (CV, EIS, polarization). A battery cycler is built for one job: long-term charge-discharge testing of many cells in parallel, at a much lower cost per channel.

What it measures

• Specific capacity (mAh/g): stored charge divided by active material mass, the first number compared with the material's theoretical capacity.
• C-rate: current relative to capacity. 1C charges or discharges the cell in about 1 hour, C/10 in 10 hours, 2C in 30 minutes.
• Coulombic efficiency (CE): charge out divided by charge in during the same cycle, in percent. CE below 100% means charge is lost to side reactions such as SEI formation or electrolyte decomposition.
• Capacity retention: capacity at cycle n compared with the initial capacity. A common end-of-life threshold is 80%.
• Rate capability: how much capacity remains as current increases.
• Energy efficiency and overpotential: the gap between average charge and discharge voltages.

Calculating the test current

Test current follows from the active material mass and its theoretical capacity. An example for a LiFePO₄ electrode in a coin cell:

• Active material on the electrode disc: 2.0 mg
• Theoretical capacity of LiFePO₄: about 170 mAh/g
• Cell capacity: 2.0 mg × 170 mAh/g = 0.34 mAh
• 1C current = 0.34 mA; C/10 = 34 µA

Currents this small are why channel range matters. Cycler accuracy is stated as a percentage of full scale (% FS), so measuring 34 µA on a 6 A channel is far less precise than on a 10 mA channel. Use low-current channels for research coin cells; amp-range channels are meant for pouch cells, cylindrical cells and modules.

Stepped current ranges

Milliamp-class cyclers usually split each channel into sub-ranges, and the instrument selects the sub-range that fits the programmed current. The % FS accuracy applies to that sub-range, not to the channel maximum. From the Neware catalogue for the CT-4000/CT-8000 series:

Channel	Range 1	Range 2	Range 3	Range 4
5V10mA	5 µA–1 mA	1–5 mA	5–10 mA	–
5V20mA	5 µA–1 mA	1–10 mA	10–20 mA	–
5V50mA	5 µA–1 mA	1–25 mA	25–50 mA	–
5V100mA	0.2 µA–0.1 mA	0.1–1 mA	1–10 mA	10–100 mA

In the LiFePO₄ example above, both the 34 µA C/10 current and the 0.34 mA 1C current fall in range 1 of a 5V10mA channel. The Neware catalogue lists voltage and current accuracy for this series as ±0.01% / 0.02% / 0.05% FS (depending on model), with recording frequency up to 1000 Hz.

Common test protocols

1. Initial rest (OCV). Let the cell rest for a few hours after assembly so the electrolyte wets the electrode pores and separator.
2. Formation cycles. Two to three cycles at low current (for example C/10 or C/20) to build a stable SEI layer.
3. Rate test. A few cycles at each rate, for example C/10, C/5, C/2, 1C, 2C, then back to C/10 to see whether capacity recovers.
4. Cycle-life test. Repeated cycles at a fixed rate (for example 1C) until retention reaches 80% or a target cycle count is met.

Charging usually follows a CC-CV profile: constant current up to the upper voltage limit, then that voltage is held until the current falls to a set value (for example C/20). Discharge uses constant current down to the lower voltage limit.

The voltage window depends on the material. Typical ranges seen in the literature:

System	Voltage window (example)
LiFePO4 vs Li (half-cell)	2.5–4.2 V
NMC vs Li (half-cell)	3.0–4.3 V
Graphite vs Li (half-cell)	0.01–1.5 V
Li4Ti5O12 (LTO) vs Li	1.0–2.5 V
LiFePO4 / graphite (full cell)	2.5–3.65 V

Always check the literature for your own material and formulation.

Reading the data

• Voltage vs capacity curves: a long flat plateau indicates a two-phase reaction, as in LiFePO₄; sloping or shifting plateaus from cycle to cycle point to rising resistance.
• dQ/dV (incremental capacity): the derivative of capacity with respect to voltage. Its peaks represent phase transitions, and shifting or shrinking peaks help identify the type of degradation.
• Coulombic efficiency trend: CE rising towards 100% after the first few cycles indicates a stable SEI. CE that stays low means side reactions continue.
• GITT and DCIR: GITT is a sequence of current pulses and rests used to estimate ion diffusion coefficients; DCIR calculates resistance from the voltage step under a current pulse. Both are available as test steps on modern cyclers. The CT-4008Tn-5V10mA specification, for example, supports pulse mode with a 500 ms minimum width and configurable DCIR calculation points.

For impedance spectroscopy (EIS), which separates ohmic, charge-transfer and diffusion resistance, use a potentiostat with an EIS module. The same cell can be measured by EIS between cycling steps.

Why data fails to reproduce

• Uncontrolled temperature. Capacity and resistance change with room temperature from day to night. Serious tests run inside a constant-temperature chamber.
• Poor contacts. Loose clips or holders add resistance and cut curves short.
• Cell assembly. Moisture during assembly, poorly sealed crimping and inconsistent electrolyte volume are the largest sources of variation.
• Mis-weighed active mass. A 0.1 mg error on a 2 mg electrode is already a 5% error in specific capacity.
• One cell per variation. Test several identical cells per variation so reported differences are not assembly accidents.

How to choose a battery cycler

1. Current range per channel. Coin cells: microamps to milliamps. Pouch and 18650 cells: amps. Modules and packs: tens of amps at higher voltage.
2. Maximum voltage. 5 V is enough for a single Li-ion cell; series strings need more.
3. Number of channels. Multiply variations by replicate cells, and remember that cycle-life tests lock channels for weeks.
4. Software and data protection. How easily you can build steps (CC, CV, rest, loops) and export raw data. Make sure tests keep running if the PC goes down: the CT-4008Tn specification, for example, lists power-off data protection and off-line operation, a MySQL database, and export to Excel and TXT.
5. Accessories. Coin cell holders or clips, pouch clamps, racks and temperature chambers.

Neware battery cyclers and battery test equipment at AMI Scientific

Neware is a battery test equipment manufacturer founded in 1998 in Shenzhen, China. Its range runs from milliamp research cyclers to module and pack test systems rated at hundreds of kilowatts. Our Neware collection covers cyclers for different cell formats:

• Coin cells and half-cells: CT-4008 series with 5V10mA, 5V20mA and 5V50mA channels, and the CT-8002Q 5V100mA.
• Pouch and cylindrical cells: CT-4008 5V6A, BTS-4008 5V6A and BTS 10V6A.
• Modules: CE-6004n 150V50A.
• Accessories: coin cell clips, pouch clamps, racks, middle machines and the MHW-200 temperature chamber.

For cell assembly we also carry CR2032 coin cell cases, crimping machines and electrode disc cutters. Electrode and electrolyte materials are in our battery materials collection, and EIS can be measured with Corrtest potentiostats and the CS951 coin cell cable.

Not ready to buy an instrument? AMI also offers electrochemical analysis services, including charge-discharge testing, CV and EIS. Send us your battery chemistry, cell format and the number of channels you need via WhatsApp, and we will recommend a configuration and prepare a formal quotation.