Not All Cell Cyclers Are Equal: How to Match Battery Cell Tests to the Right Equipment
- Kelly Martin
- Sep 30
- 6 min read
When there are numerous types of cell cyclers & multiple channel configurations available, it can feel daunting deciding which to use - here we delve into why it's important to consider your battery cell testing project before picking your cycler and configuration to do the task.
Dr Geraint Minton, Testing Services Manager
When testing a cell, a cycler is used to control the current passing through the cell, to monitor the voltage, to record additional inputs (e.g. temperature data), and possibly to also feed output to other hardware. Furthermore, the cycler software is also used to programme the cycler to run the test and to monitor tests as they are run. All of these can be factors when choosing what cycler channel to run a particular test on.
Why is current a crucial factor?
The primary concern when choosing a channel to run a test cell is whether the channel in question is able to provide the required current. The maximum current of a test depends on what the test is designed to measure, but is typically linked to the cell capacity by the C-rate, where 1C is the current required to discharge a cell in 1 hour. For example, a 3Ah cylindrical cell tested at 1C requires a 3A current, while a 300Ah prismatic cell tested at the same C-rate requires a 300A current. On this basis alone, it is clear that a low-current channel would be incapable of testing the prismatic cell, but a high-current channel could run both. However, should a high current channel be used for a low-current test? In short, the answer is probably no, due the accuracy of the current measurement.
The accuracy to which a channel applies the current is a function of the full-scale range (FSR) of the channel. Two cyclers with accuracies of ±0.05% FSR but different maximum currents will have different absolute errors: on a 300A channel the error is ±0.15A, while on a 3A channel it would be ±0.0015A. On this basis, using a high-current channel for a low-current test introduces a large error relative to the magnitude of the current. This impacts the accuracy of the test (i.e. the cell may be discharged at the wrong current), but also introduces error into measurement of the cell capacity, since this is calculated from the total current passed through the cell as it is cycled.
To manage this error, many high-current cyclers have one or more additional current ranges, which reduces the error when passing lower currents. However, because high-current cyclers are generally targeted towards testing high capacity cells, the lower-current additional ranges may still have an unfavourably high error at very low currents. Consequently, it is better (and often cheaper!) to use low-current channels for low-current tests.
One thing to bear in mind is that some cyclers allow channels to be connected in parallel and run as a single channel. This increases the maximum current which can be passed, at the expense of reducing the number of cells which can be tested at the same time, and potentially negatively impacting the accuracy.
Why is voltage range important?
The other clearest factor in selecting a cycler channel is the voltage range of the channel. If the voltage range of the test (i.e. the minimum and maximum voltages the cell can be operated at) are outside the operational range of the cycler, the maximum current output by the cycler will taper or simply fail to be provided. For the most part, all Li-ion cells operate in a voltage range between 2V (lower limit for LFP cells) and 4.2V (upper limit for NMC cells), and most cyclers function across this whole range. However, some next-generation chemistries, such as lithium-sulfur or sodium-ion can be discharged as low as 1V. A cycler which is not specified to operate below 2V, for example, will be unable to test these cells across the full voltage range.
One important thing to consider is that the output voltage of the cycler is usually specified at the front of the cycler, meaning that voltage drops in the cycler cables are not accounted for. If the cables are under-sized (or too long), voltage drops in the cable impact the cycler’s ability to provide current, potentially limiting the current the cycler can pass, even if the voltage window of the test is technically within the range of the cycler. Managing this requires consideration to be given to the physical setup of the lab.
Any other additional data needed?
Beyond the basics of current and voltage, testing commonly requires additional data to be recorded, and in many cases used, by the cycler when running a test. The most obvious of these is temperature data, which is important for understanding cell behaviour during a test, but is also used to improve safety by providing a thermal cut-off: when the cell temperature exceeds a defined value, the test is stopped to prevent cell damage and, potentially, thermal runaway occurring. To achieve this, the cycler must allow temperature data to be recorded and used by each test, meaning that auxiliary inputs are needed. Again, the size of the cell is a reasonable guide to the number of inputs needed: a small cylindrical cell might only need one temperature sensor, while a larger prismatic cell might have at least three. While separate data loggers could be used for gathering the extra data, it is often simpler to have the cycler record it all, meaning that the needs of the test setup can therefore limit the channels which can be used for the testing.
A similar constraint exists for controlling external devices. Most commonly, this is the temperature of the thermal chamber in which the cell under test is located, but control of other devices may be desired as well. For many tests, control of the external device is infrequent enough that manual control may be sufficient, but if frequent interaction is required, it would be much more efficient to use a channel which can signal the device and control it.
What type of tests does the cycler need to run?
Other factors which may limit the choice of channel include the structure of the test and data acquisition requirements. All cyclers should be able to carry out most types of test, but limitations may arise in the case of very short pulses or drive cycle/power profile testing, where the current changes very frequently. How quickly a cycler can switch on or off a current should be considered in these cases, to ensure that the test profile can be followed correctly. Linked to this is the data sampling rate: if the test profile has many quick variations in the applied current, it’s likely you’ll want to see it, making it a requirement that the cycler can sample the data quickly enough.
Other factors to consider?
From a power use perspective, there may be a choice of using channels on a regen cycler, which recycle the energy generated during the discharge of a cell into energy used to charge other cells on the same cycler. This setup can significantly reduce the overall power consumption of the cycler, especially when using high current channels.
Factors which are external to the cycler may also need to be taken into consideration. For example, test channels in a laboratory are usually linked to specific thermal chambers, meaning that space in the chamber, the required test temperature (i.e. can the chamber hold the required temperature?), and the capability of the chamber to be controlled (if required), will all impact the choice of channel.
Getting the right cell cycler for each battery cell testing project
Overall, assigning a cell test to a cycler channel requires some thought and planning, as there is usually an optimal channel on which to run a test. This feeds into the overall setup of the lab: at scale, when running multiple testing projects and a variety of tests, each with their own requirements, there is a need to ensure that enough of the right types of test channel are available. In this wider context, using the wrong type of channel on one project can cause delays in other projects, which may not be able to use the remaining available channels.
Our team at Cognition Energy brings extensive experience to every battery testing project, with a focus on efficient execution and high-quality data collection. With over 600 testing channels (up to a maximum of 1600A), we have a range of various cell cyclers enabling us to ensure that each project uses the best cell cycler, ensuring that we create reliable, relevant data to inform and advance your project.
To learn more about the battery cell testing services offered: