Grading 21700 lithium-ion batteries for accurate performance testing. As batteries become ubiquitous for everything from electric vehicle(W) powertrains to stationary power storage, the need for accurate grading of performance of these devices grows. While 18650 format cells were an appropriate tradeoff between performance and footprint, 21700 cells provide the additional density and capacity that manufacturers crave. Grading these cells – whether for EV / battery PCB assembly, or secondary use for e-bikes, scooters and other applications – is critical to optimal use, as a poorly-graded cell can mean jerkier performance and reduced battery life down the road. Here is how it’s done, in short – focusing on machine settings, and testing best practices.
Why grading is important to 21700 cells. Grading li-ion battery packs as 21700 cells for big vehicles, or powerwalls as most call them—rides to other power discoverers, Jerick Arbuckle writes: “On an individual cell basis, the function of grading can be performed by measuring such battery characteristics such as: capacity, internal resistance, voltage, and cycle life. Ideally, when a cell has been properly graded, it is used in an application for which it is deemed to have the appropriate performance, minimizing the risk of failure and optimizing overall system efficiency. Improperly graded batteries may exhibit shortened lifespans, capacity fade, or more extreme failures, i.e,” and how it’s done, in short – focusing on machine settings, and testing best practices, starting with machine setup. Voltage ranges and cut-off limits” the voltage range is one of the most important settings when grading 21700 batteries. For proper grading to be achieved, test machines should be arranged to test the same cells in their recommended voltage range, usually between 3.0V and 4.2V. A cut-off is usually used to prevent overcharging and tricks – allowing accurate grading of the cell without pushing into deep discharge which damages the battery.Settings for Discharge and Charge Cycles
21700 batteries are typically graded by going through charge and discharge cycles. Set the discharge current as recommended by the supplier, typically around 0.5C to 1C for most 21700 cells. (C denoting the cell capacity in amp-hours). Over-discharge and speed could lead to decreased capacity.
Internal Resistance
Regardless of their capacity, the internal resistance of every battery is a crucial factor that should be accurately measured for grading purposes. The recent trend in production has seen batteries being loaded with higher discharge rates to provide higher power. Hence the problem lies in identifying batteries’ with internally higher resistance which may lead to inappropriate heating in use. Some grading machines include accurate internal resistance testers.
Temperature
Temperature is critical for lithium-ion batteries. It should be enforced in the grading machine that a controlled temperature tests cells for grading.Cycle Life Testing
Some of the more sophisticated grading systems even have cycle life testing capability, so the battery is fully charged/discharged a few times more to see what it’s like over longer term use. Accurate cycle life testing of 21700 batteries is essential for those applications where long life is an important advantage, like EV battery systems, renewable energy storage systems etc. The number of cycles and the current & voltage points at which this is measured, must match the intended application for the battery. Optimising Grading Settings for Different Applications
For different applications the grading settings for a 21700 may need to change too; for noise reduction in smaller capacity cells, a consumer electronics device may not need such high capacity low resistance cells as in an EV application where compact power is an important product feature.
Best Practices for Accurate 21700 Battery Grading
Regularly calibration of the grading machine and thoroughly testing of multiple parameters such as voltage, capacity, internal resistance, cycle life etc., logging data and tracking performance. Testing in a very temperature controlled environment is even better. Conclusion
Setting correctly specifying suitably-to-the-purpose settings on a 21700 battery grading machine and then keeping it correctly adjusted optimally to grading the applications it’s set to, namely EV batteries, energy storage, consumer electronics, is vital to quality.
