Why is Voltage Sorting Not Enough for Lithium Cells?

Many people assume when sorting lithium cells that voltage is the only necessary metric we need to care about to determine if a specific cell is a good or bad one.
Voltage alone simply cannot sort/test a lithium cell to ready it for use in applications like energy storage, electric vehicles, consumer electronics and more.
Why Sorting on Voltage Is Not Enough
18650, 21700, prismatic and pouch lithium batteries all can look the same when we consider voltage alone.
These cells might have the same voltage even at the end of their life. And voltage alone leaves us utterly in the dark about a battery’s health and performance capabilities. Here are just some of the reasons we can’t rely on voltage sorting:
Voltage Doesn’t Show the Capacity of the Battery
Voltage tells us that the cell might be charged, and that the cell is at a certain point in the charging curve.
But the voltage is not an actual test of capacity of the battery. If two cells have exactly the same voltage, one might have lost much of its capacity due to heat, aging, charge/discharge cycles, etc. And unless that discharged cell undergoes a capacity check, it will be grouped and placed in with cells of the same voltage but of higher capacity, for example.The capacity of lithium cells is often tested with an SOH/health machine that better represents the “true” picture of the cell in real-world operation.
Voltage Doesn’t Tell You Everything Due to Internal Resistance
Every lithium cell has internal resistance. This can vary with the manufacturer of the cell and also its age and usage. This resistance plays a large part in characterising how well the battery performs under load and dis/charge. Sorting by voltage only doesn’t account for this and could mean poorly matched cells will be used together, reducing performance or possibly creating unsafe conditions.
Cells also react differently to temperature – a cold cell will read a lower voltage and a hot cell higher. Testing batteries by voltage results in misremembering the cell (e.g. when hot or cold). This is all left out of the equation.
Real Insights Require Better Sorting Systems
Things you can add to the equation to better test your lithium battery include:
Capacity testing: Making sure only the cells with near-same true energy capacity are sorted into the same category, leading to far better performance overall.
State of health (SOH) testing. Testing the health of the battery on an SOH machine guarantees that the cell with very little degradation is being used. It also improves battery pack life and long-term performance.
Internal resistance. By measuring this aspect of the cells, making sure they are correctly matched means that charge/discharge cycles are maximised and the battery pack operating with long term efficiency.
Why a Multi-Parameter Battery Sorting Machine?
Advanced multi-parameter battery sorting machines eliminate the “mistake” of testing by voltage alone. You get a greater guarantee of only quality, cell-matched batteries being used in the final pack. In industries with safety implications like EVs and renewable energy storage, this is crucial. A semi-automatic / even fully-automatic sorting machine increases the accuracy of your testing massively.
Voltage testing on its own is a poor method for proper classifying lithium cells. While it forms the bedrock of the method, a number of other testing methods are leveraged to get the full picture. For highly-reliable lithium batteries ensure you’re using a comprehensive sorting system that is going to contribute better to the overall performance of the cell long-term.