Before filling the electrolyte into the cell, defective products need to be removed from production. To identify defective products, you can run a test on the insulator (also called the separator) that involves a charging-dwelling-discharging sequence and measure the leakage current.
When issues with the separator exist (membrane problems, decomposition etc.), the failure is easy to detect as the level of current is in the range of tens of mA. When insulation is good, the leakage current can be quite low (in the pico Ampere range). The insulation resistance test is also repeated in the module assembly and pack assembly stages to prove the insulation is good for different parts with respect to ground (chassis).
The Keithley model 6517B Electrometer/ High Resistance Meter offers insulation resistance measurements at various calibrated insulation test voltages. The 6517B has very low current sensitivity and a built-in 1kV voltage source with sweep capabilities. This simplifies performing leakage, breakdown, and high-resistance testing as well as surface resistivity measurement on insulating materials.
The formation and electrical testing of individual battery cells occurs in the last steps of the production line and generally represent a significant bottleneck for mass production.
One of the key characteristics of a battery cell is its performance over the course of its life, so cells are put through an aging process to investigate cell degradation.
A typical characterization of performance over time is achieved by cycling the cells. Cycling is essentially a repeated charging and discharging process.
A current and voltage profile representative of the intended application under typical application-related stress conditions (e.g. temperature variation) is applied to cells.
It is important to note that lifetime requirements for cells, modules, and systems are determined by the application where they will be used (e.g. electrified mobility, Consumer Electronics, etc.).
There are several cycling (charging-discharging) protocols defined by standards for each specific application (e.g. IEC 62660-2).
Depending on the methodology, a programmable power supply source is used to apply a constant current - constant voltage (CC-CV) charge procedure, taking the cell from an initial set voltage to a final set voltage. Then the voltage is fixed, and the current is reduced until a certain capacity is reached.
A discharge cycle is generally applied via a programmable electronic load.
All these cycles are repeated several times with some variable “relaxation” time between charge and discharge.
These tests are typically executed in a temperature chamber; the variation in temperature that a cell experiences is also dependent on the intrinsic thermal variation during charging and discharging.
A multi-channel system based on several source measure units like Keithley SMUs, which capable of measuring while sourcing or sinking, can be used in this stage. The system typically requires multichannel DMMs with specific resolution, accuracy, and stability over time and across environmental conditions.
Players compete in introducing the most efficient methodology on end-of-line testing and formation. Meanwhile system integrators challenge themselves in developing efficient test procedures and parameter extraction regarding cell quality monitoring.
Test racks in this space are generally highly automated, involve multiple instrument units, and generally perform the following measurements:
The automation of data collection is critical here and helps the following grading phase, when cells are sorted according to their performance test results.