Energy storage system testing is a trending topic today. Commonly referred to as “battery testing,” it ranges from small portable format batteries to the larger ones used in electric vehicles (EVs) to those used in backup systems for high energy supply in so-called “stationary applications.” Depending on the specific context and manufacturing cycle phase of these systems, Tektronix Keithley serves the market with test solutions, like those aimed to serve the pressing needs of system integrators designing automated test systems (ATEs) for EV OEMs. We continue to grow our experience on various test cases and production quality requirements as technology progresses.
Safety, performance, and system management
The “battery testing” context can be really wide, spanning from the characterization of the smallest-possible cell in portable devices to large vehicles batteries operating at 1,000 V or even higher. The battery system is of paramount importance for electrified mobility. Nowadays, lithium-ion batteries are among the most commonly used types in EVs due to their high energy and power density. There are different nomenclatures used for the “batteries” that depend on the market context. In automotive, for instance, depending on the state of integration in the vehicle, the EV battery as a device under test and the related test procedures can differ if you refer to cell manufacturing, modules, or pack manufacturing.
The cell is generally a single electrochemical device, an individual storage unit typically ranging not more than 5 V maximum. The module is composed of several connected cells and some other electronics to control the entire system. A module is somehow packaged, so tests generally involve the whole thing as a single element. A pack is a larger element composed by several modules, again connected by some wiring and with more sophisticated control and communication electronics on board to communicate with other processing units, like in the case of vehicles.
As mentioned, testing cells is not the same as testing modules, or testing packs, and test setup can vary at each stage of the manufacturing value chain. Tests can finally differ by the test methodology used, as in the case of impedance measurement.
Tektronix Keithley is supplying solutions for test system designers covering electrical tests, concentrating on wherever a potential voltage, current, and resistance measurement are needed in complex ATEs for system integration testing in both battery manufacturing (e.g., cells, modules, and pack assembly lines) and final application integration (e.g., automotive battery management systems [BMS] and battery pack integration).
Tests generally refer to three main areas: safety testing, critical for a system built as a combination of several cells arranged in series/parallel topology to deliver a higher power density; performance testing of the battery cell/module/pack, closely related to the number of charging/discharging cycles, running time, and temperature; and management testing, when performance optimization and EOL test validation is key.
Example 1: Busbar weld impedance safety test workstation in battery pack manufacturing
The multiple cells composing a battery module are connected in parallel or series to achieve the desired voltage output. All cells are laser-welded to a busbar, a long conductor that is isolated from ground and is responsible for carrying high current for distribution of power from the battery. The VSH-busbar weld impedance test characterizes the impedance of the weld. Small resistances in the weld can generate enough heat to degrade the batteries and lead to early failures or unsafe operating conditions. By measuring the resistance before testing the battery operation, defective modules can be quickly removed from the line.
A generic representation of busbars in batteries
Measuring the impedance of the weld involves sourcing a current across the weld and measuring the voltage to calculate the resistance. Test execution speed and measurement accuracy are the two most important considerations when measuring the weld impedance. This can be done using source measure units (SMUs) like the Keithley models 2460 or 2461 and either the model 3706A system switch and multimeter or the model DAQ6510 data acquisition and logging multimeter system.
click for full size image
A schematic example of an automated system for impedance test in battery production
The model 2460 and 2461 SMUs are capable of sourcing up to 7 A for battery systems requiring high current. The impedance of the weld can be as small as a few milliohms. Therefore, it’s important to use a sensitive-enough meter to measure very small voltages. The model 3706A features a 7.5-digit digital multimeter (DMM) and can measure tens of nanovolts in the 100-mV range. Because a battery pack could have close to 80 welds on one busbar, we support mainframes with configurable slots for multi-channel plug-in modules, eliminating the need for rewiring. The process of closing each channel to measure is obviously automated for speed and efficiency.
Example 2: Internal resistance measurement and open-circuit voltage in cell performance testing
The performance of a battery and its efficiency during the charge and discharge process can be evaluated in few different ways and looking at several indicators. The battery internal resistance characterization is one of them, and it basically means accurately characterizing its changes under several charge/discharge current rates, state of charge, temperature, and other aging indicators.
Open-circuit voltage (OCV) is the voltage measured at the terminals of the battery after enough rest time (sometimes called “relaxation”), and it is a key measurement for Li-ion battery cells.
OCV also varies mostly according to battery state of charge and at a less extent according to the temperature, and it can be used to create a battery-equivalent model to design a BMS not only to evaluate/assess battery specs and condition.
Internal resistance in the battery accounts for the voltage drop across the battery’s terminals when a load is connected compared with no-load voltage and can be derived from OCV measurements.
OCV is generally not just a measurement but a set of measurements. In fact, we call it the “OCV characterization of the battery,” and we trace an exhaustive analysis derived from a curve on a state of charge versus OCV plane.
To trace this curve, you need to bring the battery to specific states of charge, typically by charging or discharging current in a pulsed way using a smart source/load, waiting for some settlement time, and then measuring the open-circuit potential at the electrodes.
A Keithley SMU like the 2460 or the 2461 (with 10-A pulse capability and digitizers) is a perfect solution to perform this test. In fact, it can either source or sink the cell current in a controlled way while measuring the cell current and voltage with a four-wire (Kelvin) connection with contact check. All this is easily automated and controlled by a programmed embedded microprocessor.
The accuracy of the voltage measurement in OCV is a discriminating factor for the choice of the instrument. In some cases, the typical 6 ½-digit measurement resolution, the thermal stability, but mostly the accuracy of an SMU can be considered insufficient.
For this reason, some test setups involve a special DMM, the Keithley DMM7510, that became a standard in Li-ion battery cell testing. Its low-noise 32-bit A/D converter allows 7 ½-digit resolution and metrology-grade accuracy.
Tektronix’s Keithley DMM7510 is a reference 7 ½-digit multimeter with great accuracy specs for situations where small voltage drops or tiny leakage currents have to be detected.
Example 3: BMS testing and a special case for collision switch detection
A BMS is a specific element in charge of performing critical battery functions like cell monitoring, cell balancing, charge and discharge control, safety control, and communication with external units. Several ATE designers work to squeeze into a compact and reliable platform all the needed test units to control the interaction between the BMS and the battery.
These ATEs for validation are typically modular elements, with items from multiple vendors combined to operate together as a system. The system needs to track and log multiple input signals coming from the battery and the BMS. The sensing units and the I/O communication stage are indeed critical and must be implemented for proper screening. In some cases, the choice of the individual instruments to compose the system is partially driven by the test management software environment, but in general, system integrators prefer to design custom solutions based on OEMs’ requirements, which are independent from a specific environment granting an interchangeable and fast execution in parallel of several automated validation test systems.
To validate the BMS prior to the real battery system interaction, you may need to simulate the battery pack voltage. This means to control a precise 1,000-V (or more) voltage source or even the simulation of hundreds of individual cell voltages. Environmental stress chamber is another key sub-element to include for temperature and test atmosphere control. From an SMU perspective, Keithley support with model 2470 is capable of exceeding 1-kV test capability.
Aside from the specific portfolio of data loggers and acquisition switch cards like the DAQ6510, for instance, we will focus now on the need for voltage and current pulsers chosen for the specific design of test setup for BMS reaction to low energy collision during a DC fast charge.
Let’s consider the case of a vehicle plugged into a DC charger in a parking lot that suffers a low speed collision. How will the BMS react? How do you exclude critical faults like isolation? The collision signal to the BMS may be a voltage pulse or a current pulse, depending on the actual situation. Regardless of the type, the signal is required to be clear and stable enough to resist interference. We also supply solutions with our AFGs to simulate error frames in the CAN bus message communications to recreate potential fault situations and test the system’s robustness against it.
Tektronix’s AFG31000 series can be programmed to replicate real-world bus signals with customized impairments to replicate specific potential failure conditions
Electrical measurement of Li-ion batteries is a very broad topic, and depending on the manufacturing stage or the stage of testing in the application integration, different sets of measurements with different requirements need to be implemented by smart automated test equipment. Tektronix is historically a key supplier with oscilloscopes and probing solutions to test the battery behavior when loaded by the motor drive inverter, but the Tektronix and Keithley portfolio supports the space of battery manufacturing in particular when highly accurate resistance, isolation, or voltage and current measurements must be implemented while collecting data across multiple sensing entry points. In particular, special source measure units like the 2470 SMU capable of sourcing over 1 kV while accurately measuring current, or the 2461 covering high-current pulsed sinking for battery cycling, as well as industry reference digital multimeters like the DMM7510 for OCV and discharge current characterization, are specifically designed to seamlessly integrate in test automation solutions.
For More Information
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- Measurement challenges with Wide Band Gap devices
- Please visit the ebook for the complete article
>> This article was originally published on our sister site, Power Electronics News.
|Andrea Vinci has an M.Sc. in Electronic Engineering from the University of Padova, Italy, where he specialized in Metrology and Instruments. After ten (10) years in the Telecommunication industry across several large corporates as a technical design consultant, Andrea joined Tektronix in 2011. He covered roles as Field Applications Engineer, Account Manager, Business Developer and Solutions Manager in the Power and Automotive Business Unit at Tektronix before joining the Marketing department in 2019.|
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