Electric vehicles (EVs) are rapidly becoming the standard in transportation, driving an unprecedented demand for high-performance battery packs. Ensuring these batteries meet stringent requirements for power, charging speed, safety, and longevity is paramount. Battery and automotive manufacturers, as well as repair professionals, must rigorously test battery packs for defects and performance issues. This testing phase can be a bottleneck in production and maintenance, making efficient test methods highly valuable. One crucial measurement in battery assessment is the Open Voltage Test, also known as the open circuit voltage (OCV) test. Understanding how to perform this test at the battery module or pack level is different from testing individual cells. This guide, tailored for auto repair experts, will explore various methods for measuring open voltage on EV battery packs, whether at the full pack level, on individual cells in parallel, or cells in series.
Understanding Open Circuit Voltage (OCV) in Battery Diagnostics
The open circuit voltage (OCV) of any electrical device is the voltage measured when no external load is connected. In a battery context, the OCV reading reflects the potential difference between its positive and negative terminals. This potential difference is a direct consequence of the battery’s internal chemistry and serves as a key indicator of its state of charge (SOC)—essentially, how much energy is currently stored within the battery. In fact, OCV measurement is frequently employed to define and characterize the relationship between SOC and OCV at the individual cell level. Monitoring the OCV of a cell over time, particularly as it self-discharges, can also reveal underlying defects, enabling manufacturers and technicians to accurately assess battery quality and health.
EV battery packs are complex assemblies of numerous cells, interconnected in series or parallel configurations to achieve specific voltage, current output, and energy storage capacities. These cells are often grouped into modules, which are then combined to form the complete battery pack. The open voltage test on a pack or module level provides insight into the overall energy storage capacity of the entire unit. This information is vital for verifying charging and discharging behavior within the vehicle, monitoring self-discharge rates to identify potential issues, and accurately modeling battery pack performance under diverse environmental conditions.
Furthermore, measuring the open voltage of individual cells within a pack is often as important as assessing the entire pack voltage. This granular approach is particularly useful for evaluating the effectiveness of cell balancing during charging and discharging cycles. Proper cell balancing is crucial to prevent stress on individual cells, extend battery life, and validate the accuracy of battery management systems (BMS). Individual cell OCV measurements can also pinpoint defective cells within a pack, streamlining diagnostics and repair processes. A significant difference between cell-level and pack-level measurements is the voltage range. Individual cells typically operate below 10V, requiring high-resolution measurements, whereas pack voltages can reach hundreds or even thousands of volts, necessitating different measurement considerations and equipment. Therefore, specialized methods are needed to accurately isolate and measure the voltage of each cell within the complex interconnected structure of a battery pack.
Open Voltage Testing of the Entire Battery Pack
Despite being composed of numerous cells, a battery module or pack can be treated as a single, larger battery unit when measuring its overall open voltage. The voltage can be directly measured across the main positive and negative terminals using a digital multimeter (DMM), as illustrated in Figure 1.
Figure 1 (a). Representation of battery cells within a pack. (b). Equivalent circuit diagram for (a). (c). Direct connection of a DMM to a battery pack for OCV measurement. (d) Equivalent circuit diagram for (c).
At the pack or module level, voltages and currents are significantly higher than at the individual cell level. When selecting a DMM for pack OCV measurements, it is crucial to ensure it has a high input impedance (10 MΩ or greater). This high impedance minimizes battery discharge during measurement, which could alter the reading or, in high-current scenarios, potentially damage the test system. Always verify that the expected voltage is within the DMM’s specified voltage limitations. For battery modules or packs capable of high current output, incorporating fuses into the test setup is a critical safety measure to protect the DMM and other test equipment from potential damage due to overcurrent conditions.
Open Voltage Testing on Parallel-Connected Cells
Battery cells are connected in parallel to increase the overall current capacity of the battery system. In a parallel configuration, the open circuit voltage across each cell and the combination remains theoretically the same. To measure the open voltage of an individual cell within a parallel arrangement, connect the DMM directly across the terminals of that specific cell, as shown in Figure 2.
Figure 2: Illustration of OCV measurement for a single cell within a parallel battery configuration.
The considerations for this measurement are similar to those for testing a single, isolated cell. High voltage exposure is generally not a primary concern in parallel configurations. However, the risk of high current events due to short circuits is significantly elevated, as depicted in Figure 3.
Figure 3: Depiction of potential high current flow resulting from a short circuit across the DMM in a parallel cell configuration.
To prevent damage to the batteries or test equipment, meticulous care must be taken to avoid any short circuits during test connections. Implementing fuses within the test system is strongly recommended to provide crucial protection against high current surges, safeguarding the test equipment from potential damage.
Open Voltage Testing on Series-Connected Cells
Cells are connected in series to achieve higher voltage potentials within the battery system, while the current capacity remains consistent across all cells in the series string. In series configurations, while short circuits are less likely to cause immediate, severe high-current events compared to parallel setups, the primary concern shifts to managing high voltage exposure for the measuring instruments. Fusing, while still good practice, becomes less critical for immediate safety than in parallel circuits.
Figure 4a illustrates a configuration with multiple cells connected in series. If one terminal of the battery pack is connected to ground, a common approach, measuring the open voltage across each individual cell might seem straightforward. This works on the principle that DMMs measure differential voltage—the potential difference between the HI and LO terminals. However, with one end of the battery pack grounded, the LO terminal of the DMM can float at a voltage potential significantly above the common ground reference. This voltage difference is known as the common mode voltage. If this common mode voltage exceeds the DMM’s specifications, it can lead to damage to the instrument’s input circuitry. For instance, with a DMM rated for a maximum common mode voltage of 500V, attempting to measure the OCV of the last cell in a long series string, where the cumulative common mode voltage exceeds this limit, could damage the DMM’s LO terminal.
Figure 4: (a) Measuring OCV on series-connected cells with a common ground at one end of the pack, leading to high common mode voltage. (b) Relocating the ground connection with the DMM’s LO terminal to mitigate high common mode voltage. (c) Grounding the pack at its center to keep common mode voltage within acceptable limits.
This risk can be effectively mitigated by adjusting the grounding strategy. As shown in Figure 4b, by moving the common ground connection to coincide with the LO terminal of the DMM as it moves along the series string, we prevent the buildup of high common mode voltage relative to the DMM’s LO terminal. Battery potential is inherently relative to its electrodes. A fully charged battery has a cathode at a higher potential than the anode, typically by the battery’s nominal voltage. In series connections, this potential difference accumulates across each cell, but there is still no absolute ground reference until we establish one externally. This flexibility allows us to connect any point in the circuit to common ground, ensuring our DMM’s LO terminal is effectively at 0V relative to its measurement point, thus preventing damage from excessive common mode voltage.
Another variation of this method involves permanently grounding the electrical center of the battery pack, as shown in Figure 4c. In this scenario, the battery pack potential is symmetrically distributed around ground, with half of the pack voltage potential below ground at the low potential end and half above ground at the high potential end. As long as this voltage range stays within the common mode voltage specifications of the DMM, the instrument remains protected from damage. This approach can simplify the measurement setup for series-connected cells by providing a fixed ground reference point.
Testing Series and Parallel Battery Combinations
Most practical battery packs utilize a combination of series and parallel cell configurations. For instance, modules might consist of groups of parallel-connected cells that are then connected in series to form the module, and these modules are subsequently connected in series to create the final battery pack. In such complex arrangements, it is crucial to consider both the need for fuse protection against high current events (relevant to parallel connections or potential shorts) and proper grounding techniques to prevent high common mode voltage exposure (relevant to series connections). At each measurement point, carefully assess the potential voltage levels and possible short-circuit current magnitudes to implement appropriate safety and measurement strategies.
Multichannel Open Voltage Test Solutions
Battery packs in EVs can contain hundreds of cells, making efficient testing solutions essential. Multichannel DMMs offer a way to significantly reduce testing time and equipment costs. These systems use integrated switching matrices to sequentially measure multiple signals without requiring manual rewiring for each measurement point. When employing a multichannel system for open voltage tests, it’s important to be aware that the switching pathways and components introduce some level of measurement uncertainty. Relays within switch cards have maximum signal level and common mode voltage ratings that must be taken into account, especially when measuring numerous cells connected in series. Proper fuse selection and voltage rating considerations are critical for the entire multichannel system.
Common mode voltage specifications are particularly crucial in multichannel setups. The specified limit typically applies across all channels and might also apply between signal paths or between signal paths and chassis ground. For cell configurations that could expose the DMM or switching system to high common mode voltages, consistently routing the LO terminal to ground can protect the DMM’s measurement circuitry. However, the unused switch terminals may still be exposed to high common mode voltages. In such cases, the entire switch system, including the switch cards and backplane, must be rated to safely handle the highest voltage potentials present in the battery pack. Figure 5 illustrates an example of using a multichannel system to measure multiple series-connected cells.
Figure 5: (a) Multichannel DMM configuration for measuring open voltage on series-connected cells. (b) Potential high common mode voltage exposure on channel 2 terminals during measurement.
Advanced Tools for Open Voltage Testing
Keithley Instruments offers several advanced solutions specifically designed for accurate and efficient open voltage testing of battery packs, as shown in Figure 6. The DMM7510, with its 7.5-digit resolution and high accuracy, is ideal for critical applications like battery characterization and self-discharge monitoring, providing a highly detailed view of even minute changes in open voltage. The DMM6500, offering 6.5-digit resolution, provides a balance of accuracy and cost-effectiveness, suitable for routine OCV monitoring as part of broader battery test sequences where extremely high resolution is not always necessary. For high-throughput, multichannel testing, the Keithley 3706A System Switch and Multimeter stands out. This system features a modular design with six slots for various switch cards, including high-density cards supporting up to 576 two-wire channels and high-voltage cards capable of measuring up to 1000V.
Figure 6. Overview of Keithley single-channel and multichannel solutions for open voltage testing.
Conclusion
Measuring the open voltage of a battery pack is essential for a multitude of reasons, from initial quality checks to ongoing diagnostics and performance monitoring in EVs. Several methods are available for conducting these open voltage tests, each tailored to different battery configurations and measurement objectives. In any high-energy system, especially EV batteries, the paramount consideration is always safety. Understanding the potential energy levels, voltage and current, within the system is crucial. Implementing appropriate test procedures, safety protocols, and protective measures is vital to ensure accurate measurements while safeguarding both personnel and equipment. By carefully considering the specific battery configuration and measurement goals, and by employing the right techniques and tools, automotive professionals can effectively utilize open voltage testing for comprehensive EV battery diagnostics and maintenance.
