Using DKCMS™ and DCIR to Improve Battery Safety

We’ve previously discussed the importance of cell-level temperature measurement in enhancing a high-power battery pack’s operational safety, performance, and lifespan¹. By utilizing the Dukosi Cell Monitoring System (DKCMS™), battery designers can proactively monitor the granular cell temperature profile in a pack, gaining valuable insights for diagnostics and preventive maintenance, enabling early detection of abnormalities or potential issues.

Dukosi Cell Monitors are typically installed directly on each cell (chip-on-cell) and can measure up to three temperature points on the cell’s exterior surface. One sensor is integrated into the chip itself, and, if required, two additional thermistors can be connected to extend coverage, which is particularly useful for large-format cells with extensive surface areas.

Another non-invasive method for gaining insights into cell temperature is Direct Current Internal Resistance (DCIR). This technique measures the internal resistance, which exhibits a temperature-dependent variation, allowing us to infer the internal cell temperature.

The Dukosi Cell Monitor can perform on-cell measurements

Unlike other solutions, Dukosi Cell Monitors are capable of performing DCIR without external stimuli, and can uniquely capture a direct external temperature reading and an internal resistance test at the same time, providing complementary, synchronous data.

This enhances safety and reliability, offers an additional data point for more accurate State-of-X (SoX) estimations during the lifetime of the cell, and deepens understanding of how cells generate and propagate heat, which is valuable information for both cell and battery design.

What affects DCIR measurement?

Both environmental factors and cell state affect DCIR measurements:

• DCIR varies significantly with temperature
• DCIR varies less with State of Charge (SoC)
• DCIR increases with age
• DCIR accuracy depends on minimizing parasitic losses

We can map the relationship between internal resistance and temperature, but we must also account for cell aging. This requires a large dataset, as no two cells degrade in exactly the same way. Directly measuring the cell temperature provides the advantage of tracking an actual measured value, allowing us to validate inferred assumptions. While silicon chips also age over time, their variance is well documented and accounted for in their design.

Battery- versus cell-level in DCIR measurement

DCIR can be measured at both the battery-level (all cells simultaneously) and the cell-level (every cell individually). Measuring at the battery-level includes not only the internal cell resistance, but also the resistances of all ancillary materials, such as cell-to-cell busbars. In contrast, measuring at the cell-level includes only the sense leads between the Dukosi Cell Monitor and the cell terminals.

A 50x20x2mm copper busbar has a 10 µΩ impedance, while a bolted contact might vary between 10 µΩ to 1 mΩ, depending on its cleanliness and contact pressure, but a 100mm long / 2mm diameter sense wire has a much larger 500 µΩ impedance.

An example of a sense lead and copper busbar

In comparison, large format LFP cells typically used in BESS modules have typical DCIR values of under 500 µΩ, so clearly ancillary resistance can significantly limit the sensitivity and accuracy of measurements. In addition, in-field measurements taken throughout the life of a battery are subject to material changes like corrosion and connection wear, affecting measurement accuracy over time.

Dukosi Cell Monitors can measure cell-level DCIR, providing highly accurate and repeatable results, because it minimizes the distance and wiring between itself and the cell terminals. In addition, thanks to DKCMS’ simplicity, it is cost-effective to implement for system integrators and battery designers, while also eliminating the additional processing overhead that the BMS host would otherwise require for battery-level measurement.

The Dukosi Cell Monitor EVK² with DCIR and EIS is now available for early stage battery development

Furthermore, cell-level DCIR is self-contained and isolated from other loads (contactors open). By eliminating the need for external stimulus, each cell is measured without outside influence, enabling more accurate comparisons and easier identification of outlier cells. This contrasts with battery-level DCIR, which requires waiting for when the battery is in use (contactors closed) to use a current pulse, for example, during vehicle acceleration, HVAC operation, or fast charging.

When compared directly, battery-level DCIR may be less accurate than cell-level DCIR, since the steadiness and duration of the current pulse may not be as precisely controlled and in many cases the cell voltage measurements are also not fully synchronized with the current pulse.

Nevertheless, despite the higher accuracy of cell-level DCIR, the two measurements are ultimately complementary. When cell-level DCIR is captured alongside battery-level DCIR, the BMS can compare operating and non-operating battery states, identify the sources of resistance changes, and provide additional insights for operators.

What else can we use temperature data for?

Temperature data plays a crucial role in several battery management functions:

For State of Charge (SoC) estimation, temperature directly affects accuracy by influencing cell performance and allowing the reduction of safety margins.

In State of Health (SoH) tracking, temperature data can be used when comparing a cell’s DCIR against other cells (SoH-R) or independently, using an external temperature sensor as a reference point.

Finally, in State of Power (SoP) estimation, temperature information helps dynamically determine the battery’s safe operating power range, preventing cells from exceeding safe voltage or internal temperature limits.

Conclusions

Capturing two temperature data points using two different but complementary methods can significantly enhance battery safety and reliability across various scenarios and applications:

• Measuring both external and internal cell temperatures can create a detailed thermal propagation map, helping to determine whether a cell is heating due to external or internal factors, without the need for physical inspection.
• Providing a layer of redundancy by comparing temperature sensor data with internal resistance as the cell ages can help identify whether a sensor reading is erroneous, or if unexpected changes have occurred within the cell or its environment. This is particularly important in mission-critical applications such as marine or aerospace.
• Improving the accuracy of State-of-X (SoX) calculations that incorporate cell temperature as an input variable.
• Supplying additional data points for analyzing heat propagation and cooling performance during cell or battery development, without requiring expensive equipment.

1. https://www.dukosi.com/blog/monitoring-the-temperature-of-every-cell-to-maximize-safety-and-performance-of-high-power-batteries ↩︎
2. https://www.dukosi.com/evk ↩︎

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