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Convection isn’t cool: precision thermal control of the Molicel P45B (Part 2 of 2)


a baker putting batteries in an oven in a EV manufacturing plant

Understanding the impact of temperature on battery performance is critical for battery system development, but in most labs, this parameter is not controlled effectively. The second edition in our two-part series delves into how our thermal control hardware aids battery model development with reference to lab data on the Molicel P45B (a cell we have reported on here and here). This case study examines one of the most commonly used high-power 21700 cylindrical cells across motorsports, aerospace, and other performance industries. In case you missed it, the first part explored the reasons for these challenges and, more importantly, how we overcome them to produce high-quality data and parameter sets.


Key benefits our thermal control hardware provides:

  • Temperature Range: Provides precise control from -60 to 100°C to analyse battery performance in extreme environments.

  • Voltage Response: Enhances the correlation between temperature and voltage response, aiding in pack sizing and state of charge estimation.

  • Lifetime: Reduces cell-to-cell variability in degradation experiments and enhances understanding of temperature impacts on battery lifetime.

  • Time-to-insights: Reduces the time and costs associated with data extraction through space-efficiency and cycling protocols.


Molicel P45B case study

Here, we quantify the benefits of our isothermal control method compared to traditional convection thermal chambers in battery testing and parameterisation, using the Molicel P45B. As mentioned in Part 1, About:Energy's unique thermal control method plays a critical role in industry research and development. Co-Founders Gavin White and Alastair Hales have developed, over several years, both the software and hardware for isothermal temperature control (detailed in a paper here). This hardware utilises conduction and the Peltier effect, a thermoelectric phenomenon where heat is pumped when electric current is passed.



About:Energy's thermal control rigs for cylindrical batteries, testing the Molicel P45B. These adaptable rigs provide high-quality data essential for informed engineering decisions.

Peltier Method: About:Energy's thermal control rigs for cylindrical batteries, testing the Molicel P45B. These adaptable rigs provide high-quality data essential for informed engineering decisions.


For this experiment, we subjected the cell to load conditions at various temperatures. This involved running drive cycles and comparing the voltage responses, as well as the surface temperatures of the batteries. To understand repeatability, we conducted three repeats for each cell type in the experiments. See below for the experimental setup and power profile of the drive cycle used. To understand the value of using our thermal control, we examine voltage response, temperature control, and repeatability. We also explore the direct economic benefits of using scalable hardware in lab facilities instead of traditional convection chambers.

Drive Cycle: Power profiles for the drive cycle used to test the Molicel P45B.


🌡️ Temperature Range

For automotive qualification of batteries and the development of new cell chemistries, understanding battery performance in extreme temperatures is important. In this test, we examine the Molicel P45B under extreme temperature testing at -5 °C. Measuring battery performance in these conditions is particularly crucial, as it relates to state-of-charge and state-of-health prediction. State-of-charge inaccuracy is especially relevant for many who experience sudden drops in their phone's battery from 20% to depletion, or for those driving EVs where, in cold climates, the range predictor can be inaccurate.


To study our ability to test temperature ranges, we performed a test at -5 °C. Here we noticed a wide range of temperatures in the convection chamber, significantly impacting the validity of the data. In this test, the temperature of a battery increased significantly, reaching 30 °C, a deviation of 35 °C from the desired temperature.

Thermal Response: The temperature of the Molicel P45B under a drive cycle using our thermal control method and a convection chamber.


To create the model, we aim to precisely control parameters for our training data. However, for validation, we might want to emulate a cold start, which is also achievable with our thermal control method. This approach provides relevant pack validation and emulates the thermal management system. The thermal control can be used to heat or cool both sides, one side, or even the base of the battery.




The temperature window of Molicel P45Bs when subjected to a drive cycle at -5 °C, illustrating the differences in surface temperature due to the convection and thermal control set-up.

Temperature Range: The temperature window of Molicel P45Bs when subjected to a drive cycle at -5 °C, illustrating the differences in surface temperature due to the convection and thermal control set-up.


⚡ Voltage Response

The voltage response of batteries in an experiment is crucial as it provides essential information for assessing performance. As seen above, the change in resistance between the two methods will influence the voltage behavior. In the thermal chamber, as the cell's temperature increases, the battery's voltage response will differ, increasing power output due to reduced resistance.


As a high-power cell, the P45B has a low resistance of 13.8 mΩ at 25 °C. However, this high performance can worsen at low temperatures due to resistive losses, as observed while testing our thermal control method and a convection chamber. In this experiment, the resistance nearly halves when the temperature changes from -5 °C to 30 °C. This change in resistance significantly impacts the voltage response of the battery and, therefore, affects the validity of data for subsequent R&D activities. Conducting these experiments accurately at low temperatures is crucial for developing accurate models for state-of-charge estimation, and eliminating range uncertainty in cold climates.


The temperature window of Molicel P45Bs when subjected to a drive cycle at -5°C, illustrating the differences in surface temperature due to the convection and thermal control set-up.

Resistance: The temperature window of Molicel P45Bs when subjected to a drive cycle at -5°C, illustrating the differences in surface temperature due to the convection and thermal control set-up.


Notably, the cell in the thermal chamber discharged 3.99 Ah before reaching the cut-off voltage, while the cell in the thermal control rig discharged only 3.22 Ah. The resulting data from our thermal control method offers a more accurate basis for making decisions regarding the practical application of batteries. This method reflects a more realistic scenario when the thermal management system is considered, ensuring that our tests enable accurate predictions about real-world battery performance and longevity. The voltage behaviour will vary even more in other drive cycle cases, and for larger batteries that are not tailored for high-power applications.

Voltage Response: The voltage responses of the Molicel P45B under a drive cycle using our thermal control method and a convection chamber at -5 °C.


❓ Uncertainty

Another significant benefit of our thermal control setup is its ability to reduce uncertainty in measurements and variability in results. Test variability in a thermal chamber arises from two main sources. Firstly, there is a temperature gradient within thermal chambers due to differences in convection, which leads to experimental error. Secondly, affixing thermocouples onto cells with Kapton tape impacts the robustness and reproducibility of the temperature measurements, introducing measurement error. Additionally, due to the engineering setup, the thermistor is always positioned in the same place, resulting in highly replicable temperature measurements.


As observed in the Molicel P45B drive cycle tests at -5 °C, there is a significant difference between the recorded peak temperatures of each cell in a thermal chamber, with a difference of nearly 20 °C between the three cells. Batteries experience different temperatures in the same test within convection chambers due to the uneven temperature distribution and inconsistent heat dissipation across the chamber. Initial differences in cell temperature lead to variations in resistance, resulting in disparities in heat generation across the cells. This creates a positive feedback loop where the generated heat further amplifies the initial temperature differences, perpetuating and intensifying the cycle. Our thermal control method reduces this variability because each cell is independently thermally controlled with precision.

Temperature Variability: Repeatability of the measurement of battery surface temperature using Molicel P45Bs, comparing three batteries tested using our thermal control method and convection chambers.


In our thermal control setup, there is negligible variability between repeated measurements. This contrasts with the results from the convection chamber, which exhibits greater variability and a corresponding increase in voltage error. This observation suggests that the majority of the uncertainty in measurements is due to poor thermal control rather than intrinsic variability stemming from the cell design or manufacturing process.


Comparison of voltage uncertainty between our thermal control method and thermal convection chambers.

Quantifying Voltage Uncertainty: Comparison of voltage uncertainty between our thermal control method and thermal convection chambers.


💀 Degradation

The significance of thermal control units becomes increasingly apparent when examining battery lifetime. As evident from the graphs below, batteries tested under similar operating conditions exhibit similar aging trends. This data is crucial for developing accurate State of Health (SOH) models and determining the Remaining Useful Life (RUL) of the batteries. Our findings further emphasise that at a pack level, variations in aging are predominantly attributed to temperature variability, rather than intrinsic variability of the batteries themselves.


Understanding and managing these variables – specifically temperature – is key to extending battery life and ensuring consistent performance across the board. The advantages of conduction vs convection cooling apparatus may be particularly significant under degradation experiments - as the long timescales involved make it very difficult for any analyst to account and compensate for the temperature variation exhibited in the convection chamber. The graph below shows that, over hundreds and thousands of cycles, several Molicel P45B batteries exhibit almost identical degradation. Most differences in battery degradation can be attributed to variations in the operational envelopes of these batteries.

Reducing Cell-to-Cell Variability in Battery Lifetime: This graph compares battery state-of-health using our precise thermal control method, highlighting tight grouping for cells.


💸 Economics

Developing a scalable lab architecture is crucial when your business model revolves around data. It necessitates continual innovation to enhance efficiency, speed, and cost-effectiveness. In most industries, being 'faster' and 'better' is essential for business success. This is where our thermal control innovation becomes crucial for scaling up and rapidly building models. Below is a visual comparison of our thermal control method versus thermal chambers, illustrating the advantages in terms of space efficiency.


Thermal chambers, constrained by cabling, connectors, and safety, typically test only a few cells per chamber. Our thermal control method tests more batteries in the same space with independent temperature settings for each cell, reducing costs by eliminating uniform heating and allowing for quick temperature changes.



Comparison of thermal control method vs. thermal chambers - highlighting space efficiency and temperature parallelisation. Each block represents a battery testing channel.

Comparison of thermal control method vs. thermal chambers - highlighting space efficiency and temperature parallelisation. Each block represents a battery testing channel.


🕒 Time

Our cutting-edge thermal control unit offers a substantial advantage over traditional convection chambers, particularly in its ability to modulate battery temperatures at an unprecedented rate 45 times faster than conventional methods. This rapid temperature adjustment capability is crucial for executing accelerated test plans efficiently, allowing for more tests in less time. Moreover, it ensures the highest quality of data accuracy; measurements are taken with the assurance that the battery temperature has precisely reached the set target. This reliability and speed facilitate more dynamic testing scenarios and can significantly streamline the research and development process for battery technologies. This enables us to pass savings onto our customers through a vast database of cell datasets and models at a low cost and with rapid turnaround times.

Rapid Temperature Stepping: Comparison of thermal control method vs. thermal chambers - highlighting time saving of temperature stepping with our hardware.


Summary

The advantages of our thermal control technology over traditional convection chambers are multifaceted, addressing key areas including quality, cost, flexibility, and time. This innovative approach not only enhances the efficiency and accuracy of battery testing but also provides substantial economic benefits that we pass onto customers:

  • Quality: Greater confidence in test data and simulation results enables more robust optimisation of battery systems.

  • Cost: Our method reduces both CapEx and OpEx compared to traditional thermal chambers, offering space savings and avoiding extra expenses.

  • Flexibility: Cells can be electrically cycled with an independent thermal drive cycle, streamlining tests like extensive degradation experiments.

  • Time: Conduction allows for shorter battery test plan durations, as switching between temperatures is quicker.


However, achieving perfect "isothermal" conditions is still challenging due to the inevitable thermal gradient between a cell’s core and surface. With the introduction of new high-capacity batteries like the Molicel P50B, Samsung 53G, and LG M58T, and larger-format prismatic cells from CATL, BAK, and EVE, managing thermal control during testing is increasingly crucial.


Looking ahead, we aim to deepen our exploration of thermal control through case studies on state estimation, lifetime prediction, and thermal management design. These studies will not only offer practical insights but also highlight the business advantages of our innovations. As we advance our technology, we are committed to keeping our customers informed about the progress and breakthroughs in this dynamic field.

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