How graphene-based sensors are advancing EV battery cell monitoring

Author : John Tingay | Chief Technology Officer | Paragraf

01 November 2021

Electric vehicle charging
Electric vehicle charging

Battery performance remains a crucial factor in accelerating electric vehicle (EV) adoption, with customers seeking extended range driving & fast charging times. But traditional methods for measuring current density have technical limitations that make the development of cell technology challenging.

This article was originally featured in the November 2021 issue of EPDT magazine [read the digital issue]. And sign up to receive your own copy each month.

However, as John Tingay, Chief Technology Officer at Cambridge-based spin-out Paragraf explains here, new one-atom-thick graphene-based Hall effect sensors are making it possible to obtain accurate and instantaneous measures of current density within a cell. These new sensors could facilitate the design of more optimised battery cells for EVs…

Electric vehicle (EV) adoption continues to grow at rapid rates, exceeding previous expectations. According to a 2021 study by professional services firm, Ernst & Young, EV sales will surpass internal combustion engine models in Europe, China and the US (the world’s largest auto markets) by 2033 – 5 years earlier than previously expected, as tougher regulations and rising interest drive demand for zero-emission transportation. This positive trend demonstrates the market’s confidence in EV technology, including performance, cost and charging infrastructure.  

However, some prospective buyers still have concerns about vehicle range and charging time, which have been seen as limiting factors in the past. Overcoming these customer anxieties demands further improvements to battery capacity, which means overcoming current materials and engineering challenges. In terms of range and charging time, the battery is therefore the defining component of an EV.

Limitations of existing battery cell sensors

Paragraf GHS series Hall sensors
Paragraf GHS series Hall sensors

Current density gives the best measure of internal battery condition and performance. Physical processes cause changes in cell resistivity, which alters the flow of current within the cell. Some parts of the cell experience high current flow, resulting in temperature hotspots. Monitoring temperature at multiple points on the battery surface can detect these internal battery changes. However, there is a lag between internal physical changes and resulting hotspots.

Temperature monitoring systems are usually sufficient for battery monitoring during regular operation. But this approach does not meet the needs of cell development, characterisation and testing. The lag between physical changes in the battery and temperature change on the surface may prevent researchers and engineers from identifying cell chemistry effects that could lead to thermal runaway and failure.

How graphene Hall effect sensors improve battery cell monitoring

Magnetic field sensors offer an improvement over temperature devices, because of their more direct measurement of current density. But silicon-based magnetic sensors have limitations: their three-dimensional sensing element can impact measurement accuracy.

Single-atom thick graphene sensors have unique characteristics, with significant benefits over other Hall sensors. A standout feature is their measurement of magnetic fields on the axis perpendicular to the sensor’s plane only. This characteristic enables the sensor to reject stray magnetic fields and other off-axis field components.

Pouch cell mapping set-up at WMG
Pouch cell mapping set-up at WMG

Graphene sensors have a resolution that is typically better than 10 ppm, with excellent electrical and thermal characteristics. They have a small footprint that allows for good spatial resolution, and they can be optimised for low field environments and normal ambient temperatures.

Graphene Hall sensors have the potential to revolutionise battery testing. The technology enables scientists and engineers to perform detailed cell analysis when comparing cell chemistry derivatives and form factors during development.

Cell parameters monitored by graphene-based Hall effect sensors

The demand for increased driving range and fast charging features for EVs can only be fulfilled through advanced battery cell performance analysis. Graphene-based sensors give researchers better insights into the physical changes inside a cell, allowing manufacturers to create high-performance batteries. These sensors can monitor different cell parameters during testing, including:

1. Sensors next to the cathode and anode cell connections measure the absolute current of the cell.

2. Multiple sensors on the cell’s external surface detect local variations of current within the cell.

3. Sensors detect current direction, enabling the mapping of differences in the current flow path for charging and discharging.

Figure 1. Magnetic field measurements from sensor 1, located close to the cell's cathode during charge & discharge cycles
Figure 1. Magnetic field measurements from sensor 1, located close to the cell's cathode during charge & discharge cycles

4. Identifying changes in the internal resistance of a cell enables faster identification of failure modes.

5. Measurements made during initial prototyping can enable further cell design optimisation.

6. Using a graphene-based sensor testbed during manufacturing can boost quality control by checking cell performance and leakage currents.

7. Sensors also have applications at the end of cell life. They can screen cells non-destructively to determine the best reuse and recycling options.

Cell analysis case study

Paragraf worked with WMG (formerly known as Warwick Manufacturing Group), a University of Warwick-based academic department focused on industrial innovation in science, technology and engineering, including battery R&D, to perform cell analysis and data mapping using eight GHS-A sensor probes. The probes were attached to the outside of a pouch cell in a test rig. Three sensors were located close to the terminal tabs, three in the centre and two at the far end of the cell.

Figure 1 shows the output from a sensor next to the cathode tab, in response to a series of 10-second current bursts in a set of charge and discharge cycles. Magnetic field variations correspond to the current bursts for both charging and discharging.

Figure 2. Output from sensor 1 during the 160 Amp discharge cycle demonstrates the rapid response characteristics of the Paragraf GHS-A sensor
Figure 2. Output from sensor 1 during the 160 Amp discharge cycle demonstrates the rapid response characteristics of the Paragraf GHS-A sensor

The middle three sensors detected a smaller magnetic field, indicating a lower current density at that point in the cell. As expected, the magnetic fields become even less significant at the far end of the cell, where the current density is lowest. The experiment shows how current density is distributed in a cell during fast charging or speedy EV acceleration.

Figure 2 illustrates the rapid data collection ability of Paragraf GHS-A sensors. The graph shows a sharp, instantaneous rise in the measured magnetic field due to the 160 Amp discharge event. The speed of response for GHS sensors allows for detailed transient effect analysis.

How graphene Hall effect sensors are changing the design of battery cells

While the EV market continues to gain momentum, there are still some challenges to meet the range and charging time expectations of new customers. Improving these attributes of battery cells requires a thorough understanding of their internal current density map. With this knowledge, manufacturers can develop higher density cell chemistries and formats.

Graphene Hall effect sensors make it possible for battery manufacturers to measure internal magnetic fields at a very high resolution, with an instantaneous response to current density changes. This technology has the potential to improve EV performance to meet the expectations of a wider customer base.

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