Inside the Cell: Internal thermal sensing for eVTOL battery performance

Vertical selected 21700 cylindrical battery cells to power their eVTOL aircraft, as they offer a balance between weight, performance and cost. These cells are widely produced, making them affordable and easy to integrate into new designs. Their shape also supports structural strength and simplifies manufacturing.

In an eVTOL, thousands of these cells are managed at their performance limits, especially during take-off and landing when power demands are at their highest. At these moments, it's essential to understand how heat builds up inside the cells, as managing this thermal behaviour is key to ensuring high performance and reliability of the aircraft.

However, cylindrical cells are sealed units, which makes it difficult to monitor cell internal behaviour. Most common methods only measure surface temperature, which doesn’t reflect the cell core environment where the highest temperatures are often observed.

Vertical Aerospace challenged the specialist Cell Instrumentation team within WMG’s Battery Systems group to develop a way of gathering accurate internal temperature data using sensors placed inside the cell core. This would give them the insight needed to more accurately derive temperature limits for safety, preserve lifetime and correlate model behaviour.

Solution

To help Vertical Aerospace gain a clearer picture of what’s happening inside their battery cells, WMG developed a specialised method for measuring internal temperatures. Using expertise from their Cell Instrumentation Team, WMG created custom thermistor arrays, comprising multiple sensors mounted onto a flexible printed circuit board that can be inserted directly into the core of commercially available cylindrical cells.

One of the main challenges in doing this is the sealed nature of the cell’s structure. Inside a cylindrical battery, the electrodes are tightly wound, leaving very little space. However, WMG was able to access the small central gap within the cell, without damaging the internal components. By carefully drilling into this space and using bespoke fittings, they were able to insert the sensors and reseal the cell safely without causing a short circuit or impacting the behaviour of the cell. This was validated though equivalent reference performance testing before and after instrumentation.

The instrumentation process takes around three days per batch and can be applied to a wide range of cell formats and chemistries. The instrumented cells can then be tested under typical or more extreme conditions to observe how they perform.

The data gathered from these internal sensors has revealed important details about spatially resolved heat generation during use, allowing us to build a test-based (rather than model-based) picture of intracell temperature gradients.

“We were delighted to partner with Vertical Aerospace. This project gave us the opportunity to use the new methods that we have been created on internal cell sensing and metrology to help solve a challenging real-world problem. Working to support the evolution of novel transport technologies such as eVTOL aircraft is a key element of our research.”

- James Marco, Professor of Battery Systems, WMG