An inspection technique known as Computed Tomography, or CT, uses X-rays to evaluate an object in three dimensions.

That third dimension is a valuable one, and allows a manufacturer to get volumetric data to effectively see inside an object.

The CT method reveals the internal details of objects like batteries, and does so non-destructively.

Computed tomography provides a 3D map of the X-ray absorption throughout a sample. By rotating an object 360 degrees, the CT systems receive 2D projection images from every angle. The two-dimensional pieces are then reconstructed into a 3D volume.

A computed reconstruction algorithm creates a stack of cross-sectional slices from the 2D radiographs, enabling CT users to visualize, analyze, and measure surfaces and internal structures — components like a battery’s safety vents or separators.

See the AA batteries inside this electric toothbrush, for example.

An inside look at a AA battery. Note the damage in the middle image, found via CT inspection.
An inside look at a AA battery, placed within an electric toothbrush. Note the damage in the middle image, found via CT inspection. (Image Credit: Nikon)

Computed tomography can act as a kind of “post-mortem” in the analysis phase, says Chris Peitsch, Business Development Manager, X-ray/CT Systems, at the Maple Grove, MN-based inspection manufacturer Avonix Imaging, who spoke with a Tech Briefs audience this month.

“You can generate virtual cross-sections at any plane you desire, and you can even run numerical analysis and make precise measurements,” Peitsch said in a live presentation titled X-ray Computed Tomography: Enabling Safe and Effective Battery Technology.

CT inspection shows swelling in a pouch battery after a thermal event.
CT inspection shows swelling in a pouch battery after a thermal event. (Image Credit: Nikon)

Readers had the following battery-related and inspection-relation questions for Peitsch and co-presenter Andrew Ramsey, X-ray CT Consultant at Nikon Metrology, headquartered in the UK. Read Peitsch’s and Ramsey’s edited responses below.

Does CT pick up distribution of soft substances like liquids or grease?

Andrew Ramsey: Liquids are a little tricky, unless they're contained. Our scans work by turning the object around 360 degrees. Anybody who's picked up a bucket and spun it around will realize that the liquid doesn't actually move when you do that.

If the liquid is trapped in little pores, then, yes, we can pick it up. Certainly, soft material is tricky; you have to filter your X-rays to get a very high energy in the beam so that it can penetrate the dense material around it.

Then, of course, having done that, you have to do quite a long scan to get good signal to noise but you can pick up things like rubber o-rings and grease. It all really depends on the situation. In theory, it is definitely possible to visualize liquids. It just takes a bit more time.

Is CT effective for heavier, thick metal parts like cast iron or cast aluminum parts, or is it most effective for thinner sheet metal or plastic parts?

Andrew Ramsey: It is effective for the lower-density parts. Large castings are a little bit more difficult; it really depends on the material they're made form. You mentioned aluminum or steel. They're very different animals really. Aluminum has a low atomic number, so it's much easier to penetrate than cast iron or steel.

While we can [inspect up] to a foot of aluminum, we can probably only do three or four inches of steel. Titanium is somewhere in between.

What materials are the systems suitable for: Alloys? Ferrous materials? Non-ferrous materials?

Chris Peitsch: We have systems capable of inspecting [the nickel-chrome superalloy] Inconel now. It all depends on the part size and structure. Fortunately, in aerospace, weight reduction is a big deal, so a lot of Inconel structures are already thin walled; it's all dependent on the amount of material that we have to penetrate.

Andrew Ramsey: It's not the actual sample size but it's the total path length of solid material. So, if you have a large part that's made of very thin walls, but it has lots of holes and spaces inside it, then it's going to be a lot easier to penetrate than a solid sample.

Have you used a CT scan on a battery? Share your questions and comments below.