I Donut Believe, Episode 10: The Swell Test
- Buck City Biker
- May 13
- 2 min read
Donut are back, and the latest VTT test results are in. This time the focus shifts to something a lot less glamorous than charging speed or headline power figures. It’s swelling.
More specifically, how the pouch cell physically expands and contracts (breathes) under charge and discharge cycles, something every battery does to some degree, but something that becomes very important once you start chasing high performance and ultra-fast charging.
What The VTT Test Paper Says
The VTT report looks at how the cell’s swelling behaviour holds up under repeated cycling. Across the test, expansion and contraction remained controlled, with no signs of runaway growth or instability, even as conditions became more demanding.
“Breathing” during charging and discharging cycles is expected in this type of cell, but it becomes a genuine engineering constraint in tightly packaged applications like motorcycles, where space and thermal management are critical.
As charging power increased in the test, expansion increased too, confirming that higher charge rates still introduce measurable mechanical stress within the cell.
Alongside this, temperatures remained relatively controlled throughout the test window, reinforcing the wider Donut narrative around thermal stability under load.
The BCB Take
Swelling is one of those battery behaviours that rarely gets attention, but it tells you a lot about how a cell is actually behaving under stress. Nothing in this data is dramatic, and that’s the point. The pouch cell expands and contracts in a consistent, repeatable way, without drifting into instability even under repeated cycling.
For Donut, it adds another small but useful piece to the wider picture. Fast charging, high power output, and thermal stability all sound impressive in isolation, but they only matter if the cell can physically handle everything at once.
And that’s really what this test shows. Not performance, but whether the cell stays physically consistent when it’s pushed.
Ride safe, folks.
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