Where does a battery accident actually begin?
When a lithium battery goes into runaway, we say “a battery caught fire.” But a battery is a system: cells, management electronics, an assembly, and a user at the end of the chain. So where, precisely, does the problem originate?
A breakdown of safety-incident causes recorded on a portable Li-ion technology shows proportions that speak for themselves.
The numbers: nearly 3 in 5 incidents come from the cell
| Origin | Share of incidents | What it covers |
|---|---|---|
| Cell | ≈ 59% | Internal defect, ageing, degradation of the cell itself |
| BMU — management unit | ≈ 19% | Balancing, thermal management |
| User | ≈ 11% | Use: unsuitable charger, impact, charging in the cold… |
| System / BMS | ≈ 4% | SoC/SoH supervision, system logic |
The message is obvious: the cell is by far the leading origin of incidents — about three times more than all the management electronics combined. The most “low-level” element, the hardest to inspect once the pack is sealed, is also the most at risk.
Why the cell concentrates the risk
A cell can become dangerous for reasons of its own, independent of any electronics:
- Manufacturing defects: metallic particles, impurities, a separator flaw — all possible triggers of an internal short-circuit, the most feared failure because it largely escapes the BMS.
- Ageing: lithium plating, dendrite growth, electrolyte loss — degradation changes how the cell behaves.
- Damage: impact, puncture, swelling, prior immersion.
The common thread: the short-circuit is triggered inside the cell. The BMS, which monitors voltage and temperature at the terminals, cannot always anticipate it — hence the importance of the cell's internal protections (see CID and PTC) and, upstream, of cell screening.
The role of the electronics and the user
The ≈ 19% attributed to the BMU (Battery Management Unit — balancing, thermal management) and the ≈ 4% to the system/BMS are a reminder that faulty or missing management also weighs in, for nearly one incident in four. A balancing fault lets a cell drift; poor thermal management lets temperature run away. Containing this share is exactly the job of a good BMS (see our BMS).
Finally, ≈ 11% come from the user: unsuitable charger, charging in cold weather, mechanical impact. A share that good practices can largely reduce.
What these numbers mean for a second-life battery
If the cell is the leading cause of incidents, then the most powerful safety lever is controlling the cell. And that, paradoxically, is where reuse done properly has an edge:
- at cirBATT, every cell is tested individually (capacity, internal resistance, safety) and questionable cells are removed — precisely the population that concentrates the 59%;
- the retained cells, with reduced energy, are less energetic in the event of a fault (see why a second-life battery can be safer);
- they are managed by a BMS designed for them, which addresses the “management” share.
Where a new battery has never been “opened” cell by cell, a requalified second-life battery necessarily has been. Screening attacks the risk exactly where it is most concentrated.
In short
The data are clear: ≈ 59% of incidents come from the cell, ≈ 19% from the management unit, ≈ 11% from the user and ≈ 4% from the system. A battery's safety is decided first at the cell level — and therefore in testing and screening. That is cirBATT's conviction, and its craft.