Coin cells fail differently from pouch cells under the same abuse. The hermetic stainless-steel shell is a structural element, not just a package — it changes how energy releases when something goes wrong. Designers planning safety analyses for medical, aerospace and industrial devices should understand the difference.

The three abuse cases

IEC 62133-2 and UN 38.3 both test coin cells through three primary abuse cases that reflect realistic field failures:

  1. External short-circuit — the positive and negative terminals connected through a low-resistance path
  2. Mechanical crush — direct compression force applied perpendicular to the cell axis
  3. Forced over-charge — current applied beyond the cell's rated cutoff voltage

How the cell fails in each case is determined more by the can construction than by the chemistry. A 65 mAh ML cell short-circuited dissipates roughly 0.7 Wh — meaningful, but not enough to cause cascade failure in adjacent components if the can holds.

External short-circuit behaviour

When a coin cell is shorted externally, the entire stored energy dissipates as heat in the cell internals and the short-circuit path. For a healthy ML2032 the surface temperature peaks at 85 to 110 °C within 30 to 60 seconds, then declines as the cell discharges to flat. For LIR2032 the same test peaks at 100 to 130 °C because of the higher voltage and energy density.

The hermetic can does not vent during a normal short. The internal pressure builds slightly from electrolyte vapour but stays below the can's burst pressure (typically > 30 bar). The cell goes flat and stays sealed. This is why coin cells pass IEC 62133-2 short-circuit testing reliably — there is no post-test cleanup of vented electrolyte, no fire risk, no neighbouring component damage.

Mechanical crush behaviour

Under MIL-STD-810H mechanical crush at 13 kN applied across the flat faces, the can deforms before the internal stack ruptures. The deformation alone short-circuits the internal positive and negative tabs, and the cell discharges through the internal short. Surface temperature spikes briefly to 80 to 100 °C, then declines.

The risk in mechanical crush is electrolyte leakage if the deformation breaks the crimp seal. For the LIR cells we ship, < 0.5% of crushed cells in a typical 100-cell test batch leak measurable electrolyte. For ML cells, < 0.2% — the crimp seal on ML is more conservative because of the reflow tolerance requirement.

Compare to a pouch cell of the same capacity (e.g. a 65 mAh thin Li-Po): mechanical crush typically punctures the aluminium-laminate pouch, releases vaporised electrolyte, and may ignite if the crush is fast enough. Coin cells do not have this failure mode because the can is structurally orders of magnitude stiffer than a pouch.

Forced over-charge behaviour

Forced over-charge — applying current after the cell reaches cutoff — drives the cathode beyond its stable potential. For LIR2032 above 4.30 V the LCO releases oxygen and the cell can vent or burst. For ML2032 above 3.50 V the LiMn2O4 cathode is more stable; the cell tolerates a wider over-charge envelope before venting.

Both chemistries have a cell-internal over-charge protection mechanism: a current interrupt device (CID) inside the can that breaks the circuit when internal pressure reaches a threshold. The CID is mechanical; once tripped, the cell is permanently disabled but does not vent flame. This is the primary safety feature that lets coin cells pass UN 38.3 forced over-charge testing without external BMS.

Vent design and what happens after

If internal pressure exceeds the CID threshold and continues rising (rare, but possible under extreme abuse like sustained over-charge from a mis-configured charger after CID activation), the can has a designed vent point — a thinned section of the metal that bursts at a controlled pressure (40 to 60 bar). The vent releases vapourised electrolyte through a defined direction (typically downward through the negative terminal pad).

For SMD-mounted ML cells, the vent direction matters for the PCB layout: keep heat-sensitive components 10 mm clear of the negative terminal pad, and avoid placing the cell directly above any IC that can fail open under thermal exposure.

What we test on every lot

Each production lot is sampled for:

  • External short-circuit: 5 cells for 24 hours, surface temperature monitored, no venting allowed
  • Crimp seal integrity: 5 cells held at 60 °C / 80% RH for 7 days, weight loss < 0.1%
  • Forced over-charge: 3 cells charged to 1.5× rated voltage, CID must trip, no flame
  • Mechanical drop: 5 cells dropped 1.5 m onto concrete, no leakage, capacity retention > 95%

For medical and aerospace customers the SOP adds two more: vibration test per MIL-STD-810H and thermal cycling -40 °C ↔ +85 °C for 200 cycles. Test reports retained 10 years per ISO 13485.