An experiment that upends the conventional wisdom

In 2021, a team from Underwriters Laboratories (UL) and Purdue University published a study in the Journal of The Electrochemical Society on how ageing affects the safety of lithium batteries. The protocol: subject both fresh and aged cells and modules (up to 20% capacity loss) to abuse tests (overcharge, external short-circuit), then compare their behaviour.

The most striking result concerns the modules — assemblies of cells, like a real pack:

  • The module made of fresh cells went into catastrophic thermal runaway — a fire.
  • The modules made of aged cells (20% loss) did not go into thermal runaway.

In other words, under identical abuse conditions, the “used” pack behaved more safely than the fresh one.

Why ageing can reduce the hazard

Three mechanisms, identified by the authors, explain this apparent paradox.

1. Less stored energy = less energy to release

An aged cell has lost capacity: it holds less energy and less free electrolyte. Thermal runaway is precisely the sudden, uncontrolled release of stored energy. The less there is, the less violent the event. A cell at 80% SOH mechanically has less “fuel” to feed a fire than a full, new cell.

2. The protections trigger earlier

Remember the CID, the mechanical device that cuts the current under internal pressure (see our article CID and PTC: how a cell protects itself). The study measured that the CID in aged cells activates faster than in fresh cells. The reason: ageing is accompanied by a gradual build-up of gas from degradation reactions, so the starting pressure is already closer to the trigger threshold. The cut-off happens sooner, before the temperature becomes critical.

3. A short-circuit with no nasty surprises

For the external short-circuit tests, the study found no disadvantage from ageing: the PTC regulated the current and protected fresh and aged cells alike.

The reliability argument: the bathtub curve

There is a second reason, well known to reliability engineers: the “bathtub curve.” A component's failure rate is high at the very start of its life (manufacturing defects, “infant mortality”), then drops and stays very low for a long time, before rising again at end of life.

A cell that has already worked for several years without incident has therefore passed the riskiest period: any manufacturing defects would already have shown up. A brand-new cell, by contrast, has yet to prove itself.

The word that changes everything: “tested”

A caveat: this does not mean that any old battery picked up at random is safe. The study's own authors warn that one cannot conclude that any level of ageing makes a cell harmless. A cell that is damaged, swollen, has suffered lithium plating, or was stored carelessly can be very dangerous.

The observed safety benefit only holds for cells that are characterised and selected. That is the whole difference between a “used” battery and a “second-life” battery:

  • each cell is tested individually (real capacity, internal resistance, safety behaviour);
  • questionable cells are removed — the bad ones are weeded out rather than tolerated;
  • the cells retained are matched by performance, then managed by a dedicated BMS.

You then combine the best of both worlds: the reduced energy and early-triggering protections of a mature cell, plus the screening that eliminates defects. That is exactly cirBATT's requalification process.

In short

The science shows it: in key abuse scenarios, an aged cell can be safer than a new one — because it stores less energy and its internal safeguards trigger earlier. Add cell-by-cell screening and a reliable BMS, and a well-designed second-life battery is not a compromise on safety: it is often a guarantee of it.