Battery Circularity Needs a Common Language

Why battery second life needs verifiable health, application-specific remaining useful life and confidence — not just an SOH percentage.

As electric-vehicle fleets mature, a growing number of batteries will reach the end of their first application without necessarily reaching the end of their useful technical life. The difficult question is not simply whether a battery has “80% SOH.” It is: what can this battery safely do next, for how long, and with what level of confidence?

A single SOH number is not enough

State of Health is useful, but a single scalar can hide important differences between energy retention, power capability, internal resistance, cell consistency, thermal behaviour and diagnostic condition. Two packs carrying the same reported SOH may therefore present very different technical risks and very different suitability for a second application.

Battery history matters

A defensible circularity decision should use evidence from the battery's full lifecycle, rather than relying only on a test performed after removal from the vehicle.

Cell-level reference information establishes chemistry, rated performance, operating limits and degradation context. Pack and vehicle data establish configuration, usable energy, BMS and thermal limits and the OEM's end-of-life criteria. First-life data adds energy throughput, equivalent full cycles, SOC and temperature exposure, fast charging, peak currents, imbalance and abnormal events. Storage and logistics history matters because degradation does not stop when the vehicle stops operating.

We cannot comprehensively retest every cell

For large-scale circularity, dismantling every pack and independently characterising every cell is neither an efficient nor necessarily desirable default. The assessment should instead combine multiple independent indicators and use them to identify where health or safety cannot be established with adequate confidence.

I propose a three-stage qualification process:

1. Historical screening — provenance, operating history, thermal and SOC exposure, charging history, abnormal events, storage history and data integrity.
2. Non-destructive pack diagnostics — usable energy or capacity, resistance at defined conditions, pulse power, voltage dispersion, thermal dispersion, relaxation/self-discharge behaviour and electrical safety.
3. Anomaly-driven investigation — deeper module or cell testing only where historical or diagnostic evidence identifies uncertainty or abnormal behaviour.

From SOH to a Battery Health State

Battery health should be represented as a multidimensional state rather than compressed prematurely into one percentage. A practical health vector can include energy retention, capacity retention, power capability, resistance or impedance condition, cell/module balance, thermal behaviour and safety/diagnostic condition.

The next application is part of the calculation

A battery does not have one universally meaningful remaining useful life. RUL depends on what the battery will be asked to do next: expected energy throughput, power demand, C-rate, ambient and thermal conditions, cycling pattern, required service life and the application's end-of-life threshold.

Suitability is therefore a relationship between the observed battery health state and the requirement vector of the proposed application.

Confidence should be explicit

Two assessments can produce similar health estimates while being supported by very different evidence. One may have complete BMS history, verified provenance and controlled diagnostics; another may have large gaps in operating or storage records. The decision framework should expose this difference rather than hide it.

Battery Circularity Qualification Framework

The proposed Battery Circularity Qualification Framework (BCQF) therefore reports four distinct outputs:

H represents the multidimensional battery health state; S_A represents suitability for application A; RUL_A represents application-specific remaining useful life; and C represents confidence in the assessment.

From battery passport to battery decision

Battery passports and lifecycle data systems can establish identity and preserve evidence. The next challenge is converting that evidence into a technically defensible decision. A useful chain is:

The objective is not to replace existing standards or safety requirements. It is to develop a common technical language that can connect cell manufacturers, vehicle OEMs, fleet operators, second-life integrators, recyclers, financiers, insurers, regulators and standards bodies.

A credible methodology will ultimately need validation against physical ageing, field outcomes and independent datasets. The framework proposed here is intended as a basis for technical discussion, validation and peer review.