Battery State of Health in the Passport

Almost everything in a product passport is fixed at manufacture. Composition does not change, origin does not change, and the carbon footprint declaration is calculated once. State of health is the exception, and that single difference has consequences throughout the architecture.

Regulation (EU) 2023/1542 requires the battery passport to carry information on state of health, which means the passport has to accept data from a battery in service rather than only from a factory.

What it actually measures

At its simplest, state of health is the ratio of a battery’s present capacity to its rated capacity when new, expressed as a percentage. A battery at eighty per cent state of health holds four fifths of the energy it originally could.

The complication is that capacity is not the only thing that degrades, and different applications care about different degradation. Internal resistance rises, which limits power delivery even where capacity is intact. For a power tool that matters more than capacity; for stationary storage it matters less.

A single state of health percentage collapses several of these into one figure, and how that collapse is done varies by manufacturer. That is why two batteries both reported at eighty-five per cent may have materially different remaining useful lives.

Why the definition problem matters commercially

The second-life market for batteries depends on being able to price a used pack. Pricing requires comparison, and comparison requires a common definition.

For the passport, the implication is that the measurement method belongs in the record alongside the value, in the same way a carbon footprint needs its boundary. Without it, aggregating state of health across a fleet produces a number that looks precise and is not.

How a live attribute changes the architecture

Static attributes are written once and read many times. A live attribute has to be updated by something in the field, which raises questions the rest of the passport never faces.

• Who is permitted to write it. The battery management system, a service centre, or a second-life assessor — and how each proves it is entitled to.
• How often it updates. Continuous reporting is unnecessary and expensive; an update on service events and ownership transfer is usually sufficient.
• What history is retained. A single current value hides a degradation curve, and the curve is what predicts remaining life.
• What happens offline. A battery in a vehicle without connectivity still degrades, so the passport must tolerate a stale value with a clear timestamp.

The timestamp point is the one most often missed. A state of health figure without a date is unusable, because the reader cannot tell whether it describes the battery today or three years ago.

The second-life decision

The commercial reason this attribute matters is that it decides what happens to a battery when it leaves its first application. That decision has three outcomes with very different values.

That last row is the one the passport exists to prevent. A pack with no reliable health data is assessed as scrap regardless of its actual condition, because individually testing a used pack costs more than the difference in value. Reliable reported state of health is what makes the second-life route economic at all.

What to put in place

Three practical measures cover most of the requirement and are considerably easier to design in than to retrofit.

First, record the measurement method alongside the value, so the figure remains interpretable by a buyer who has never seen your battery management system. Second, retain a history rather than only the current value, because the degradation trend predicts remaining life better than any single reading. Third, timestamp every reading and surface the age of the data, so a stale value is visibly stale rather than silently wrong.

None of these require continuous connectivity. A reading captured at each service event, at fault diagnosis and at ownership transfer produces a usable curve for most applications, and those are moments when the battery is already being interrogated.