State of health fields: what a passport shows about a battery as it ages

State of health fields record the condition of one individual battery at a stated time. Annex XIII point 4 requires state of health information for the individual battery, and Article 14 with Annex VII requires the battery management system to provide data that supports assessment of state of health and expected lifetime for stationary storage, LMT and EV batteries. State of health is not a single number with a single meaning.

Different systems define it differently: remaining capacity, remaining power capability, internal resistance increase, capacity fade, power fade, round trip efficiency change, and self discharge evolution are all parameters that can contribute to a state of health picture. A record that shows one figure without saying which parameter it represents, how it was determined, and when, shows a label rather than a measurement.

Which parameters make up state of health?

Annex VII Part A and Annex XIII point 4 contemplate a set of parameters, applied differently by battery category. For EV batteries, state of certified energy is the parameter that the battery management system must make accessible. For LMT batteries and stationary storage, remaining capacity and, where possible, remaining power capability, round trip efficiency, self discharge evolution and ohmic resistance are contemplated. Capacity fade, measured as capacity throughput or as loss against the rated value, and power fade are tracked as change over time. No single parameter captures health on its own. A battery can retain capacity while losing power capability, or retain power while self discharge rises. The record should therefore show the parameters that were measured, with the method for each, rather than collapsing them into one unexplained percentage. A percentage without a parameter definition is not comparable across batteries, models or systems.

How is state of health measured and dated?

State of health is measured through the battery management system, through testing, or through a combination of both, depending on the battery and the system. The record must state the measurement basis: battery management system estimate, laboratory or workshop test, or another stated method. It must state the date of measurement, the reference conditions, and the value for each parameter measured. A measurement taken at an unknown temperature, at an unknown state of charge, or after an unknown rest period cannot be compared with a later measurement. The record should show the sequence of measurements, in date order, so that trend can be read. A single measurement is a snapshot. A sequence is a history. Decisions about repair, repurposing or recycling should rest on a sequence where one is available, not on a single snapshot quoted out of context.

What does a state of health series look like?

Values in the table are illustrative positions, not measurements from a real battery. A real record completes each cell with a verified value, a method and a date.

How does state of health relate to expected lifetime?

Expected lifetime information in Annex XIII point 1 is a model level statement about the expected service life of the battery model under stated assumptions. State of health in point 4 is a unit level measurement of condition at a point in time. The two must not be merged. A battery with a high state of health figure may still be near the end of its expected lifetime on age grounds. A battery with a lower figure may have years of suitable use in a less demanding application, subject to assessment. Expected lifetime is a planning input. State of health is a condition input. A decision about further use needs both, together with safety assessment, the intended application and the operating conditions. The record keeps the two inputs in their separate layers so that neither is mistaken for the other.

What errors appear in state of health fields?

Common errors include a single percentage with no parameter, no method and no date; a battery management system estimate presented as a laboratory test; a measurement from one battery copied to another unit of the same model; and a trend drawn from measurements taken under different conditions without a method note. A further error is the presentation of a state of health figure as a guarantee of remaining life. Each error has the same remedy: restore the parameter, the method, the date and the conditions, and label any projection as a projection. Where a measurement is missing for a period, the gap is shown as a gap. A gap in measurement history is not evidence of stable health. It is evidence that no measurement was recorded.

What should a reader do with a state of health entry?

A reader should check the parameter, the method, the date and the conditions before drawing any conclusion, and should seek the sequence rather than a single value. A reader should not compare figures across batteries unless the parameter, method and conditions match. A reader should not treat a state of health figure as a safety assessment or as proof of suitability for a new application. Those conclusions require separate assessment. This guide gives general educational information only and does not give legal advice for a particular battery, measurement or decision.