(EU) 2023/1542 · Annex XIII(4)(a) -> Annex IV, Part A(3)
Internal resistance: cell, pack and the question of method
Internal resistance is the one Part A parameter split across two levels inside its own definition. Annex IV speaks of opposition to current within a cell or a battery, while public point 1(o) of Annex XIII asks for cell and pack resistance. The first comes from your cell supplier; the second only exists in your product, with busbars, contactors, fuses and wiring in the loop. And since the Regulation names no measurement method, a figure without its procedure cannot be compared with anything.
The definition spans both levels at once
Point 5 of the introductory part of Annex IV provides that internal resistance means the opposition to the flow of current within a cell or a battery under reference conditions, that is, the sum of electronic and ionic resistance contributing to total effective resistance, including inductive and capacitive properties. The definition does not separate the levels, so the documentation has to say which one the stated value describes. Point 1(o) of Annex XIII confirms this from the other side: publicly it asks for internal battery cell and pack resistance, not a single number.
The value depends on method, and the Regulation picks none
Because the definition takes in inductive and capacitive properties, the result depends on whether you measure with a DC pulse or AC impedance, at which pulse length or frequency, and at which state of charge and temperature. The Regulation prescribes none of those choices. Article 15(1) sets a requirement on the method instead: reliable, accurate and reproducible, taking generally recognised state-of-the-art methods into account, with results deemed to be of low uncertainty. Presumption of conformity arrives, under Article 15(3), only through a harmonised standard published in the Official Journal of the European Union.
An increase needs a baseline and a moment
Point 3 of Part A of Annex IV asks for resistance in ohms and for its increase in per cent. A percentage with no baseline value and no statement of where in the service life it was taken is the appearance of data rather than data. Part B of Annex IV is therefore not an optional extra: Article 10(1) requires the technical documentation to explain the specifications, standards and conditions used to measure, calculate or estimate.
The Annex VII ohmic resistance is a different obligation
Part A of Annex VII lists ohmic resistance among the state-of-health parameters for stationary systems and LMT batteries, and lists it only where possible. That is not this parameter. It is a different annex, a different article — Article 14 rather than Article 10 — and a conditional requirement rather than an unconditional one. Copying whatever the battery management system reports into this field swaps two obligations with two different deadlines.
Where it goes wrong
- A single value with no indication of whether it describes the cell or the pack.
- Taking the supplier's cell resistance as the battery's; busbars, contactors, fuses and wiring are not in that number.
- An increase in per cent with no baseline value and no measurement point in the service life.
- Copying the Annex VII ohmic resistance, which is a state-of-health parameter under Article 14, not an Article 10 parameter.
- Comparing two suppliers' figures obtained by different procedures as though they were like for like.
Frequently asked
Does the value refer to the cell or to the battery?
The definition in point 5 of the introductory part of Annex IV covers current within a cell or a battery, so the documentation must state the level. Point 1(o) of Annex XIII asks publicly for cell and pack resistance.
Which measurement method does the Regulation prescribe?
None specifically. Article 15(1) requires reliable, accurate and reproducible methods with results of low uncertainty, and Article 15(3) grants presumption of conformity only through a harmonised standard published in the Official Journal of the European Union.
Is the Annex VII ohmic resistance the same figure?
No. Annex VII lists it among state-of-health parameters under Article 14 and only where possible; this parameter comes from point 3 of Part A of Annex IV under Article 10.
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