(EU) 2023/1542 · Annex XIII(1)(o)
Internal cell and pack resistance: two values, two owners
The conjunction in the text hides two separate values. Point 1(o) of Annex XIII requires internal battery cell and pack resistance, meaning two measurements at two levels made by two different parties. The cell figure belongs to the cell supplier. Nobody but you holds the pack figure, because busbars, welded joints, connectors, the fuse and contactors determine it. The usual way this field stays unproven is a single number — normally the cell one — and because the text states no unit, it is not immediately visible which one it is.
Two levels the Regulation keeps apart
Article 3 separately defines the battery cell, the battery module and the battery pack. Point 1(o) names the cell and the pack, but not the module. A value measured on a module is therefore not an answer to either part. If the battery placed on the market is a pack, the field must carry both figures: one from the cell supplier's data and one from a measurement on the assembled pack.
The pack is not the sum of the cells
Pack resistance is the resistance of the cells in series plus everything between them: joints, busbars, the fuse, contactors and wiring. In a well-built pack that addition is small; in a poor one it is not — which is exactly why a calculation cannot replace a measurement. Enter cell resistance multiplied by the number of cells and you publish a value nobody has ever compared with the pack.
The method changes the number
Annex IV defines internal resistance as 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. But the Regulation does not prescribe how to measure it. A DC pulse and an AC method give noticeably different values for the same cell. Point 5 of Part B of Annex IV therefore requires any calculations performed with the measured parameters to be stated, and Article 10(1) requires the specifications, standards and conditions to be explained in the Annex VIII technical documentation.
The unit does not come from Annex XIII
Point 1(o) states no unit. It is in point 3 of Part A of Annex IV, where internal resistance is required in ohms together with internal resistance increase in per cent. Because cell suppliers quote milliohms and those add up at pack level, an order-of-magnitude mix-up is more common in this field than in any other.
Three places, three meanings
The same quantity appears three times in the Regulation with different meanings. Point 1(o) of Annex XIII is the public model-level value. Point 3 of Part A of Annex IV is a performance and durability parameter under Article 10(1), which under point 4(a) of Annex XIII applies to the individual battery and is not public. Point 5 of Part A of Annex VII requires the ohmic resistance “where possible” as a state-of-health parameter in the battery management system — and only for stationary battery energy storage systems and LMT batteries, not for electric vehicle batteries.
Where it goes wrong
- A single value when the field requires both cell and pack.
- A calculated pack resistance from cell resistance and cell count instead of a measurement.
- A value with no method stated; DC and AC measurements produce different numbers.
- Confusing milliohms and ohms; the unit comes from point 3 of Part A of Annex IV, not from Annex XIII.
- Using the battery management system's ohmic resistance under Annex VII as the model value — that is individual-battery data, and it is not even foreseen for electric vehicle batteries.
Frequently asked
Is the cell datasheet resistance sufficient?
No. Point 1(o) of Annex XIII requires cell and pack resistance. The cell value covers only the first half.
Which measurement method does the Regulation prescribe?
None. Annex IV defines the concept but not the method. That is why the method must be explained in the technical documentation, as Article 10(1) requires with the elements of Part B of Annex IV.
In which unit is it stated?
Point 3 of Part A of Annex IV requires ohms, together with internal resistance increase in per cent. Point 1(o) of Annex XIII states no unit.
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