Responsibility when a battery is modified: when a modifier becomes the manufacturer

A battery is modified when it is changed after placing, in a way that affects its design, construction, performance, safety or intended use. Modification covers a wide range: replacing cells with cells of a different chemistry, changing the battery management system software in a way that alters safety functions, rebuilding a pack to a new specification, or repurposing a battery for a different application with a new housing and control system.

The regulatory question is whether the modification is substantial, in the sense used in the Blue Guide for product rules, and whether the modifier therefore becomes the manufacturer of the modified battery. The answer decides who holds conformity, labelling and passport duties for the battery as modified, and whether a new placing on the market occurs.

What counts as a substantial modification?

A substantial modification is a change that affects conformity with the applicable requirements, that creates a new product in the sense that the modified battery no longer corresponds to the original technical documentation and declaration, or that changes the intended use in a way that requires a new assessment. Examples that will often be substantial include a change in chemistry, a change in rated capacity or voltage that alters the model characteristics, a change in safety functions, and a change in application from traction to stationary storage where the safety and performance assessment differs. Examples that will often not be substantial include a like for like repair with identical parts, a software update that corrects a fault without altering safety functions, and a cosmetic change that does not affect the battery, the marking or the record. The test is applied to the facts of the modification, not to the label the modifier gives it. A rebuild labelled as a repair is assessed as a rebuild.

When does a modifier become the manufacturer?

A modifier becomes the manufacturer where the modifier places the modified battery on the market under its own name or trade mark, or where the modification is substantial and the modifier puts the modified battery into service or places it as a new product. In that position, the modifier must meet manufacturer duties: ensure design and manufacture in accordance with the applicable articles, draw up technical documentation, carry out the conformity assessment, draw up the declaration, affix marking and labelling, and ensure the passport record for the modified battery. The original manufacturer is not responsible for the modified battery as modified, although the original manufacturer remains responsible for the original placing. The passport record must reflect the new position: a new model or remanufactured model identifier as applicable, a new unique identifier where required, a new declaration reference, and a link to the prior record. A record that continues to show the original manufacturer as the placing operator for a substantially modified battery misstates responsibility.

How are repair and modification distinguished in the record?

Repair preserves the original identifier, the original model layer and the original placing, and adds a dated repair entry. Modification that is substantial creates a new placing, a new record or a new record version as applicable, and a link to the prior record. The record distinction follows the facts distinction: repair restores, modification creates. The operation record should describe what was done, what was changed, and what assessment was performed, so that a later reader can tell which position applies. A record that labels a substantial rebuild as a repair, with no new declaration and no new identifier, hides the new placing and leaves the modified battery without a responsible operator for the modified configuration. A record that labels a minor repair as a modification, with a new declaration and a new identifier, creates unnecessary duties and breaks history without a reason.

What must the new record contain?

The new record contains the modified model identity, the new unique identifier where required, the identity of the modifier as manufacturer, the declaration reference for the modified battery, the performance and safety values for the modified specification with test conditions, and the dismantling and safety information for the modified configuration. It contains a link to the prior identifier and prior record, so that history is not lost. It contains carbon and recycled content information where the duties apply to the new placing, assessed on the modified battery, not carried forward from the original declaration without assessment. The table below summarises the two positions.

How is software modification assessed?

Software modification is assessed by its effect on the battery, not by the size of the code change. A software change that alters charge limits, thermal management, safety cut offs, state of health calculation, or the software reset function for EV batteries can be substantial even where the change is described as a minor update. A software change that corrects a display fault without altering battery behaviour may not be substantial. The assessment should record the software version before and after, the functions affected, the testing performed, and the safety assessment relied on. The record should show the software version for the unit where the system provides for it, so that a later reader can tell which software the battery runs. A battery whose software has changed without a record entry presents a safety and assessment gap, because the next person to work on the battery cannot know what the battery management system will do.

What should a reader check on a modified battery?

A reader should check the operation record, the identifier linkage, the declaration reference, and the model version, and should ask whether the modification described matches the record position. A reader who sees a new housing, new cells or new software, together with an unchanged original record and no repair or modification entry, should treat the record as incomplete for the battery as it stands. A reader should not assume that a modified battery is covered by the original declaration, or that the original safety information describes the modified configuration. This guide gives general educational information only and does not give legal advice for a particular battery, modification or transaction.