Using a subsystem¶
The 2025 edition adds a definition for "battery subsystem" (3.5, new) and lets the manufacturer substitute one for the full pack/system in tests where the full object is impractical (6.1.6). The substitution is conditional — the subsystem must include the safety-relevant interfaces, and the manufacturer must show the results generalise to the full pack.
Definition (3.5, new)¶
Battery Subsystem A collection of any energy storage components formed from the components of a battery pack or system.
Source: clause 3.5 (PDF page 10). New in 2025.
When substitution is allowed (6.1.6)¶
If a battery pack or system is unsuitable for some tests due to its size or mass, the manufacturer and testing agency can agree to use a battery subsystem as a replacement test object for all or part of the tests. However, the battery subsystem used for testing should include all parts related to the vehicle's requirements (such as connecting components or protective components) and may include the battery management system. If the battery subsystem is selected for testing, it should be proven that the test results can represent the safety performance of the battery pack or system under the same conditions.
Source: clause 6.1.6 (PDF page 13).
Requirements summary¶
| Requirement | Detail |
|---|---|
| Trigger condition | Pack/system size or mass makes the test impractical |
| Authority to substitute | Agreement between manufacturer and testing agency |
| Scope | All or part of the test items |
| Mandatory inclusions | All parts related to the vehicle's requirements (e.g. connectors, protective components) |
| Optional inclusion | Battery management system |
| Burden of proof | Manufacturer must demonstrate that subsystem results represent the full pack/system safety performance under the same conditions |
What "represent the full pack" looks like in practice¶
Engineering note (non-normative): The standard does not prescribe how the manufacturer proves equivalence. In type-approval submissions accepted to date, this typically means:
- A modular pack architecture where the subsystem is one repeat unit (e.g. one module string of N).
- Documentation that the failure modes excited by the test (mechanical, thermal, electrical) propagate at the module-string level and are bounded by the protections in the subsystem under test.
- For thermal propagation specifically (the test most often run on a subsystem), the subsystem must contain enough cells to demonstrate that propagation either does not start or is contained — a single-module subsystem is rarely accepted.
Tests where subsystem substitution is most commonly invoked: external fire (8.2.7.1), thermal propagation (8.2.7.2), and bottom impact (8.2.16) on very large packs (commercial vehicles, ESS-derived designs). Tests on the full vehicle (the upper end of bottom impact and thermal propagation) are not subsystem-eligible by their nature.
Cross-reference¶
- Same type determination — subsystem testing does not affect the same-type rules.
- Testing with the vehicle body — the related substitution at the other end (whole-vehicle inclusion, not subsystem).
Source¶
Clauses 3.5, 6.1.6 (PDF pages 10, 13).