Fault diagnostics in automotive battery systems rely heavily on standardized protocols to ensure interoperability, safety, and efficient troubleshooting. Among the most critical standards are SAE J1939-91 and ISO 6469-3, which define fault code classification, diagnostic interfaces, and failure mode coverage for battery electric vehicles (BEVs) and hybrid systems. These frameworks enable consistent communication between battery management systems (BMS) and onboard diagnostics (OBD), though gaps remain in harmonizing fault definitions and addressing emerging failure modes.

SAE J1939-91, part of the broader SAE J1939 series for heavy-duty vehicles, specifies diagnostic messages and fault codes for hybrid and electric powertrains. The standard organizes fault codes using a hierarchical structure based on suspect parameter numbers (SPNs) and failure mode identifiers (FMIs). SPNs identify the specific component or parameter, such as cell voltage deviation (SPN 5256) or insulation resistance (SPN 5272), while FMIs classify the failure type, such as "voltage below normal" (FMI 3) or "erratic signal" (FMI 8). This structure allows for granular fault reporting but requires manufacturers to map proprietary BMS data to standardized SPN-FMI pairs, which can lead to inconsistencies in interpretation.

ISO 6469-3, focused on electrical safety in BEVs, complements SAE J1939-91 by defining fault conditions related to high-voltage systems. It categorizes faults into three tiers: Class A (immediate hazard, e.g., isolation breach), Class B (potential hazard, e.g., coolant leakage), and Class C (non-hazardous, e.g., sensor drift). Unlike SAE J1939-91, ISO 6469-3 does not prescribe specific fault codes but mandates that faults trigger appropriate safety actions, such as contactor opening or derating. This performance-based approach ensures safety but leaves fault reporting granularity to manufacturers.

A key interoperability challenge arises from differing implementations of these standards. While SAE J1939-91 provides a common language for fault codes, manufacturers often extend the standard with proprietary SPNs or modify FMIs to cover edge cases. For example, thermal runaway detection may use SPN 5280 in one system but a custom SPN in another, complicating third-party diagnostics. Similarly, ISO 6469-3’s tiered classification lacks prescriptive fault codes, leading to variability in how Class B and C faults are logged across OEMs.

Onboard diagnostic interfaces further highlight these disparities. SAE J1939-91 mandates the use of CAN bus for fault reporting, with standardized data lengths and transmission rates. However, the interpretation of fault data often requires manufacturer-specific software tools, as critical context (e.g., environmental conditions during fault triggering) may not be fully captured in SPN-FMI pairs. ISO 6469-3, by contrast, does not specify a communication protocol, allowing for proprietary OBD interfaces that may not expose raw fault data to standardized scanners.

Failure mode coverage gaps persist in both standards. SAE J1939-91 lacks explicit fault codes for emerging issues like lithium plating or solid-electrolyte interphase (SEI) degradation, relying instead on generic voltage or temperature deviations. ISO 6469-3’s safety focus omits non-hazardous but performance-critical faults, such as capacity fade or charge acceptance loss. These gaps force manufacturers to either misuse existing codes or bypass standards entirely, undermining diagnostic consistency.

Thermal fault handling illustrates these limitations. SAE J1939-91 provides SPNs for overtemperature events (e.g., SPN 5270 for pack temperature), but does not distinguish between causes like cooling pump failure or cell imbalance. ISO 6469-3 requires thermal runaway mitigation but does not standardize early warning indicators, such as gas evolution or pressure buildup. This ambiguity delays root cause analysis and complicates cross-OEM benchmarking.

Cybersecurity adds another layer of complexity. While neither standard addresses fault code authentication, malicious spoofing of SPN-FMI pairs or ISO 6469-3 alerts could trigger false safety actions. The lack of cryptographic signing in SAE J1939-91’s CAN messages leaves fault reporting vulnerable to manipulation, a growing concern as BEVs integrate with vehicle-to-grid (V2G) networks.

Efforts to bridge these gaps are ongoing. SAE J1939-91’s periodic updates increasingly incorporate BEV-specific SPNs, such as those for DC-DC converter faults, while ISO 6469-3’s revisions clarify fault response timelines. However, convergence remains slow due to competing OEM priorities and regional regulatory differences. For instance, European manufacturers often prioritize ISO 6469-3 compliance, while North American OEMs lean toward SAE J1939-91, creating siloed diagnostic ecosystems.

The path forward requires tighter alignment between the two standards. Harmonizing SPN-FMI definitions with ISO 6469-3’s safety tiers could reduce redundancy, while expanding fault coverage to include predictive failure modes would enhance diagnostic utility. Standardizing OBD interfaces for raw fault data access, without compromising proprietary algorithms, would further improve interoperability. Until then, technicians must rely on multi-standard scanners and OEM-specific training to navigate the fragmented landscape of automotive battery diagnostics.