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Insulation resistance

Almost every pack/system pass criterion in clause 5.2 ends with the same two-part insulation requirement. The thresholds are normalized per volt of nominal voltage, and the standard provides two measurement methods in Appendix B.

The thresholds

The insulation resistance after testing should not be less than 100 Ω/V. If there is an AC circuit, the insulation resistance should not be less than 500 Ω/V.

This wording is repeated almost verbatim across clauses 5.2.1, 5.2.2, 5.2.3, 5.2.4, 5.2.5, 5.2.6 b), 5.2.8, 5.2.9, 5.2.10, 5.2.11, 5.2.12, 5.2.13, 5.2.14, 5.2.15, and 5.2.16.

Circuit type Minimum insulation resistance (per volt of nominal voltage)
DC circuit ≥ 100 Ω/V
AC circuit (if present) ≥ 500 Ω/V

For a 400 V DC pack: minimum 40 kΩ. For a 800 V DC pack: minimum 80 kΩ. For an AC circuit on a 400 V system: minimum 200 kΩ.

Source: GB 38031-2025 clause 5.2.x (PDF page 12).

Where the thresholds apply

The insulation requirement attaches to every pack/system test whose pass criterion uses the STD pattern. It does not apply to:

  • Cell-level tests (5.1.1 to 5.1.7) — cells are not characterized by insulation resistance in this standard.
  • External fire (5.2.7 a) — only "no explosion" is required.
  • Thermal propagation (5.2.7 b) — uses the no-fire/no-explosion + 5-min warning criteria; insulation is not required.
  • Immersion Method 1 (5.2.6 a) — only "no fire or explosion" is required for the 3.5 % NaCl submersion path.

Source: GB 38031-2025 clauses 5.2.6, 5.2.7 (PDF page 12).

When insulation is measured

The standard requires insulation testing before all pack/system tests and after certain tests:

The battery pack or system should undergo insulation resistance testing before all tests and after certain tests. The test position is between the positive and negative output terminals and the electric platform. Specific test methods are provided in Appendix B.

Source: GB 38031-2025 clause 6.1.5 (PDF page 13).

Special timing — damp heat (5.2.5)

For damp heat cycling, the insulation measurement must happen within 30 minutes after the test ends:

A battery pack or system, after undergoing the damp heat cycling test as per 8.2.5, should show no leakage, housing cracks, fire, or explosion. The insulation resistance within 30 minutes after testing should not be less than 100 Ω/V. If there is an AC circuit, the insulation resistance should not be less than 500 Ω/V.

This is the only clause in 5.2 that prescribes a measurement window. Other clauses simply say "after testing" without a tight bound.

Source: GB 38031-2025 clause 5.2.5 (PDF page 12).

Engineering note (non-normative): The 30-minute limit exists because the moisture absorbed during damp heat will dry off, and dry insulation reads higher than wet insulation. Measuring later inflates the result. Plan the test sequence so the insulation rig is set up and zeroed before the chamber door opens.

Measurement method (Appendix B)

Appendix B is normative. It provides two equivalent measurement methods.

B.1 — Test conditions

The battery pack or system should be in a fully charged state as specified by the manufacturer. The test environment temperature should be 22 °C ± 5 °C, and the relative humidity should be 10 % ~ 90 %.

The internal resistance of the voltage detection tool should be no less than 10 MΩ. If the insulation monitoring function affects the insulation resistance test of the battery pack or system during measurement, the insulation monitoring function should be turned off, or the insulation resistance monitoring unit should be disconnected from the B-level voltage circuit to avoid affecting the measurement value.

Source: GB 38031-2025 Appendix B.1 (PDF page 32).

B.2.1 — Method 1 (voltage divider with R₀)

The pack remains energized and connected. Two identical voltage detection tools (≥ 10 MΩ internal resistance) measure the voltage from each output terminal to the electric platform (the conductive outer shell, tied to the vehicle's electric platform).

Steps:

  1. Activate the internal switches so the battery system is in the connected state.
  2. Measure U₁ (higher reading) and U₁′ (lower reading) from the two terminals to the electric platform.
  3. Add a known parallel resistance R₀ — recommended value 1 MΩ — between the U₁-side terminal and the electric platform. Re-measure to get U₂ and U₂′.
  4. Calculate the insulation resistance Rᵢ from R₀, U₁, U₁′, U₂, U₂′, and the internal resistance r of the voltage detector, using Formula B.1 or B.2.

Source: GB 38031-2025 Appendix B.2.1 (PDF pages 32–33).

Engineering note (non-normative): Method 1 measures the live, in-system insulation resistance without applying an external high voltage. It is the path of least disruption to the BMS and is the standard choice for pre-test screening.

B.2.2 — Method 2 (insulation resistance meter)

An external insulation-resistance meter measures from each output terminal to the electric platform.

Steps:

  1. Ensure the battery pack or system is in the connected state.
  2. Use an insulation resistance meter to measure separately between each output terminal and the electric platform.
  3. Measurement voltage: the higher of 1.5 × nominal pack/system voltage or 500 V DC.
  4. Measurement time: apply the voltage for at least 30 seconds to obtain stable readings.

Source: GB 38031-2025 Appendix B.2.2 (PDF page 33).

Engineering note (non-normative): Method 2 is the conventional megohmmeter test. The "1.5× nominal or 500 V, whichever is higher" rule means an 800 V pack is tested at 1 200 V DC, while any pack ≤ 333 V nominal is tested at 500 V. The 30-second hold ensures dielectric absorption settles before the reading is logged.

Cross-references