Acceptance Quality Limit (AQL)

How it is measured and specified

International standard ISO 2859-1 (Sampling procedures for inspection by attributes — Part 1: Sampling schemes indexed by acceptance quality limit for lot-by-lot inspection) uses a two-step lookup to govern factory lot release. First, the inspector cross-references the total production lot size against the specified inspection level—most commonly General Inspection Level II for normal commercial shipments—to obtain a sample-size code letter and the corresponding number of units (n) to pull at random from the lot.

Second, for each defect severity tier (Critical, Major, and Minor), the inspector looks up the chosen AQL value to find two integer thresholds: the acceptance number (Ac), which is the maximum count of non-conforming units that still permits lot release, and the rejection number (Re), which triggers immediate lot rejection, quarantine, and 100% factory sorting or rework.

How it matters when sourcing from China

When purchasing LFP battery modules, high-voltage packs, solar PV pallets, or EV chargers from Chinese factories, writing ISO 2859-1 AQL thresholds directly into the purchase contract and pre-shipment inspection (PSI) protocol prevents subjective disputes over whether a batch passes or fails before the balance payment is released. Across Tradvolt's factory inspection SOPs, defects are partitioned into three strict severity classes:

• Critical Defects (AQL 0 — Ac = 0, Re = 1): Any defect that creates a safety hazard, regulatory non-compliance, or dielectric breakdown results in immediate lot quarantine upon a single occurrence. In the battery pack hipot dielectric test SOP (China-to-Pakistan corridor), flashover, arcing, leakage current exceeding the 5 mA threshold, or insulation resistance below 100 MΩ at 1000 V DC is classified as Critical (AQL 0), halting shipment until 100% of packs are re-screened.
• Major Defects (AQL 1.0): Functional or performance deviations that fall outside datasheet tolerances. In the battery pack capacity-retention test SOP (China-to-Jamaica corridor), for a 50-pack shipment lot inspected at General Inspection Level II (Code Letter D, sample size n = 8), Major defects at AQL 1.0 allow Ac = 1, Re = 2 for packs failing the ≥95% discharge capacity threshold or busbar torque specification, while Critical defects (electrolyte leakage, thermal runaway alarm, or cell swelling >1.5 mm) remain at AQL 0 (Ac = 0, Re = 1).
• Minor Defects (AQL 2.5): Cosmetic scratches, minor carton scuffs, or non-safety label alignment tolerances that do not impair electrical performance or customs clearance, as applied alongside electrical grading in the battery module EOL grading SOP (China-to-Spain corridor) and physical nameplate checks in the CE technical-file compilation SOP (China-to-Georgia corridor).

What to confirm before procurement

• Attach an explicit Defect Classification Matrix (Critical = AQL 0, Major = AQL 1.0, Minor = AQL 2.5 under ISO 2859-1 General Inspection Level II) as an annex to the sales contract before paying the deposit.
• Require at least 80%–100% of the export lot to be fully assembled, tested, and packed before the third-party inspector draws the random sample.
• Specify that any lot reaching the rejection number (Re) must undergo 100% factory sorting and a paid re-inspection at the supplier's expense prior to container loading.