EOL and leak testing for ESS modules and PACKs: what to test, which parameters, how to judge a pass

Where on the line does EOL testing happen?

End-of-line (EOL) testing is the final check before a product leaves the line. An ESS module and PACK line usually has four test points:

1. Module EOL: after busbar welding and post-weld cleaning, before modules go into the housing.
2. Cooling-plate leak test: after the cooling plate is loaded, before adhesive and assembly, so a leak is found before it is built in.
3. PACK EOL: after PACK assembly and the top cover.
4. Full-pack leak test: before the PACK leaves the line, to confirm the housing seal meets its protection rating.

The 8 PPM line on this site follows this layout: module EOL is fully automatic; cooling-plate leak test, PACK EOL and full-pack leak test are semi-automatic stations.

What does module EOL test?

Test	Purpose	Typical parameters
Total and cell voltages	Confirm series count and cell consistency	Cell spread per matching rules, e.g. ≤ 5 mV
Internal resistance (AC or DC)	Catch poor welds and high connection resistance	Compared with the statistical range of the same module type
Insulation resistance	Positive and negative each to housing and structure	DC 500–1000 V, commonly ≥ 500 MΩ
Withstand voltage	Positive and negative to housing, insulation strength	Graded by charge cut-off voltage (see below), leakage < 10 mA
Temperature sensing	NTC readings plausible, no open or short circuit	Spread between points commonly ≤ 3 °C
Sensing channels	Each sense wire maps to its cell, no open circuits	All channels
Polarity	Prevent reversed connection	—

Module EOL on the 8 PPM line on this site: insulation test at DC 1000 V, ≥ 500 MΩ; withstand test at DC 2800 V, leakage ≤ 10 mA.

What does PACK EOL test?

Beyond the module items, PACK EOL focuses on the BMS and electrical safety:

Test	Purpose	Typical parameters
Insulation, withstand	PACK positive and negative to housing	Graded by the PACK’s charge cut-off voltage
Equipotential bonding	Exposed conductive parts reliably earthed	Four-wire at 25–40 A, commonly ≤ 0.1 Ω
BMS communication	CAN / RS485, firmware version, parameter writing	Version and configuration match the work order
Sensing calibration	BMS voltage, current and temperature readings against reference meters	Cell voltage error ≤ 5 mV; current ≤ 2 A (below 200 A) or ≤ 1 %; temperature ≤ 1 °C
Relays, fuses, pre-charge	Operation and welded-contact detection	Per BMS design
Short charge-discharge and DC resistance	Main-circuit connections and temperature rise	e.g. a 10-second 1C pulse for DC resistance
Balancing, SOC setting	Verify balancing; set state of charge for shipping	Per customer requirement

Insulation and withstand: which voltages?

GB/T 36276-2023, the Chinese national standard for lithium-ion batteries in power storage, sets test voltages by the battery’s charge cut-off voltage U.

Insulation resistance (positive and negative each to exposed conductive parts, held 1 minute), requirement ≥ 1000 Ω/V:

Charge cut-off voltage U	Test voltage
U < 500 V	500 V
500 V ≤ U < 1000 V	1000 V
U ≥ 1000 V	2500 V

Withstand voltage (no breakdown or flashover, DC leakage < 10 mA; start at no more than half the test voltage, reach it within 10 seconds, hold 60 seconds):

Charge cut-off voltage U	DC test voltage	AC test voltage
≤ 60 V	1530 V	1080 V
60–300 V	2010 V	1420 V
300–690 V	2800 V	1970 V
690–800 V	3000 V	2120 V
800–1000 V	3390 V	2400 V
1000–1500 V	4380 V	3100 V
> 1500 V	5370 V	3800 V

Lines usually keep these voltage grades, shorten the dwell to fit takt (the Chinese BMS standard GB/T 34131 allows 1 second for factory withstand tests), and set pass limits that are often stricter than the type test.

Leak testing: how to test and judge cooling plates and packs

Object	Usual method	Typical test pressure	Common limit	Cycle time
Cooling plate / coolant circuit	Pressure decay or differential pressure; helium where tight	1.5–2 × maximum working pressure, commonly 200–350 kPa	Hold 30–60 s with a drop within tens of pascals to 100 Pa; helium around 1×10⁻³ mbar·L/s	1–3 min
PACK housing (IP67)	Pressure decay or mass flow; helium where tight	3–20 kPa	Derived from enclosure volume and protection rating, commonly tens of pascals per minute; helium around 5×10⁻³ mbar·L/s	1–2 min

In the national standard, the coolant-circuit test is a type test: pressurise with gas to 1.2 times maximum working pressure, stabilise for 2 minutes, hold for 1 minute; the drop may not exceed 0.1 % of maximum working pressure for a module, 0.2 % for a cluster.

Do not copy someone else’s pack limit: for the same leak rate, a larger enclosure shows a smaller pressure drop. One set of test data for reference: an 85-litre pack leaking no more than 27 mL/min at 5 kPa (about 32 Pa per minute of pressure drop) stayed dry after 1 hour under 1 metre of water, while a sample dropping 76 Pa per minute let water in. Run your own immersion comparison with your housing to set the line limit.

From experience:

• Let housing temperature settle before and during the test; a 1 °C change can move pressure more than the leak itself.
• Check the tester regularly with a calibrated leak so the limit stays valid.
• Locate leaks in rejected parts with helium or immersion.

Matching test takt to the line

• An 8 PPM line with 1P13S modules produces a module about every 97.5 seconds. Module EOL takes about 30 seconds, so one station is enough.
• PACK EOL includes charge-discharge and BMS calibration and often takes several minutes, so two or more stations usually run in parallel. The 8 PPM line on this site has two PACK test stations.
• Leak tests have long hold times; size the number of cooling-plate and pack leak stations against takt.
• With large cells, DC resistance and charge-discharge tests run at higher currents; check the testers’ current ranges. See 500 Ah-plus cells: what changes.

Data, limits and traceability

• Pass limits are managed and sent from MES, with an approval record for every change.
• All test data for each module and PACK is tied to its barcode and traces back to the sorting data of every cell in it.
• Define retest rules: which tests may be repeated and how often; count first-pass and retest passes separately.
• Run statistical process control on continuous values such as resistance, insulation and pressure drop, and act when distributions drift.

Type tests versus factory inspection

Standard	Status	What matters for line testing
GB/T 36276-2023, Li-ion batteries for power storage	Recommended national standard	Type tests include insulation, withstand and coolant-circuit leak tests; factory inspection covers only appearance, dimensions, initial charge-discharge energy and cluster protection
GB/T 34131-2023, BMS for power storage	Recommended national standard	BMS sensing accuracy, insulation and withstand; factory withstand test may be 1 second
GB 44240-2024, lithium batteries for energy storage systems — safety	Mandatory national standard, in force from 1 August 2025	Safety type tests
UL 9540	Energy storage system safety (North America)	Production-line dielectric withstand test for systems above 60 V working voltage; bonding resistance no more than 0.1 Ω (check the clauses in the current edition)
UL 9540A	Thermal runaway and fire propagation test method	Not a production test

A type test validates a design once; factory inspection checks every unit. National factory-inspection lists are short. Insulation, withstand and leak tests are run on 100 % of production because they bear directly on safety, and customers and certification bodies widely require it.

What we can supply

The 8 PPM line on this site includes module EOL, cooling-plate leak test, PACK EOL and full-pack leak test stations. As a project needs, we can also supply higher-voltage insulation and withstand testers, equipotential testers, charge-discharge testing, helium leak testers and MES-connected test data systems through our supply-chain service.