Hipot Testing for Lithium-Ion Battery Packs

EEC Huayi has manufactured electrical safety testers since 1978, and our EEC-31 and EEC-41 hipot testers are built for small and medium-sized manufacturers, with manufacturing and service based in the Asia-Pacific region. In this article we explain where a hipot test for lithium-ion battery packs fits in production, which parts you can test with a general-purpose tester, and how to choose the test voltage. If you would like the background on test methods first, our guide to choosing between AC hipot, DC hipot and insulation resistance testing covers it.

A common production test for a lithium-ion battery module is an insulation resistance test between the positive or negative terminal and the enclosure. Many of our battery customers specify 500 V DC for 60 seconds, with a pass limit of more than 5 MΩ. The EEC-31 hipot tester runs this test, and it can also test pack parts before the cells are installed.

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Why a General-Purpose Hipot Tester Should Not Be Used on a Charged Battery Pack

Lithium-ion cells hold a voltage from the moment they are formed, so a battery module becomes energized equipment as soon as its cells are connected together. The EEC-41 user manual states plainly that a hipot test must never be performed on energized circuitry or equipment, and we give the same advice for the EEC-31.

Why Insulation Faults Cost Less to Find Early

Insulation faults in a battery pack can begin in an individual part, for example a busbar sleeve cut during handling, a cable pinched under a cover, or an insulating film missing from a cooling plate. When the fault is found at incoming inspection or at sub-assembly, the part is replaced. When it is found after the cells are connected, the pack has to be made safe and partly dismantled, which takes considerably longer and puts the people doing the work at greater risk.

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Which Lithium-Ion Battery Pack Parts Can You Hipot Test?

Any part that insulates the high-voltage circuit from the enclosure can be tested on its own before the cells are fitted. Your customer’s specification and the certification spec determines which parts are tested and at what voltage, but the following parts are commonly tested by customers.

Enclosures, Cooling Plates and Module Frames

When an enclosure, cooling plate or module frame has an insulating coating or film, a DC withstand test between the conductive body and a test electrode confirms that the insulation is complete. We suggest an insulation resistance test as well, as this gives you a measured value to record against each part in addition to a pass or fail result, but you should always make sure that the required test standards take precedence.

Busbars and High-Voltage Cable Assemblies

Insulated busbars and high-voltage cable assemblies can be tested between the conductor and a grounded fixture. As assembled packs operate on DC, a DC withstand test is the normal choice for these components and it also avoids false failures that capacitance can cause during an AC withstand test.

Enclosure Bonding with a Ground Bond Test

Exposed conductive parts of the pack, such as the enclosure, must have a low-resistance connection to the vehicle chassis. UN Regulation 100 sets the limit for this connection. The resistance must be lower than 0.1 Ω when a current of at least 0.2 A flows.

You can test this on the empty enclosure before the cells go in. The tester passes a current between each exposed conductive part and the point where the enclosure connects to the chassis. The EEC-41 ground bond function delivers 1.0 to 32.0 A, which is well above the 0.2 A minimum.

Selecting the Insulation Resistance Test Voltage

For battery pack parts, we suggest testing at a voltage above the pack’s nominal voltage, so that each part is checked under at least the stress it will see in service.

The EEC-41 is rated for insulation resistance tests at 1 kV and measures up to 1,000 MΩ, which covers parts for packs with a nominal voltage below 1 kV, including 400 V and 800 V designs. The EEC-31 is rated for insulation resistance tests at 4 kV and measures up to 200 GΩ, which leaves a margin for higher-voltage designs or a customer specification that requires a higher test voltage.

Discharge the Part After Every DC Test

A DC withstand or insulation resistance test leaves a charge on the part and on the test fixture. The EEC-41 user manual instructs operators to always discharge the item under test, and anything the high voltage has contacted before handling it or disconnecting the test leads. Adding this to the standard process for that production cell will help to avoid it being missed, especially by newer operators.

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Which EEC Huayi Tester Do You Need for Your Battery Pack Parts Line?

The EEC-31 and EEC-41 have the same DC withstand rating but they differ in insulation resistance range and in ground bond capabilities.

When the EEC-41 Is the Better Choice

If you assemble enclosures as well as cable assemblies, wire harnesses and busbars, the EEC-41 4-in-1 hipot and ground bond tester runs the bonding check and the insulation tests from one station. Its three test memories store the test settings, so the operator does not have to enter them again at the start of each batch. Our article on choosing a hipot tester with ground bond provides more information on the ground bond function.

When the EEC-31 Is the Better Choice

If your production line tests insulated parts only, or your customer specifies an insulation resistance test voltage above 1 kV, the EEC-31 3-in-1 hipot tester covers the work in a much smaller unit weighing 5.46 kg. Both models are CE certified and IEC compliant, and both can be started from a PLC on an automated line, as our article on PLC control for hipot testers explains.

If you let us know your customer’s test specification for the battery components you manufacture, we will tell you which model can run it. Please get in touch with our application engineers with the specification and your expected production volumes to determine the best model for your needs.

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Frequently Asked Questions