Battery testing for electric vehicles grows in importance
Edited by Motorized Vehicle Manufacturing Yearbook Editor Bill Koenig from information supplied by INFICON (Instruments for Intelligent Control).
California’s goal to have 1 million Zero Emission Vehicles (ZEVs) on the road in less than three years has vast implications. Fuel Cell Vehicles (FCVs), Battery Electric Vehicles (BEVs) and Plug-in Hybrid Electric Vehicles (PHEVs) have ushered in a new generation of battery technology and quality control requirements.
With BEVs and PHEVs expected to dominate sales due to the cost of fuel-cell-powered vehicles, the demand for batteries to power electric vehicles is exploding. Consider that Tesla’s Nevada gigafactory is predicted to deliver more lithium-ion batteries in one year than were produced globally in 2014, according to the Rechargeable Battery Association. The question is how will the auto industry assure battery quality and ensure sustained problem-free operation?
New Batteries Require Leak Detection
The quality of battery components will be a critical concern of automakers and their suppliers as the number of electric and hybrid electric vehicles increases.
The requirements for the physical integrity of BEV and PHEV batteries and electric motors are strict. We have only to think of recent events with air bag inflators to understand the importance of manufacturing quality standards and accurate and reliable test methods.
Fortunately, manufacturers and suppliers have access to well-known methods of leak detection. They include helium vacuum testing of individual battery cells; or cooling and refrigeration sniffer leak detection after final assembly.
Electrolytes and Water Don’t Mix
Most people have experienced a mobile phone’s loss of load capacity or the amount of charge it can hold. Sooner or later a phone’s load capacity will decrease. Suppliers, manufacturers and customers all want to minimize this loss of capacity in the expensive batteries used in electric and various forms of hybrid electric vehicles.
Auto industry standards generally call for batteries to maintain a storage capacity of 80% or more after 10,000 charging cycles. If the EV/HEV battery is to reach this target during a lengthy service life, OEMs and suppliers must ensure leak tightness down to the smallest battery component — the individual battery cell. Electrolytes must not leak from the battery cell and moisture should not be able to enter the cell.
Each battery cell must be gas-tight. If this is not the case, not only will battery power be reduced over the long term, but an improperly enclosed battery cell also could be destroyed by the penetration of atmospheric humidity. The results could be catastrophic.
Helium Leak-Detection Testing
There are three different basic battery-cell designs: 1) round cells (also known as type 18650, 20700, or Tesla’s 21700) that resemble a soda can; 2) prismatic cells which are rectangular in sizes from a candy bar-sized mobile phone battery to larger ones used for laptop batteries, all with stable housings, and 3) pouch cells which have no housing, may look like zip-closure bags, and often are used to create automotive multi-cell batteries.
Leak-detection tests should be performed before these various battery cells are sealed into a final battery container. Each battery cell filled with electrolyte also must be tested for leaks after it is sealed into a container to prevent moisture from entering from the outside. Permissible atmospheric leakage rates are in the range of 10-5-10-6 mbar·l/s.
Commonly used leak-detection methods are simply unsuitable for today’s new generation of electric vehicle batteries. Best practice calls for the use of modern integral-test-gas testing which is both fast and accurate. During the manufacturing process, small amounts of helium, usually a blend of about 5%, may be added to the electrolyte filling each cell. Therefore every battery cell to be tested would contain test gas.
Pouch Cell Bombing Tests
Sometimes it is not possible to add helium to the individual battery cell. In that case a process called “bombing” is used. The battery cell is exposed to helium under high pressure prior to testing: helium can then penetrate into the cell through any leak. The part or cell is placed in a vacuum chamber, and test gas leaking out of the part can be detected.
The bombing method is mainly used for prismatic and pouch cells. It is important to test pouch cells in a support structure so that their sealing seams are not damaged by the vacuum since vacuum pressure could deform the pouch-cell’s flexible envelope.
There are definite advantages to performing helium testing in a vacuum chamber on a production line. The process can be highly automated; it is a highly accurate test method, and cycle times are short and free from user interference.
Testing Incoming Cells
Many automotive manufacturers are purchasing their battery cells from Asian suppliers. These suppliers test the leak tightness of their cells during production. However, cell damage in transit from Asia to the US or Europe is not an uncommon occurrence.
In many cases lithium-ion batteries and cells cannot be shipped on passenger aircraft because of a potential fire risk. Shipments of this type were embargoed after several laptop computers using lithium-ion batteries severely overheated. Catastrophic aircraft crashes due to cargo fires also have occurred. As a result, various flight-control authorities have banned the transport of lithium-ion batteries as cargo on passenger aircraft.
Some air transport companies such as FedEx are using special fire-suppressant systems in their cargo bays – systems that deploy fire-suppressant foam to extinguish fires in battery containers.
Even ocean-going vessels are not immune to potential fires. On-board cargo containers, for example, have been completely destroyed by fire. The so-called “thermal runaway” of a single battery cell caused by the short circuit of internal electrodes can lead to a fire or the explosion of an entire cargo container as temperatures reach 1100° C or more.
Against this background it is not surprising that many experts suggest that auto manufacturers carefully inspect incoming battery cells for quality. The recommendation is meant to cover any OEM or supplier using an overseas battery-cell manufacturer.
How Much is Too Much?
Any manufacturer who adds value or works with battery cells should be conducting leak-detection tests. Typically battery cells from Asia are assembled into battery modules which are then used to produce battery packs. Some OEMs perform these production steps on their own, while others source the assembly of battery packs from Tier-1 suppliers.
Battery modules and battery packs normally are equipped with a number of cooling channels which are filled either with a water-glycol mixture or refrigerant from the vehicle‘s air-conditioning system. As a rule, the power electronics that regulate battery operation in a vehicle are cooled in one of these two ways. System leak tightness on a long-term basis is critical since leakage of the cooling medium could lead to short circuits. If a water-glycol mixture is used as the refrigerant, the normal marginal leak rate is defined as 10-3 mbar·l/s. In refrigerant circuits, required leak tightness is measured by testing to a leakage rate of 10-5 mbar·l/s.
The integrity of the battery-pack case also needs to be tested according to Protection Class IP67 or IP69K. Water-leak tightness is ensured by testing against an approximate leakage rate of 10-3 mbar·l/s.
Forming Gas or Refrigerant Sniffer Leak Testing
To test the cooling circuit of battery packs it is not necessary to use helium as a test gas. Tests with a different forming gas often are more economical. A less expensive forming gas, for example, is a mixture consisting of 95% nitrogen and five percent hydrogen. This mixture ratio results in forming gas that is not combustible (and in fact is often used as a shielding gas in welding). Using this method, the hydrogen in the mixture serves as the test gas.
When leak testing battery-pack cooling circuits, all of the cooling circuits should be evacuated before they are filled with forming gas. Welded seams and soldered joints then can be tested to determine whether the forming gas leaks out.
In pre-production and small-scale series production the sniffer probe of a hydrogen leak detector can be operated manually to find potential leaks.
In large-scale production an automated procedure is recommended. For example, when using a helium sniffer leak detection device such as the INFICON Protec P3000 (XL) with a sniffer probe on a robotic arm, precise and reliable leak detection is possible from a greater distance. The location of potential leaks also can be precisely determined for possible rework.
If a refrigerant is used instead of a water-glycol coolant, leaks can be located precisely without evacuating the cooling circuits and filling them with forming gas. Sniffer leak detection devices such as INFICON‘s Ecotec E3000 are able to use common refrigerants like R1234a, R1234yf or CO² as test gases, further reducing cost.
Battery Case Leak Detection
When testing battery housings, the ideal method depends on the size of the battery pack. For large enclosures, sniffer leak detection is the method of choice. For small batches, manual testing with forming gas is recommended. Serial production of larger housings is best accomplished by an automated helium sniffer with a robotic arm. If, however, the test part is small-to-medium in size with an approximate internal free volume of 70 liters, an accumulation test might serve as an alternative.
While a single sniffer test takes two to five minutes, an accumulation test typically requires one to two minutes. Although accumulation tests also use helium as a test gas, they are not as expensive as helium vacuum tests. In comparison to a helium vacuum test, a sniffer test does not require an expensive vacuum chamber. For a simple accumulation chamber, an air-tight container is sufficient.
Should automatic integral leak testing be required for medium-sized battery-pack housings, the housing is first pumped down to a pressure range of approximately minus 100-250 mbar and then filled with helium to a pressure of about 100-250 mbar. The resulting helium concentration in the housings is 20–50%. Helium, which emerges from leakage points in the test part, collects in the accumulation chamber where it can be detected with a device such as the INFICON T-Guard sensor at atmospheric pressure. If a case fails an integral leak test, it would then need to be removed from the accumulation chamber and subjected to a sniffer test to locate the leak or leaks involved.
EV/HEV Batteries Require Test Gas Methods
Quality-control test gas methods for drivetrain batteries and battery components used on EV/HEV vehicles have distinct advantages compared to older methods such as water-bath or pressure-drop tests.
Test-gas methods provide accurate, traceable and repeatable measurements to ensure reliable test results. Unlike pressure-drop tests, test-gas methods also are not affected by changes in temperature or humidity during the test process. And because test parts do not come into contact with water, there is no risk of water penetration or need to dry test components.
Older conventional testing methods simply can’t compete with the high sensitivity of modern test-gas methods, which can easily detect even the smallest leaks in the range of 10-4-10-6 mbar·l/s.
Adopting modern test-gas leak-detection methodology to assure the quality of today’s new generation of battery components, batteries and battery systems is becoming critically important as more and more electric vehicles hit the road in the US and elsewhere around the world.