Lube-Tech
Combustibility may be assessed by heating the test fluid in a specially-designed enclosure and introducing a “blank” cell that has been heated to 800+ °C (representative of thermal runaway conditions). The blank is immersed in the fluid adjacent to “dummy” cells and the temperature of the blank, dummy cells, fluid, and air are monitored. A spark source is active throughout the test to ignite any combustible gases. Resistance to physical damage is evaluated using the Nail Penetration Test. For this test, thermocouples are installed on each battery cell and in the test fluid. The test module is conditioned and charged to 100% State of Charge (SOC) at 25 ˚C, at which point a nail physically penetrates the first cell in a group. The test is then monitored to determine if a thermal runaway event occurs due to the nail penetration and if the runaway propagates to nearby cells.
Heat induced failure testing uses a test apparatus similar to the nail penetration test but without the use of the nail. Instead, a heater wire is placed on the target cell and the module is conditioned and charged to 100% SOC at 25 ˚C. The heater wire is then energised and the test monitored for thermal runaway induced by the heated wire. If thermal runaway occurs, the test can determine the fluid’s ability to sufficiently transfer heat away and prevent propagation to adjacent cells.
Performance and life testing Tests may also be performed to assess an immersive coolant’s ability to improve charging performance, and prolong the life of the battery. For each of the following tests, a module and container are housed in a test chamber, plumbed to a circulation system, and wired to a battery cycler.
Performance testing targets a temperature rise when the module is subjected to a fast charge profile and is used to determine the degree to which charging and discharging performance are improved in the presence of an immersive coolant. Life testing
28 LUBE MAGAZINE NO.170 AUGUST 2022
The Advanced Fluids for Electrified Vehicles Consortium (
afev.swri.org) is a joint industry project whereby member companies pool resources to explore precompetitive concepts to properly characterise EV fluids.
LINK
www.swri.org
PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE
No.141 page 5
involves subjecting the module to a repeated fast charge profile in order to accelerate aging under high stress. Module health is assessed at intervals using a reference performance test to quantify degradation.
Conclusion There is little debate that electrified vehicles will ultimately dominate the automotive landscape. As the industry transitions away from traditional applications, it is vital to understand the unique fluid needs of EVs of various design. Electrical and heat transfer properties, high speed durability, material compatibility, oxidation and aeration performance are all of considerable importance when selecting a fluid for use in an EV drive unit.
Fluids for battery cooling must be able to remove heat from the system while protecting against flame propagation and thermal runaway, and can have positive effects on battery life and performance. Fluids currently in use were likely designed for legacy drivetrains and may not be optimised for electrified vehicles. The lubricants industry is rapidly developing the test methods that will be needed to assess the performance of the next generation of EV fluids.
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