?Electric Vehicle???? Battery?? Pack Test Strands???

`The rapidly emerging electric vehicle (EV) market is poised for growth and change. Since high voltage (HV) power battery system plays a critical role and it is very sensitive to scenarios i.e. shock/vibration/temperature swings/penetration, it is a must that from cells to battery packs, the performance, durability, safety, and abuse testing's are required to meet the latest industry and national and international standards and regulations. This article is a brief introduction to a testing procedure for EV's HV battery, based on my understanding and experience.

???Regulatory Bodies

There are many standards for EVs dictated by regulatory bodies, such as ISO (International Organization for Standardization), IEC (International Electrotechnical Commission), UL (Underwriters Laboratories), SAE (Society of Automotive Engineers), ECE (the United Nations Economic Commission for Europe), etc. These standards differ in the scope of the market, application, test content, and judgment criteria. For example, UL or SAE testing is usually required in the US, while GB/T is needed in China.

??Cell :-

As a basic building unit of a battery system, a cell requires many fundamental tests, such as over-discharge, overcharge, short-circuit, drop, heating, crush, nail-penetration, seawater immersion, thermal-cycle, low-pressure, etc. Here I'd like to highlight three types of testing as examples. (1) Seawater immersion. The cell is immersed in 3.5% NaCl solution (used to simulate seawater environment) for 2-hour and the capacity and cell mass are recorded within 1-hour after completing the test. (2) Crush. The cell is crushed by a solid cylinder at a certain speed and the test stops until the voltage drops to zero or the deformation amount increases to the target percentage or the extrusion force reaches the target value. (3) Nail-penetration (see the test video below). A stainless steel nail penetrates into the cell vertically at a certain speed with respect to the electrode plates with jigs until the nail penetrates through the cell. The packaging protection level is relatively low for polymer batteries, so the nail-penetration test result is quite important for the improvement in structure design of battery module and pack. For all testing mentioned above, besides recording some typical parameters history like the voltage, temperature, and current, the general judgment criteria are if vent/fire/explosion occurs.

? Module :-

A battery module is composed of multiple cells. The testing standards for a module is similar to a cell, although there are differences in some specific details. For example, module-level testing includes not only some fundamental tests that prevail in cell-level, like over-discharge, over-charge, and heating, but also sample accuracy and compression, etc. Note that the module testing typically is conducted at room temperature of 25+/-5 centi-degree, the relative humidity of 15%-90%, and an atmospheric pressure of 86-106 KPa, based on most international standards.

However, the module-level testing items are decreasing because the battery development trend is larger-sized cells and the "Cell-to-Pack" concept, that is, modules being removed to increase pack energy density and lower the cost. The picture below is BYD's blade battery pack without modules, with high space utilization of 60% and low space with 40% utilisation.

?? Pack :-

A battery pack is typically composed of modules, hardness, cooling channels, structural enclosures, and ECUs. Some testing contents are different from module or cell-level, especially the protection performance (shock, crash, vibration), thermal management capability (thermal propagation), electricity leakage protection (short-circuit). Take the crash test as an example: 25g+ accelerations are applied in X-Y-Z three directions for ~100ms to see there are mechanical failing and capacity amount. Another thing worthwhile to mention is thermal propagation control. Nowadays with the wide use of higher energy density cell chemistry, the chance of thermal runaway seems to go higher. We need to check the pack's thermal propagation control capability, i.e. say 5-minute remained for passengers' leave ahead of burning.

???? Full-vehicle :-

The last step is to test at the full-vehicle level after the pack-level, i.e. crash safety, waterproofing, environment (i.e. low/high temperature) during the service. For the crush test, the battery pack is evaluated during full-vehicle crush, typically located under the body bottom, protected by aluminum or steel-made tray, plus the added protection from the body frame. Particularly, side crush is challenging and necessary due to the limited energy absorption space (see the picture below). The pack will be evaluated if there is smoke or leakage after a certain incursion amount in Y-direction on the pack and cells. Also, the crush sensors embedded in the pack are used to cut off the HV circuit at the moment of crash accidents. For the extreme severe environmental test, a series of durability tests of a vehicle will be conducted in extremely hot or cold regions.

The performance and safety testing of EV's HV battery at different levels (i.e. cell, module, pack, full-vehicle) are critical. With the partnership of regulators, suppliers, and OEMs, the EV industry will keep moving forward, with more confidence brought to the public users.