Someday we may all be driving – or be driven by – electric vehicles, but before that can happen, lithium ion batteries need to be perfected. In fact there are other alternatives, as exhibits at the Battery Show in Novi, Michigan, made clear this week, but those are stories for another day.
Sticking for the moment with lithium ion batteries I posed the broad question “are they getting better?” to someone who would know – Erik Spek, chief engineer at TÜV SÜD, the global third-party testing, inspection, and certification organization. As it happens, earlier this month he and Dr. Mehdi Hosseinifar, also with TÜV SÜD, presented a paper at the IEEE Symposium on Product Compliance Engineering in Portland, Oregon.
The paper, “Lithium Ion Abuse Test Methods Improvement,” focused on improvements in the mechanical testing of Li-Ion cells; specifically, nail penetration abuse testing, which the authors say is typically the most reactive of all the mechanical abuse tests. The paper describes “shortcomings” in standardized nail penetration abuse testing and outlines what TÜV SÜD believes to be a robust testing procedure.
Sandia National Labs and SAE J2464 [“Electric and Hybrid Electric Vehicle Rechargeable Energy Storage System (RESS) Safety and Abuse Testing”] use the EUCAR (European Council for Automotive R&D) Hazard Severity Levels (HSL) to describe to what extent a cell can react to a specific abuse.
The TÜV SÜD engineers conclude that “Li‐ion technologies are generally becoming less sensitive to abuse conditions.” The trend of HSL values over the past two and one-half years for HSL< 3 “is in a favorable direction.” HSL< 3 is defined as worst case evidence of minor cell leakage or venting with RESS (Rechargeable Energy Storage Systems) weight loss less than 50% of electrolyte weight.
Cells tested at lower SOCs (State of Charge) of 60‐70% in this population are strongly showing HSLs < 3. “There appears to be an opportunity for risk reduction in the area of penetration abuse at high SOCs (90‐100%),” the authors write. “Until cells are robust enough to withstand high velocity projectiles, battery packs must be designed to protect cells from such events.”
There is obviously much more work to be done, but it’s nice to get confirmation that technology is moving in the right direction.