Standard electrical tests reliably predict how a lithium-ion battery behaves under load but fail to pinpoint its internal aging condition. Engineers usually assume that matching a cell’s electrical output means the underlying mathematical model correctly reflects the physical degradation inside. Instead, widely divergent parameter values for protective surface films and ion movement through porous electrodes generate identical electrical impedance curves.
As lithium ions travel between electrodes, growing chemical coatings and electrolyte friction resist current flow. A thick surface layer paired with rapid ion flow produces the exact same electrical resistance as a thin layer paired with sluggish flow, just as narrow plumbing with high water pressure can match wide pipes under low pressure. Cooling the battery changes each physical reaction rate by different amounts, breaking the mathematical tie. At colder temperatures, slower ion migration highlights film resistance differences that room temperature measurements conceal.
Researchers evaluated a 48-parameter porous-electrode model against 82 experimental records collected from 20 aging commercial cells. The team found that ten of the eleven least constrained model parameters directly tracked battery aging markers, varying by 0.60 decades across equally valid model fits. Adding an impedance measurement taken at minus ten degrees Celsius separated 62 percent of those competing parameter sets on three independent test cells.
Testing laboratories can now distinguish between rival degradation models by pairing standard room temperature data with targeted cold measurements. This protocol ranks a cold test ahead of longer low-frequency scans to capture the maximum number of true physical coordinates per hour.