Impact of high-amplitude alternating current on LiFePO₄ battery life performance: investigation of AC-preheating and microcycling effects

Superimposed Alternating Current (AC) imposed by electric machines and power electronics components in renewable energy systems and electric vehicles (EVs) exposes Li-ion batteries (LIBs) to high-amplitude AC harmonics (i.e., ripple currents). These AC perturbations make LIBs experience shallow charge-discharge cycles or microcycles, resulting in significantly increased battery charge throughput. Recently high amplitude AC-only waveforms have also attracted significant attention as they can be used to warm up/preheat EV batteries in low-temperature environments. To understand what happens when the amplitude of AC-bearing duty cycles is pushed to higher levels, beyond the recommended constant current value set by the manufacturer, a set of fresh LiFePO₄ (LFP) cells were subjected to high-amplitude AC-only profiles at low and high frequencies for extended periods. Subsequently, LFP cells were cycled on dynamic current profiles (superimposed AC profiles) for ∼ 200 days, representing real situations encountered by LIBs in renewable energy and electromobility systems. Long-term aging results revealed that as long as the AC current induces a voltage oscillation that remains below the calculated threshold polarization overpotential, the cycle-life of LFP cells is not significantly affected for AC duties as high as the recommended constant current values. Furthermore, for high-frequency AC, a significant amount of microcycling is effectively filtered out by the battery's double-layer capacitance. However, for low-frequency superimposed AC, for which most of the AC current flows through the charge-transfer resistance, higher charge throughput induced by microcycles causes accelerated degradation.