Performance and durability analysis of electric bicycle batteries with separation of pack- and cell-level contributions to capacity fade

Powered by Li-ion batteries, electrically assisted bicycles (e-bikes) provide a sustainable way of transportation. Evaluating the performance and durability of these batteries is key to understand their functional capabilities and lifespan. However, while Li-ion cells have been extensively studied, there is scarcity of data on commercial e-bike battery packs regarding their performance and long-term durability, particularly in the context of new regulatory requirements. This study addresses this gap with an exploratory analysis of four commercially available e-bike batteries through cycling tests under laboratory conditions, conducting a comparative assessment of pack- and cell-level behaviour. The capacity, internal resistance, power, and energy round-trip efficiency were monitored over up to 500 charge-discharge cycles. Durability testing reveals capacity fade of less than 10% after 400 cycles except for one battery, which exhibits a pronounced increase in capacity fade from around 200 cycles onwards. Voltage curve analysis and additional experiments suggest that this is due to inadvertent repetitive overdischarge of a series-connected cell group. Cell- and pack-level contributions to capacity fade are disentangled by using a power-law model, revealing that cells predominantly drive overall capacity fade.