Aging estimation of lithium ion cells under real-world conditions through mechanical stress measurements

Abstract

Lithium-ion batteries are the storage technology employed in today's electric vehicles; however, they still have some technological limitations. Aging strongly influences battery performance and cycling induces volume variations in unconstrained cells during their life. Deformation becomes pressure as soon as there is a constraint, such as the typical cell frame used in vehicle battery modules. This work proposes to use a simple mechanical stress measurement as a marker to assess the State of Health (SOH) of a cell under real cycling and operative conditions. An experimental set-up for pouch cells was specifically designed to identify correlations between mechanical deformation, stress on the cell and SOH, in terms of capacity and internal resistance. Aging tests applied to two cells showed that cell's resistance increased by 25 % after 287 cycles and is the parameter that likely defines the battery end of life. The results revealed a relationship between capacity and cell resistance with cell pressure and deformation. Another outcome was that age-induced swelling should be considered when designing the module structure, as mechanical stresses could increase up to 31 %. The correlations identified in this work could be implemented in a Battery Management System to estimate the SOH, directly or in combination with more complex algorithms.