Lithium-ion is increasingly the technology of choice for battery-powered systems. Current cell performance monitoring, which relies on measurements of full cell voltage and sporadic surface temperature, does not provide a reliable information on the true internal battery state. Here, we address this issue by transforming off the shelf cells into smart systems by embedding flexible distributed sensors for long-term in-situ and operando thermodynamic data collection. Our approach, which enables the monitoring of the true battery state, does not impact its performance. In particular, our results show that this unprecedented methodology can be used to optimise the performance and map the safety limits of lithium-ion cells. We find that the cell core temperature is consistently and significantly higher than the surface temperature, and reveal a breach of safety limits during a rapid discharge test. We also demonstrate an application of a current considerably higher than the manufacturers’ specification, enabling a significant decrease in charging time, without compromising the cell’s thermal stability. Consequently, this work on cell instrumentation methodology has the potential to facilitate significant advances in battery technology.