  The rapid pace of replacing fossil fuel propelled transport by electric
vehicles is critically dependent on high-performing, high energy density and
efficient batteries. Optimal and safe use of existing battery cells and
development of much-needed novel battery chemistries and geometries require a
large range of diagnostic and monitoring tools. While structural and chemical
information is readily extracted through a host of imaging techniques,
non-invasive functional detection of interior battery processes remains
limited. Here we introduce sensitive magnetometry performed outside the battery
that reveals a battery cell's internal current distribution. As a key
application, we use an array of sensors to image the magnetic field present
under cycling of a pouch cell between charge states. We find good agreement
between measured and modelled fields with sufficient resolution to detect
percent-level deviations around areas of high current density. This opens the
path towards rapid and reliable assessment throughout the battery life cycle,
from battery development and manufacturing quality assurance to operational
safety and optimised use.
