A new method has been developed to estimate the charge capacity of LiFePO4. This method does not rely on assembly of active material, carbon and polymer binder composite electrodes, and can be used on nanoparticles with or without carbon coating. Charge capacity is derived from LiFePO4 oxidation by a solution redox mediator, 10-methylphenothiazine, in a bulk electrolysis system. The mediator serves the role of connecting the working electrode with the LiFePO4 particles suspended in the electrolyte. XRD and IR spectroscopy identified the oxidized species as heterosite-FePO4, as in standard coin cell tests. Analyses with 6.8% relative standard deviation were routinely obtained.