This study investigates ac parameter characterization to detect partial shading within a series string of photovoltaic (PV) cells. Partial shading detection is needed to prevent hot spotting, a condition that reduces panel power performance and accelerates cell degradation. A PV cell is comprised of series and parallel resistances and parallel capacitance, which are affected by voltage bias, illumination, and temperature. When the string is under a maximum power point tracker or constant voltage controller, partial shading results in a clear increase in capacitance that is detectable by measuring the ac impedance magnitude in the 10-70 kHz frequency range. When the shaded cells become moderately negatively biased, there is a clear increase in parallel resistance. These trends in the PV cell physical characteristics can be used to develop partial shading detection methods that utilize the switching ripple of the power converter that controls the PV string.