We present a discontinuous harvesting approach for switch capacitor dc-dc converters that enables ultralow-power energy harvesting. Smart sensor applications rely on ultralow-power energy harvesters to scavenge energy across a wide range of ambient power levels and charge the battery. Based on the key observation that energy source efficiency is higher than charge pump efficiency, we present a discontinuous harvesting technique that decouples the two efficiencies for a better tradeoff. By slowly accumulating charge on an input capacitor and then transferring it to a battery in burst mode, dc-dc converter switching and leakage losses can be optimally traded off with the loss incurred by nonideal maximum power point tracking operation. Harvester duty cycle is automatically modulated instead of charge pump operating frequency to match with the energy source input power level. The harvester uses a hybrid structure called a moving-sum charge pump for low startup energy upon a mode switch, an automatic conversion ratio modulator based on conduction loss optimization for fast conversion ratio increment, and a $<$15-pW asynchronous mode controller for ultralow-power operation. In 180-nm CMOS, the harvester achieves $>$40% end-to-end efficiency from 113 pW to 1.5 $μ$W with 20-pW minimum harvestable input power.