Self-powered microsystems like wireless microsensors and biomedical implants derive power from in-package minibatteries that can only store sufficient energy to sustain the system for a short life. The environment, however, is a rich source of energy that, when harnessed, can replenish the otherwise exhausted battery. The problem is harvesters generate low power levels and the electronics required to transfer the energy to charge a battery can easily demand more than the power produced. This paper presents how a 1 Ã 1 mm2 0.7-μm BiCMOS vibration-supplied electrostatic energy-harvesting system IC produces usable energy. The IC charges and holds the voltage across a vibration-driven variable capacitor CVAR so that ambient kinetic energy can induce CVAR to generate current into the battery when capacitance decreases, as the plates separate. The precharger, harvester, monitoring, and control microelectronics draw enough power to operate, yet allow the system to yield (experimentally) 1.27, 2.14, and 2.87 nJ per vibration cycle for battery voltages at 2.7, 3.5, and 4.2 V, which at 30 Hz produce 38.1, 64.2, and 86.1 nW. Experiments further show that the harvester system prototype charges 1 μF (emulating a small thin-film Li Ion) from 3.5 to 3.81 V in 35 s.