Harvesting energy into lithium-ion batteries

Figure 3. Pre-charge phase

During the harvesting phase, the MEMS capacitor is connected to the rechargeable battery with S5 so that charge can be driven into the battery (Figure 4). Since the battery is a low impedance source, the voltage across the battery is practically constant and the decreasing capacitance therefore produces a harvesting current (I = C dV/dt + V dC/dt ≈ V dC/dt). Although this current charges the battery, the resulting increase in voltage is minimal because the magnitude of the current is low, which is why the capacitor voltage is still constrained.

Figure 4. Harvesting phase

To complete the cycle, after the minimum capacitance is reached and harvesting ends, the energy remaining in the capacitor is recovered with the same inductor used in the pre-charge phase (Figure 5). In essence, the pre-charge sequence is reversed: the residual energy is transferred into the inductor with S₂ and S₄ and later delivered to the battery with S₁ and S₃. After the capacitor is fully discharged, its electric field no longer exists and its plates are free to move and return to their minimum separation state without requiring any electrical energy.

Figure 5. Recovery phase