The challenge with charging the battery vBAT directly from nocharge conditions is its large capacity. In other words，

　　replenishing vBAT to practical levels from a harvesting source

　　that outputs little power PH requires considerable time. This is

　　a problem because the starter draws a small fraction of the

　　power that the system can harness from PH in steady state，

　　which means the harvester is by and large inefficient. Quickly

　　charging a lower capacity source from which to power the

　　steady-state harvester is therefore an appealing alternative.

　　The purpose of startup supply vST in Fig. 2 is to store and

　　dispense just enough energy to operate the high-efficiency

　　harvester across one or more energy-transfer cycles. This way，

　　the starter energizes vST and the power conditioner then

　　charges vBAT across several cycles until vBAT is high enough to

　　drive the switches on its own. PH therefore supplies the power

　　vBAT receives as PBAT， the gate-drive power vST delivers to the

　　drivers， and the conduction and quiescent losses the switches

　　and starter dissipate. Note that vST must rise to practical levels

　　（e.g.， 1 V） to switch the system， just as vBAT must climb to

　　ultimately take control of the system.