This paper introduces an optimized approach to RF
energy harvesting from low available power (e.g. below -
20dBm at the antenna feedpoint) in the 2.4GHz (WiFi)
frequency band. The energy harvester captures the input RF
energy, rectifies it to DC, boosts the DC to a higher voltage,
and ultimately stores that energy into a long-term energy
storage unit (NiMH battery in this case). The rectenna
design is optimized for the expected RF power levels
available from ambient WiFi signals in a typical office
environment. The DC-DC boost converter has a cold-start
voltage of less than 100mV and is optimized to support the
battery charger from the extremely small input voltages
provided by the rectenna. The novel charger design is
optimized to the needs of a NiMH charging system, enabling
trickle-charging without requiring a complex charge
management system as is typical for lithium or lithium
polymer batteries.