The lead-acid battery is widely used in stand-alone solar
power systems. The lead-acid battery usually operates at
11.3–12.7 V with a state-of-charge (SOC) from 10% to
100%. The discharge controller in a stand-alone solarpowered
light-emitting diode (LED) lighting system usually
consists of a DC/DC converter that converts the floating
battery voltage into a constant current which in turn drives
the LED. A 15% energy loss of the DC/DC converter is
then introduced. The DC/DC converter may also increase
system costs and reduce system reliability due to the
additional components. As shown in Fig.5, the currentvoltage
(I-V) curve of an LED indicates that the LED will
be easily overdriven and damaged by the voltage variation
from the battery and the junction temperature of the LED.