This paper presents the thesis work for the design of solar-cell based power supply for smart sensors. The design has involved a constant current, constant voltage Li-ion battery charge controller and a DC-DC buck converter. The low power control circuits of the system have been implemented as Application Specific Integrated Circuit (ASIC) with total area of 1mm * 0.9mm. The design has involved several building blocks such as differential amplifiers, operational amplifiers, bandgap reference (BGR), ramp generator, current sensing circuit, error amplifier, comparators, inverters, NOR gate, multiplexers, DC-DC converter, and a solar cell model. The battery charge voltage measured is 3.98V at extremely low temperature of -20oC, 4.17V at the typical temperature of 25oC and 4.2V at extremely high temperature of 85oC. The design has been implemented with AMS 0.35µM CMOS technology. The electronic circuitry can be used as battery chargers in devices such as cellular phones, PDAs (personal digital assistants), handheld equipment, battery operated portable devices and can be powered by stand-alone PV (photovoltaic) system. The system can equally serve as power supply unit for remote wireless sensor networks (WSN).