In many merging economies in some countries as in Egypt, the demand for electricity is growing at least as fast as the growth rate of the economy. For the economic growth to continue, the rising demand for electricity has to be met. Whilst the estimates of the additional generation capacity required vary in detail and in principle. Fortunately, there is at least a partial solution through the use of the battery system. Nowadays, portable devices have become a wide of applications due to their small size, and light weight. Li-ion batteries are well suited to portable electronic manufactures such as cell phones. Li-ion possesses the greatest potential for future development and optimization. The Li-ion batteries offer the highest energy density and storage efficiency close to 100%, which makes the best suited for portable devices.

 Modern battery chargers require high quality, and many parameters are different from one to another such as type, charge rate, maximum charging current, internal resistance, voltage at the outlet point, temperature, service life and so on. Some parameters can make arbitrary changes in the process of battery charge at any moment. Each battery cell has its own composition and its  charging curve/characteristics. The battery charger should be suitable to fulfill such battery specific requirements.

This thesis focuses on the design of a DC- DC converter for the low voltage battery charger. The proposed design uses fly- back converter circuit because it requires fewer components and at lower power levels, the total component cost is less than the other techniques. The used control circuit is simpler and cheaper than other control mechanisms, which require many components. The charger was simulated using PSIM and MULTISIM software. Experimental results are closely matching the simulation results indicating that proposed DC-DC battery charger has high efficiency.

The operation principle and the design-oriented analysis of the proposed converter have been presented through obtaining the power loss analysis. The experimental results with a hardware prototype are also included to show that the proposed DC-DC battery charger has a high efficiency greater than 82%.

The self-oscillating flyback converter is a popular circuit for cost-sensitive applications due to its simplicity and low component count. A self-oscillating fly-back converter for mobile batteries charging applications was presented in this thesis. The converter is designed to step-down the high voltage input to an acceptable level from 220 VAC to 7VDC. By adding inductor and capacitor, the power factor and input currant increased. The current became 500mA, where the old was 250mA. Diode was connected between emitter and collector, so that output voltage was improved, and has less ripple, in addition to the soft switching was obtained.  Simulation model is developed by using MULTISIM software. The operation principle of this converter is described using some operating waveforms based on simulation results and a detailed circuit operation analysis. Analytical results are verified by laboratory experiment. With an input of 220 VAC, 500mA and an output of 7V DC, the efficiency of circuit is about 75%.