This is the simple and easy way to tap current from a Motor Bike battery to charge the Mobile Phone. Most of the Mobile Phone battery pack has three 1.2 volt cells making the voltage to 3.6 volts. For fast charging of the battery pack, each cell requires more than 1.2 volts and sufficient current. This circuit can deliver 5 volts at 300 mA to charge the battery quickly. The circuit is Voltage and Current regulated and with Polarity and Surge protection components.
The circuit uses a Positive Voltage regulator IC 7805 to give 5 volt regulated output. The components around IC1 are meant for protecting the IC so as to give smooth output power. Zener diode ZD act as the Input Surge protecting diode. It provides 7.2 volts to the input of the regulator IC from the 12 volt bike battery. Diode D1 is the Output Surge and Input Short protecting diode.
When a surge voltage exceeding maximum voltage rating of the regulator is applied to the input or when a voltage in excess of the input voltage is applied to the output, the regulator will be destroyed. If the input terminal shorts with the ground, the output voltage increases above the input voltage(ground potential)and the charge in the capacitor connected to the output flows into the input side which is also fatal to the regulator. Both these situations can be avoided by using the Zener at the input and the diode D1 across the regulator. Capacitor C1 and C2 provide stability to the regulator and these should be soldered close to the legs of the regulator. Capacitor C3 act as a buffer to give constant voltage in the output.
7805 IC can tolerate maximum 35 volts and its current rating is 1 Amps maximum. Resistor R1 restricts the charging current to around 330 mA as per the Ohms law. Even if the current is low, charging process will not be affected. Slow charging with 80 to 100 mA current is generally advised. But in case of an emergency, quick charging can be done with high current
Assemble the circuit on a Perfboard and enclose in a small case that can be fitted near the Bike battery. Use suitable pins to connect with the Mobile phone. Charging current can be tapped from the battery using Alligator Clips. Before using the circuit, double check the connections especially the polarity of connectors and measure output voltage and current using a Digital Multi Meter. The same circuit can be used for charging Mobile battery from 12 volt Car battery or from a 12 volt Solar panel.