Hallo Sachin,

again: Don't toy arround with an unsuited charging circuit! Serious consequences can result...

By the way, if you try to draw any current out of your 6V battery, using the circuit presented in one of your last replies in combination with this Darlington stuff, your battery will not be able to deliver even one electron. You must totally redesign your circuit!

As you need much morer current than mentioned earlier, you could use traditional recitifier circuit plus storage capacitor to produce enough voltage to charge a 12V battery by the help of LM317, as I recommended in my last reply. At output side arrange a big storage capacitor also, fed by the battery voltage and by the recitifier circuit via two diodes. To this second storage capacitor you could connect a switcher, which steps-down this voltage to 5V.

Using a switcher seems to be the only suited way to handle currents of more than 3A. Nevertheless, you should redesign your circuit, if really 3.5A is to be delivered by your battery. Reducing supply current to absolute minimum is highly recommended.

Kai