I need a circuit that can convert a 12V 12 Amp/hour battery voltage to 6V 4.5Amp/hour . Or how i can obtain 6V 4.5A from 12V 12 Amp/hour battery

 

Try MC34167/33167 from OnSemi.com

Datasheet: Fig 19 shows a 12 volt to 5 volt 5 amp design. Add R1 to move up to 6 volts.
Vin 10 to 36 volts (will pull under 3 amps from battery)
Vout 5 (or 6 volts) @ 5 amps

 

Your goal is too modest. You are asking how to convert something that can deliver 144 watt-hours to something that only delivers 27 watt-hours, so you are asking for something very inefficient. This can easily be done with a series voltage regulator rated at 5 amps and with a big heatsink. Consider the LM338K mounted on a heatsink that can deliver less than 1 degree C per watt of thermal convection.

A better goal might be to ask for 6V but with 20 A-H capacity.

Or, even easier, you can buy one of these:
http://www.powerstream.com/dc-3318.htm

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RadioRon