HI ! i have a audio speaker of about 100 watt and i want to connect it with my car battery for a portable audio system. I just want to know that is the voltage of battery is very high for it is there any need to reduced the voltage of battery. So, plz suggest me the best way to do so.

In technical terms, they are not supposed to work with your car. Car gives 12V DC output and your speaker must be for AC input, however they might work but may have side effect. Otherway is to remove speaker's AC to DC conversion mechanism and check what is the requirement of your speaker ie. input voltage it need, they you can will need a modulator kind of device to give it right amount supply through DC input of car.

Best way is to stick with system made for car itself.