I need it to charge my laptop?
I guess the car battery is 12-14V but dont know amperes and power(wattage).
also please I dont want the output current to exceed 3.25 apmeres and not to exceed 20V.

thanx to all

Car batteries are rated for a very large amount of current, much more than 3.25 amps. If you hook it directly up to the car battery the voltage is not stable and will occasionally jump very high, in the hundreds of volts I understand for short periods of time. I expect putting a sufficiently large filter cap would alleviate this problem, but I'm sure there are better, and more complex ways to filter it. It will for the most part remain stable at running voltage, which I think is about 14v.

The output current is not really affected by the battery, the laptop will draw however much current it needs from the car battery so long as the voltage is in the acceptable range, usually from about 10-20v.

Did you check the prices of the 12VDC/120VAC inverters?
Maybe it's not worth the trouble (unless you prefer DIY).

Yes, depend from your location inverter is your best option.
Recently I purchased for $29 a 400Watt 12-120v inverter.
You can try to open it an modify for your needs !?
If you count the parts, heatsink, transformers, can't make it under $50 unless you have many parts on stock.

Steve :idea: