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.