This is a good one for me as I have been in Automotive, Marine and  
Industrial repair for 45 years.
 
Battery voltage on an 12 volt battery varies anywhere from up to 14.0V down  
to nothing. Also connecting anything to DC current incorrectly could be a  
disaster. 
 
 If I were you, use an inverter connected to the battery and simply  plug 
your laptop power plug into that. Electric motors that run at high or too  low 
voltages will not survive. plugged into an inverter they will be  protected. 
 
 DC to AC inverters can be purchased for around $25.00 that will  handle any 
laptop.
 
Hope this helps,
                       Bud Jarman 

         PCBUILD maintains hundreds of useful files for download
                     visit our download web page at:
                  http://freepctech.com/downloads.shtml