In my experience, the laptops I've had are not really that sensitive to voltage differences on the supply. Having said that, 7V might prove to be too much. The chance of you doing damage with an undervoltage supply is smaller than overvoltage one but you need to be careful. Laptops are expensive kit to have repaired, particularly if you managed to release the magic smoke.
If you're in the UK
www.maplin.co.uk do a laptop adaptor for cars (part no. N59AC).
Having said all that, if the output of the battery (check the label) is less than 12v you might be in with a shout, if it's over 12v you'll certainly need an adaptor.
Hope this helps.