"I have been told you can run the laptop
fine at 12 volts but it wont charge the battery
. Thats fine normally on a boats nav station laptop"
That has been my experience. However - some laptops will run off the 12V independent of battery
state, and some will try to charge the battery whenever it sees it is low and overwhelm the system - now it wants upwards of 19V - shutting it down. There doesn't seem to be any way to determine how a computer will react until you buy one - it just isn't part of the specs. If the one you got needs the battery charged before it will work on 12V, I found that charging
it up with an inverter
and then running it off 12V and not letting the battery discharge will work.