Once known as Hartley18
Join Date: Aug 2007
Location: Melbourne, Australia
Thanked 85 Times in 85 Posts
Rep Power: 11
Gary, if it's discharging into your 12V system, how long the laptop will run for will depend entirely on the state of charge of both sets of batteries.
It's extremely unusual for this to happen (the power supply should have reverse polarity protection diodes in it which would stop this), but from the information you've given, to my finite mind, it can't be anything else..
A bad day on a boat beats a good day in the office