Hello! How long can I use my 12V/120W cigarette-plug power inverter to charge my laptop without having to turn on the engine?
Unfortunately, the answer is more complex than what I'm going to tell you. But here is the short version. The amount of time it would take for your battery voltage to drop below 12V would be the maximum safe time you could use your power inverter to power your laptop without starting your engine.
With a new battery, I'd say about at least an hour. To find out for sure just exactly how long, install a voltmeter. It will help determine how long the safe duration is. While I would like to tell you that a 12V 70A-hour battery will allow you to do what you have in mind for X hours before you need to start the car, I can't. Battery type, age, condition and use (or abuse) all affect how long you can safely do so.
Do car problems keep you awake at night? Send questions to email@example.com.