I have a dell laptop. Every week there will be two days where I might work from my car for about 2-3 hours each time. So I plug my laptop power cable into the car and then open up Microsoft word. That means I work from my car about 6 hours per week.
Is this bad for my car battery?
Watts divided by volts = amps
Laptops draw 70 to 100 watts. 100/12 = 8.3 amp. Check the reserve capacity rating for your battery. Most car batteries are optimized to quickly provide lots of amps for starting, but they're not designed for long periods of battery drawdown.
If the engine is running it is okay, but if not you are discharging the battery too deeply. StPaulGuy's numbers are right on the money. We use laptops for mobile data at work and our department standard is to put a one hour timer on the 12 volt source so the battery will only be discharged for one hour - and these are for large dual truck batteries. You should not go more than 1/2 hour on a car battery. It is even worse for the alternator, which gets mighty hot trying to recharge that much later.
Speaking of hot, our department recently changed from Lenovo laptops to Dell laptops. We ran into a snag because a few of the Dells have burned up - one actually caught fire - running on 12 volt supplies so they are no longer allowed to be powered off the 12 volt source. Just a word of warning.
Be careful here.
A lot of machines do not run off of 12 volts.
They require 16 or more.
Check your computer/AC adapter voltage.
If you are using a cheap or homebrew cigarette plug
Then you probably WILL let the smoke out of the
Computer.