Verified answer

Ignore the previous poster. The answer is yes, it will run down your battery, BUT it's not as bad as you think, because:

1. If your inverter is a cheap one from china or hong kong like mine is, it will automatically cut off once it reaches 10V (it's a built-in safety feature actually). At 10V a car battery is half-dead (it shortens the life of the battery), but will still have more than enough power to start the car. More expensive inverters will probably have the same cutoff but I've never used one so I can't be 100% sure :) The instruction book should tell you though

2. It takes a while to run down a car battery with a laptop. Unless you have a DC clamp meter, you won't be able to calculate exactly HOW long it would take, but here's how it is in my case (to give you an idea):

My laptop is a large and power hungry gaming laptop. Using my clamp meter, I measured 5 Amps flowing into the inverter when the laptop was playing a DVD (with the screen on normal brightness). Your laptop likely consumes less than this. Now, my car battery is a pretty small one (since my car has a 1.4 litre petrol engine) - it's got a capacity of 43Ah (that's Amp-hours). Remember what I said about car batteries being damaged if you discharge them too much? Well, a safe level of discharge is 20%. So in my case, I can discharge 20% of 43Ah, which equals (20/100)*43 = 8.6Ah

8.6 Ah of charge will power a 5 Amp load for... 8.6 / 5 = 1.72 hours (or 1 hour and 43 minutes). Your car battery is probably bigger than mine. So you can safely run the laptop for over 1 hour 45 minutes without damaging your battery at all.

3. You might have a voltmeter. In that case, measure the battery voltage periodically and power off when it falls to between 11.8 and 12V (your car battery would initially at about 12.6 to 13.3V before you power on the inverter if it's anything like my car battery)

4. Last but not least, you can always start the engine and let the alternator power the inverter (that gives you unlimited power for as long as you have fuel in the tank :).

So you could start by recharging the laptop's battery and using that until it runs out. Then take out the laptop battery (so that you don't waste electricity recharging it) and use the inverter for 2 hours. Then put the laptop battery back in the laptop, and start the car and leave it running until the laptop battery has reached 80% charge (because after this point the laptop battery starts recharging very slowly but the car engine is still burning fuel at the same rate, so you'll be wasting fuel for nothing after this point).

Power Inverter For Car Battery

so long as car is on the power inverter will not use any of the battery's juice...unless alternator issues...old/-i know yellow top optima gel cell battery's can take what your asking.

i'd 1st make sure alternator can take use...alternator test by turning car on, and disconecting pos side(carful not to touch metal(wrap in rag))car will continue to run without battery if good alt.-will fail if bad.

A: buy yellow top /keep old one in car in case...$$

B:buy jumper pack to jump start your car...$< this wont help if alternator bad...

C: setup laptop to warn of low battery at this point connect it to inverter

...IF you run power inverter with car off it will continue to suck battery dry...

Voltages? Are they the same? If not your going to ruin your lap top.Cars run on DC.