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Hi all. I have a question for you about power inverters for my car. I am going to be driving across the country soon, and I will be taking my computer and a mic with me to do some casual recording along the way.

I have a power inverter which is rated for 140 watt maximum continuous operation that will plug into the car's cigarette lighter. My car's cigarette lighter has a warning on this that says not to exceed 120 watts @ 12 v, so this raised some red flags for me. Bear in mind, my laptop's power supply only draws 90W.

I asked the toyota dealership about this and they said the fuse on it is rated for 15A, so it won't blow as long as I keep everything running lower than that. Problem is I ain't that smart with electronics/wizardry, and don't have the confidence to make a judgement call as to whether I'm in the clear or not.

My question for the smart people on the message board is, do I have anything to worry about? Here are the specs for my power inverter. http://www.12voltgifts.com/pocket-inver ... 1-0178.htm

I know nothing about electricity. So with that disclaimer out of the way, my understanding is if your laptop is only drawing 90 W, than you're well within the safe range, regardless of what the inverter is capable of putting out. I've driven many hours with my laptop plugged in with no adverse side effects. Although I believe my laptop only draws 60w from memory.

I'm sure somebody who actually knows about this stuff will chime in soon.

I know less than nothing about electricity, but when in doubt I start with the basics: Power = Current x Voltage.

120W @ 12V is 10 amps. When you say your laptop needs 90W, is that at 12V as well? Either way I don't see a problem with pluging your laptop into your car lighter. I do it all the time.

Matt pretty much got you there. P=EI, so P=12*15, P=180W, that is the maximum capability of your car's outlet based on your fuse, assuming at least 14AWG wiring from the fuse panel to the cigarette lighter. Do not upsize that fuse ever, you would cause the wiring to overheat and melt causing major issues in your harness.
FYI your laptop requires 90W at 120V which means it draws 3/4 amps. Disregarding inefficiencies in the inverter your laptop would be using half of the available headroom of your outlet.
Hint, a laptop power supply will draw more current charging an empty battery as well as running it then it would simply trickle charging a full battery while operating it, that being said keep your battery fully charged by taking every opportunity to plug it into a motel room outlet and you'l have enough power left to make toaster strudels while driving .

Cool, thanks everybody! This mostly confirms what I thought. Sounds like I should be good to go.

My girlfriend once asked me why her curling iron didn't work in her van if that makes anyone feel any better. To her, a plug is a plug and so long as it it fits she can't understand why it won't work.

Mathieu Benoit wrote:............. and so long as it it fits she can't understand why it won't work.

Pretty much my thoughts on buggery.

Alain Benoit wrote:FYI your laptop requires 90W at 120V which means it draws 3/4 amps. Disregarding inefficiencies in the inverter your laptop would be using half of the available headroom of your outlet.

Shouldn't the current being drawn be from the cigarette lighter be considered from the dc side of the circuit? Ie: 90W/12V = 7.5 amp ?

You were just testing me right?

In any case, I feel safe moving forward with the assumption that the inverter will only deliver what's being demanded from the ac outlet. So long as I keep the resultant dc current below 15A, I'm making toast.

Yup, your laptop draws .75A at 120V from your inverter, your inverter draws 7.5A at 12V from your lighter socket. In both cases it's 90 watts. The factors are inversely proportionate, when one goes up the other must come down for the product to be the same.

A.

Mathieu Benoit wrote:My girlfriend once asked me why her curling iron didn't work in her van if that makes anyone feel any better. To her, a plug is a plug and so long as it it fits she can't understand why it won't work.

Some may say you're a lucky man...