Questions for you smart people who understand power and plugs and stuff


NeuroTypical
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No, I don't think so. You would have to pull more than 250 watts (to cover conversion losses) from your car battery. At 12 V DC, that means you would be pulling well over 250/12 > 20 amps through your cigarette lighter circuit. I doubt the car's wiring for a cigarette lighter (I think it's called an auxiliary power outlet these days) is a 30A circuit. If it is, you could probably put an inverter on it and power a 250 watt gizmo. Just make sure your inverter is rated to at least 250 watts.

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When I have bought inverters, they are usually rated for a continuous watt rating with a separate peak watt rating. In the end, as Vort says, one key is to get an inverter rated high enough. In my inverters' installation instructions, there is usually something that says, "inverters above ___ watt rating must be wired direct to the battery and not wired through a cigarette lighter or aux power" for reasons that @Vort covered.

You haven't specified the kind of gizmo you want to run. DC-AC power inverters tend to create a square wave power pattern rather than the typical sine wave pattern. Most gizmos will run just fine on the inverter's square wave, but a few gizmos get grumpy with a square wave. Most commonly, I run an old laptop off of my inverters and it runs just fine. I've run power tools (up to the point of overloading the inverter, at least) and small stereos, too. Simple, relatively inexpensive things, I wouldn't worry about. If my gizmo is expensive and ruining the gizmo was going to create significant grief in my life, I would carefully check the AC wave type of the inverter and try to find out if the gizmo is going to accept a square wave.

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A little more information:

Your cig lighter is on a 10-, 15-, 20-, or (possibly) 30-amp circuit. You can tell by looking at the fuse corresponding to that cig lighter. It will say right on the fuse top what the amp rating is.

fuse-of-a-car.jpg.webp

For every ampere pulled from your car's 12-volt battery, the circuit will burn 12 watts of power. So a 10-amp fuse will allow power through at a rate of 10x12=120 watts. Any more than this, and the fuse is likely to burn out. In the same way, a 15-amp fuse will allow 15x12=180 watts of power, a 20-amp fuse will allow 240 watts, and a 30-amp fuse will allow 360 watts.

Since your gizmo pulls 250 watts, a 30-amp circuit is the only one guaranteed to be beefy enough to support it. But you might think the 20-amp circuit, which is alllllllmost big enough, should probably support it, too. And you might be right. 250 watts is likely the maximum the gizmo pulls, and that may just be a transient high wattage. Your fuse probably allows a little more than 20 amps, too. But there's more to it than this. Your car supplies unidirectional "direct" current (DC), but your gizmo requires back-and-forth "alternating" current (AC). You can perform this little piece of magic using a so-called inverter, which transforms DC to AC.

The problem is that the transformation is not 100% efficient. You always have losses with an inverter. These losses are on the order of 10% or more. That means a 250-watt gizmo would actually be pulling close to 280 watts (or more) from your battery, which means more than 23 amps of current. (280 watts/12 volts = 23.33 amps) That's almost certainly more than a 20-amp fuse will support for more than a fraction of a second.

So that's why I said that it probably wouldn't work unless your power outlet was on a 30-amp circuit. I think most power outlets/cig lighters are on 10-amp or 15-amp circuits. A 20-amp circuit is possible, but dicey. You can give it a try, but be prepared to replace a burnt-out 20-amp fuse.

Edited by Vort
Correcting some terminology errors. I am not an electrical engineer, nor do I play one on the interwebs.
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Two additional thoughts:

1. There also exist 25-amp circuits in a car, as you can see in the photograph of car fuses, which include 25-amp fuses. 25 amps will supply 300 watts of power, which is probably perfectly sufficient for your needs, even with losses.

2. Your car battery is perfectly capable of providing well over 1000 watts of power, so if you run your transformer directly off the car battery, your gizmo should work juuuuuust fine.

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Thanks for the additional info - I will try to locate my car's fuse box.

27 minutes ago, Vort said:

250 watts is likely the maximum the gizmo pulls

Actually, I did this right.  I have a Kill-o-watt, and plugged my gizmo into the house, and recorded it's actual use, rounding up slightly.  Thing only has one setting, so it's basically always pulling slightly below 130 volts, 2.2 amps, and 250 watts.

22 hours ago, MrShorty said:

I would carefully check the AC wave type of the inverter and try to find out if the gizmo is going to accept a square wave.

You sure you're not just making up geek talk to make me look dumb? :)  I've been in a boat pal, and waves ain't square.  But assuming you're right, my abilities are maxxed out by searching for stuff on Amazon.  And clicking a few options doesn't give me anything with the word "square" in it.  Can you make a suggestion?

 

 

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3 minutes ago, NeuroTypical said:

You sure you're not just making up geek talk to make me look dumb? :)  I've been in a boat pal, and waves ain't square.  But assuming you're right, my abilities are maxxed out by searching for stuff on Amazon.  And clicking a few options doesn't give me anything with the word "square" in it.  Can you make a suggestion?

Googling "auto sine wave transformer" will give you some (pricey) options for inverters that give you something better than square waves (which, as you point out, don't exist in nature, so they're baaaad).

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One thing I notice that hasn't been mentioned yet is the "amp hour" rating of the battery -- Basically, how long can the battery go before it discharges too much to continue. If memory serves, most car batteries are about 70 amp-hours -- meaning that it can sustain a 1 amp draw for 70 hours, or a 7 amp draw for 10 hours or a 70 amp draw for 1 hour (in a perfect world). As Vort explains, your gizmo is going to draw 20ish amps, giving you about 3 hours before the battery is discharged. Of course, if this is the same battery you are relying on to start the car so you can go home, you would be kind of stuck. Consider how long you need to run your gizmo and whether your car battery will run gizmo that long and whether you need to also be able to start the car and maybe you consider just letting the car idle while gizmo is running or installing a second battery or...(hello run on sentence!).

At some point, one start to wonder if it would be as good or better to buy a small gas generator.

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My gizmo is a home oxygen concentrator - a thing that lets people on supplemental oxygen breathe without needing to constantly replace tanks.   The person needing one does a lot of driving, and goes through a lot of tanks.   A portable concentrator during the drive (and maybe using the battery for short emergency use, but mostly for use while the engine is running), would cut down on tank use. 

It's kind of important I get/give good advice to this person here, because wouldn't it stink if I proposed something that ended up blowing some fuse or breaking the gizmo and then person is having an oxygen emergency on the side of the road somewhere. 

Edited by NeuroTypical
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57 minutes ago, NeuroTypical said:

You sure you're not just making up geek talk to make me look dumb? :)  I've been in a boat pal, and waves ain't square.  But assuming you're right, my abilities are maxxed out by searching for stuff on Amazon.  And clicking a few options doesn't give me anything with the word "square" in it.  Can you make a suggestion?

The term to look for in the inverter is not Square Wave...   Its Pure Sine Wave vs Modified Sine Wave...  Pure is the one that shouldn't give you any problems (and the most expensive)... a modified one might

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57 minutes ago, Emmanuel Goldstein said:

But, do you already know how to play an electric guitar flame thrower?

No, but I once practiced the ukulele enough to allow me and daughter to do one of these at the church talent night:

As for the oxygen concentrator, if it's going to be a long-term need thing for this person, there'll be a portable battery powered one coming.

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