800 watt dc to ac inverter

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This is the specs of the inverter:

Voltage In - 12VDC
Voltage Out - 115VAC
Continous POwer - 800W
Peak - 1600W
Out Frequency - 60hz
Waveform - Modified Sine Wave
No Load current - .75A

Low Battery alarm and auto shutdown
Overload and short circuit protection
Input polarity reverse protection by fuse

So my question is even if my car is off and the inverters power switch is set to "on" and I don't have anything plugged in. I will still drain my battery? If so is there something I can do so I wouldn't have to turn the inverter on by hand, but instead it only turns on when I turn my car on?
 

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  • #2
berkeman
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Welcome to PF. I have to ask, what in the world are you trying to power from your car that takes 800W? That's probably about a 10A continuous load on your car electrical system after switching efficiency is factored in. Can you just use a stand-alone gas-powered generator instead?
 
  • #3
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That's probably about a 10A continuous load on your car electrical system after switching efficiency is factored in.
To start you might as well been speaking french right there :smile: . All I want to do is power a computer and a couple of small things. I'm pretty new at working with inverters or any electrical thing in the car. The inverter was on sell for a good price so I bought it.
 
  • #4
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It may drain your battery, depending on how long you plan on running your computer. Even at 100 watts, it will draw about 7.5 amps plus whatever is lost in heat in the conversion process. That may seem like a small amount, but regular auto batteries do not like a prolonged, low current discharge.

If you plan on running it a long time between charging periods, you may consider a deep cycle marine battery in the trunk. They are designed to source a low current for a long time, such as running a bilge pump.
 
  • #5
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So say I went on an hour trip and had the computer running. It will drain the power a significant amount? I was thinking maybe using another battery. I'm going to look into a deep cycle marine one.
 
  • #6
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If you are on a road trip and your car is running, it should not pose a problem because the alternator in your car will keep the battery charged. I don't know about your car, but most vehicles have alternators that can source at least a 60amp charge. If you vehicle is off, and you had about 100watts running on your inverter and a 60ah battery, itll be dead in about 8 hours. I would even give it less than that before your battery doesnt have enough current to start your car anymore. You would be fine for a couple hours, but as mentioned above, look for a deep cycle battery if you are planning on using it a while.
 
  • #7
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Okay great! I shouldnt be driving around no more then an hour or so with the pc on. I had another question though. Am I going to have to turn the inverter on when I get in the car and off when I get out? Is there a way around that if so?
 
  • #8
berkeman
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Swingkid said:
Okay great! I shouldnt be driving around no more then an hour or so with the pc on. I had another question though. Am I going to have to turn the inverter on when I get in the car and off when I get out? Is there a way around that if so?
Find a 12V line in your fuse box that is only on when the motor is running (or the key is turned on). Use that to power a small relay that connects the battery 12V (through a suitable fuse of course) to the inverter. You probably want to use a smaller fuse than the inverter has on its input, since you won't be running it at full power.
 
  • #9
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Okay so I'm not to sure how to do what you just said. Let me add a little more info though. So I was going to run a 0awg from the battery to the trunk into a distro. I need to power my subs amp and the inverter off of it. Knowing this can someone get a clearer picture in my head or maybe a little drawing>
 
  • #10
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Swingkid said:
Okay so I'm not to sure how to do what you just said. Let me add a little more info though. So I was going to run a 0awg from the battery to the trunk into a distro. I need to power my subs amp and the inverter off of it. Knowing this can someone get a clearer picture in my head or maybe a little drawing>
berkeman is suggesting to put a relay in series with the power lead of the inverter. A relay is a switch that can trigger on some sort of voltage source, in this case, the ignition signal. This means since the relay is in series with the inverter's +12v lead and normally open (off). When you turn on the ignition, it will close the relay switch which will power up you inverter. It's always good practise to put a diode across the relay coil to prevent any flyback from possibly ruining your car's electronics.

To find an ignition triggered wire, go to your fusebox with a testlight and poke around ontop of the fuses. If the testlight turns on and off when you cycle the ignition, that fuse is on an ignition triggered circuit. Splice into the wire coming off of that circuit. Personally I would put it on an accessory wire because it turns off when the car is cranking, and you can also turn the key into accessory mode with the car off to run the computer.
 

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  • #11
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Okay great things are coming together now. I'm looking around to find where to buy a relay.http://www.robotmarketplace.com/store.html" [Broken] Will this do just fine? Thanks again everyone you've been quite helpful.
 
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  • #12
Mech_Engineer
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berkeman said:
That's probably about a 10A continuous load on your car electrical system after switching efficiency is factored in.
To generate 800W of power on a 12V electrical system, you need to draw AT LEAST 66.6 amps, probably closer to 70 depending on various efficiencies. Most cars don't have a stock alternator that can keep up with that kind of current draw when taking into account there is current draw from the engine management system, and maybe a radio. My guess is the vehicle's stock alternator is rated at about 60 amps total, but subtract about 20 amps for the ignition system, 10 for the radio, and another 10 for random other stuff (headlights, for example). That only gives you about 20 amps (240 watts) of extra current capability, or you will drain your battery while you drive.

If you're planning on running one laptop, you should be ok because it will only use perhaps 200 watts, but if you're talking about anything bigger, you could run into problems. Also, your alternator will only run at full capacity with your engine runnning above about 1500rpms, so if you're idling at a light, your battery will be draining to make up for the lack of alternator power...

Just some things to consider. Inverters are a common modification to 4x4's for camping, but really BIG inverters draw BIG current.
 
  • #13
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Mech_Engineer said:
To generate 800W of power on a 12V electrical system, you need to draw AT LEAST 66.6 amps, probably closer to 70 depending on various efficiencies. Most cars don't have a stock alternator that can keep up with that kind of current draw when taking into account there is current draw from the engine management system, and maybe a radio. My guess is the vehicle's stock alternator is rated at about 60 amps total, but subtract about 20 amps for the ignition system, 10 for the radio, and another 10 for random other stuff (headlights, for example). That only gives you about 20 amps (240 watts) of extra current capability, or you will drain your battery while you drive.

If you're planning on running one laptop, you should be ok because it will only use perhaps 200 watts, but if you're talking about anything bigger, you could run into problems. Also, your alternator will only run at full capacity with your engine runnning above about 1500rpms, so if you're idling at a light, your battery will be draining to make up for the lack of alternator power...

Just some things to consider. Inverters are a common modification to 4x4's for camping, but really BIG inverters draw BIG current.
That's really pushing it. I would think a car would draw about 10 amps total when it is running, not including headlights. I am going to put an ammeter on it when I get home just because you made me curious. At the worst, 20amps of free current will drive about 3 Laptops (my t42 thinkpad draws 72 watts at full load).
 
  • #14
russ_watters
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I use http://www.staples.com/webapp/wcs/s...051&langId=-1&productId=100284&cmArea=SEARCH" 90w inverter to power a high-end laptop with a fast hard drive, big screen and power-hungry USB peripheral (ccd camera) for astrophotography. Since it is plugged into a cigarette lighter, it is only on when the car accessories are on. I can typically run it for an hour on the car battery before the voltage drops enough to need to turn on the car and recharge the battery for a few minutes (it beeps at you).

If you start plugging too much into the one you bought, your main worry is that you're going to start blowing fuses.

And I wouldn't bother with the relay - if you're wired into the car's electrical system, you don't need to turn it off when you get out of the car. Just make sure you keep using the right fuses.
 
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  • #15
chroot
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Mech_Engineer said:
To generate 800W of power on a 12V electrical system, you need to draw AT LEAST 66.6 amps, probably closer to 70 depending on various efficiencies.
I believe berkeman just made a typo, and meant 100A, which is probably reasonable. Reasonably inexpensive power electronics are not all that efficient, perhaps 80% at best.

My guess is the vehicle's stock alternator is rated at about 60 amps total, but subtract about 20 amps for the ignition system, 10 for the radio, and another 10 for random other stuff (headlights, for example). That only gives you about 20 amps (240 watts) of extra current capability, or you will drain your battery while you drive.
That's pretty much correct. Some larger vehicles have 100A alternators, but most are in the 50-70A range. People who install very large stereo systems in their cars often have to either replace their alternator with a larger one, or sometimes even run two in parallel.

If you're planning on running one laptop, you should be ok because it will only use perhaps 200 watts, but if you're talking about anything bigger, you could run into problems.
Modern laptops use far less than this -- if they really consumed 200W, they'd be, ah, rather uncomfortable to have on your lap.

- Warren
 
  • #16
berkeman
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chroot said:
I believe berkeman just made a typo, and meant 100A, which is probably reasonable. Reasonably inexpensive power electronics are not all that efficient, perhaps 80% at best.
Yeah, thanks for the correction Warren. I had a bit of a brain fade. I had 10A of output current at 115V in my head, and didn't do the conversion back down to the 12V input current. :rolleyes:
 
  • #17
chroot
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Wow, what an honest man! That could easily have just been a typo, but you admitted to the brain fade anyway. :biggrin:

- Warren
 
  • #18
NoTime
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chroot said:
Wow, what an honest man! That could easily have just been a typo, but you admitted to the brain fade anyway. :biggrin:

- Warren
:rofl: Where did I put that lamp.

If all that is wanted is to run a laptop, why not just get the car adaptor for it.
It is sometimes nice to have AC in the car though.
 
  • #19
chroot
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Actually... in most cars, the cigarette lighter is only energized when the ignition is on, anyway.

- Warren
 
  • #20
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Well I'm going nto run a SFF Computer. I think it has a 220W power supply.
 
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