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Power inverter for car?

Discussion in 'Power Electronics' started by tomadom, Dec 3, 2018.

  1. tomadom

    tomadom

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    Jan 18, 2013
    I'm looking for a power inverter for my car as I'll need something to charge a laptop and a few other things while travelling.

    I am looking at an inverter that converts 12v to 240v 300w. Will that be enough to charge a laptop and a phone together. Is there any benefit in getting a 1000 watt version of these inverters. They don't cost much more and if it's of any benefit getting them then I'll probably do that.

    Should I get the highest wattage possible?

    Thanks
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

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    No. Get an inverter that is rated for the load you want to support plus some headroom, say 20 % more.
    The higher the output power of an inverter the higher are its internal losses. If you use a high wattage inverter with little load, the efficiency decreases as proporionally more power is "lost" within the inverter without being put to use in the load.
     
  3. kellys_eye

    kellys_eye

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    You don't need a 'mains' power converter for this. There are many laptop PSU inverters that take the 12V from the cigar socket and 'ramp it up' to the required 17V-22V that laptops need. They come with a multiplicity of plugs to fit various manufacturers connectors.

    Similarly you can get direct 12V phone charging leads.

    Those mains inverters aren't sinewave - they have a modified squarewave (at best) and often a simple 'dirty' squarewave at worst which can ruin electronic chargers.
     
  4. tomadom

    tomadom

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    Jan 18, 2013
    It's not just laptops it's using things like electric jugs and maybe other emergency items.. I need to know is a higher wattage going to be better? If I don't know what I will be running in the future besides laptops and phones should I get one of higher wattage given that it doesn't cost much more?
     
  5. Harald Kapp

    Harald Kapp Moderator Moderator

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    We don't know either. There's always the chance that your inverter will just not put out enough juice for that last gadget.
    Given that the power ratings of el cheapo electronics generally is, let' say enthusiastic, you'll probably enjoy a higher rated inverter running at low load longer than an inverter whose rating barely matches the load.
     
  6. kellys_eye

    kellys_eye

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    Electric jug? Potentially 1000W so draining 83A (but more likely 100A after inefficiencies) from your car battery? Good luck with that! The risk of fire from poor connections is high....

    When it comes to using high power equipment it is far better to go for the simplest solution involving propane gas canisters or a can of petrol running a small generator.

    When you go over a certain power level (with inverters) they often use higher input voltages too such as 24V, 48V etc that make using car batteries complicated if recharging from a car alternator.

    Seek to address your needs/usage - try to find low power version of the items you need to use before jumping in and looking to power 'what you already have'. It will work out cheaper in the end.

    On a WTSHTF basis we did this for our home and I can run ours on 700watts MAXIMUM - most of the time we could get away with 200W - and that includes TV, internet, lighting etc. But we're 'lucky' that all our heating requirements are met by wood burning .... and we live in a forest!
     
    hevans1944 likes this.
  7. Bluejets

    Bluejets

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    Get a generator.
     
  8. dave9

    dave9

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    If the 1000W version doesn't cost much more (depending on your definition of much) then I would wonder if you are just bottom basement price shopping and a quality unit would cost more than those minimal prices, that you are just observing the price difference after the majority of the cost is (anything but) MOB cost, that this is just floor pricing for the size of something shipping from China. It's difficult for me to more concisely describe that, where they can improve specs a little yet the real world benefit is only what you paid for.

    In this case I would buy a larger capacity than your originally stated need, and suffer some inefficiency when that is the case. I would not bother with anything that plugs into a lighter outlet, instead get something wired direct to the battery with an inline fuse.

    As others have mentioned "electric jug" is a significant variable. We'd need to know amps at 110V or 12VDC or whatever it's rated for.
     
    Last edited: Dec 4, 2018 at 7:18 AM
  9. tomadom

    tomadom

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    Jan 18, 2013

    Thanks for the detailed reply. So can you tell me why something which plugs into the cigarette lighter socket isn't worth buying ? I'm looking for something to charge and power things while I am driving and needed this feature for convenience. Thanks
     
  10. kellys_eye

    kellys_eye

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    The cigarette lighter socket will be fused at 10A, possibly 20A maximum therefore limiting the power to 120/240 watts (less in reality and certainly less for safety sake).

    Power inverters really should be wired directly to the battery and have uprated wiring and appropriate fusing at the battery.
     
  11. Hopup

    Hopup

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    If its for emergency use, then generator is obvious choice. Your car charger might have problems supplying enough electricity for continuous usage if the inverter is +500w and as said already you have to connect it directly to your car battery for optimum performance.

    Cigarette lighters have very thin wires in my experience, can't really handle any real current.
     
  12. dave9

    dave9

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    Most lighter outlets are fused at no more than 15A, upstream and depending on the vehicle, that 15A may supply more than just the outlet. You could run lower gauge wires to the outlet direct from the battery but the inverter may have its own 15A fuse, and the outlet itself really isn't good for much current as it's barely friction fit, not latching, and that's a new/unused outlet. If the outlet has been used for the cigarette lighter then heat and tobacco tar may have fouled it.

    Don't get me wrong, if the outlet is in good shape it should be capable of charging a laptop and lesser electronics, but anything with a significant size motor or that produces more than small amount of heat, needs a dedicated circuit added direct to the battery with fairly low gauge wire.

    Depending on use you'd also want to be sure your alternator can handle it. For example it might be a 130A alternator but that is "probably" peak output at several thousand RPM. When the engine is idling it will be producing less.

    Below is the test report from an alternator I bought a few years ago. When the vehicle I put it in is idling, it has an RPM around ~1600 so then can only produce around 75A instead of 130A.

    [​IMG]
     
    hevans1944 likes this.
  13. hevans1944

    hevans1944 Hop - AC8NS

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    We aren't saying that at all. Cell phone chargers, producing an ampere or two at five volts, are worth buying to keep your cell phone charged while it is being used as a GPS navigator on long trips. There are other low-current applications such a portable spot-lights, tire inflation compressors, and similar things that are useful and generally plug into a cigarette lighter for intermittent use. But, you need to realize two things: (1) the current or amperage capability of the alternator (used to keep the car battery charged and provide current to the cigarette lighter socket... plus anything else that draws electrical power) is quite limited compared to what is available from your electrical utility and (2) the current required to temporarily heat the element in a cigarette lighter to red-hot incandescence typically ranges from only about ten amperes to a maximum of twenty amperes and lasts for less than a minute, so compromises in wire gauge for intermittent versus continuous duty use can be, and often are, made by the car manufacturer.

    That latter limitation works out to a maximum power available from the lighter socket of 120 watts to 240 watts on an intermittent basis only. Continuous operation at the level of maximum power will likely cause the wiring to overheat, with the possibility of a fire occurring. Now consider the current rating of a typical automobile alternator is only about 90 amperes, and that only at faster idle speeds. When this available current is distributed among all the electrical loads in the car... heater fan motor, automobile lights, ignition system, air-conditioner compressor solenoid clutch, plus whatever audio/visual stuff is also there, you don't have a lot of amperes left to run anything else, much less a 1000 watt inverter. A typical alternator delivering 12 volts at 90 amperes is already maxed out at 1080 watts! Then consider (if nothing else is an electrical load on the alternator) how you are going to deliver 90 amperes or more to your 1000 watt inverter. This will require at least 4 AWG or larger copper wire. Hence the suggestion that a separate, heavy-gauge, cable be connected between the alternator (or battery terminal) and the inverter.

    In practice, an automobile alternator does not make for a very attractive alternative power source. As cars became larger and more power-hungry near the end of the previous century, alternators were up-sized to accommodate the additional load, but they never did compete with purpose-built motor-generator alternators driven from a dedicated internal combustion engine. Note those engines are generally rated at less than ten horsepower, which at 100% efficiency would yield about 7500 watts of electrical power. Larger MG sets are of course available, and you could arrange to tow one behind your automobile on a trailer, running the MG set from an external propane or gasoline tank while tooling down the road... but you might want to check local laws and regulations regarding that. You could also investigate installing another, larger, alternator or even two or more alternators on your automobile engine... maybe have to cut a hole in the hood to make room for the additional belts and pulleys though. And such a modification would seriously detract from the gas mileage and perhaps the re-sell or trade-in value, plus it would decrease the amount of power available to the wheels for acceleration, perhaps significantly.

    Lacking the availability of a Mr. Fusion personal-sized nuclear reactor that will run on garbage and stale beer, while producing 120 VAC at 200 A, I think you are SOL on your project for now. So solly! Mebbe next year!
     
    Harald Kapp likes this.
  14. tomadom

    tomadom

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    Jan 18, 2013
    Thanks, I hear what you're saying now! I'm still making a decision on this so thanks for all the feedback.
    I just knew it wasn't as simple as I originally thought. I'm not thinking of running a line to my battery as an alternative to my cigarette lighter socket.
     
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