Inverter question

Discussion in 'Off Grid Living' started by Joseph Thomas, Nov 15, 2010.


  1. Joseph Thomas

    Joseph Thomas Monkey+

    Sorry if this has been covered before. I searched older post but didn't find it. My question, Can an old car or truck or tractor with a generator versus an alternator run and charge batteries while the inverter is being used?
     
  2. melbo

    melbo Hunter Gatherer Administrator Founding Member

    I'm not sure. TN Andy or Nadja would know.
     
  3. Nadja

    Nadja RIP 3-11-2013 Forum Leader

    Now that is an interesting question. Since I don't have any personal experience on this, I will have to ask an engen. type of friend. Are you talking about one of those small invertors like you get from truck stops or sporting goods stores or a reg hard wired house unit ? In other words, is it designed to run or plug into a cig. lighter socket or to be HARD WIRED to a 12 volt battery bank ?
     
  4. Brokor

    Brokor Live Free or Cry Moderator Site Supporter+++ Founding Member

    I am no expert, but this information may prove useful. It all depends on the amount of draw, so that what is left over may be used to fill (recharge) the battery. What size inverter are you intending to use?

    Power (P), in Watts, is equal to the Voltage (V), in Volts, multiplied by the Current (I), in Amperes.

    P=V*I

    Using Algebra to solve for the Current (Amps) that are required:

    I=P/V

    Therefore: 2000 Watts / 12 Volts = ~167 Amps

    Please note, that this is for a 100% efficient inverter. If the inverter is 90% efficient the actual draw would be:

    I<sub>ideal</sub>*(1+(1-(%Efficiency/100)))=I<sub>actual</sub>
    167 Amps*(1+(1-(90/100)))=I<sub>actual</sub>
    167 A * (1+(1-0.9))=I<sub>actual</sub>
    167 A * (1+(0.1))=I<sub>actual</sub>
    167 A * (1.1)=I<sub>actual</sub>
    183 A = I<sub>actual</sub>

    The continuous current draw for a 90% efficient power inverter, operating from a 12V DC source, out putting 2000W at 120V AC, is 183 Amps.
     
  5. ghrit

    ghrit Ambulatory anachronism Administrator Founding Member

    If I'm reading it right, the answer to the question is yes. As long as the combined load of battery charging and draw on the inverter (a/c) side doesn't overload the generator, all is well. Power (amperes) out of the generator doesn't care where its used.

    In effect, the battery is a storage tank that can be filled and drawn from simultaneously. Back in the day, generator circuitry was not designed for output to inverters only, they had to be lined up to a battery to soften the changes in draw. So if you are going to use a generator, put the inverter on the battery terminals, not directly to the generator output.
     
  6. Nadja

    Nadja RIP 3-11-2013 Forum Leader

    Thanks Ghrit. I don't do much with cars except drive them. I could tell you on home use, but have never done the car /invertor thing, especially with the old generator type systems. Nadja
     
  7. Nadja

    Nadja RIP 3-11-2013 Forum Leader

    Elsur, I double checked this out with my engeenier guy today and he said you should have no problems. However, some of the older generator's would need to be up to a certain rpm before they would put out the correct currant. Ghritt is very right in this. Thanks Ghrit
     
  8. Joseph Thomas

    Joseph Thomas Monkey+

    Thanks for all the info and the mathematical equation. The inverter is a Cobra CPI 2550. 2500 watts continous and 5000 watts surge. Output current is 22 amps. Input voltage is 12.5 VDC +/- 20%. My application is to use it off my tractors batteries out in the field to power tools such as hammer drills, circular saws, etc and use it as a backup to my PTO generator for power to the house for the refrig and freezer. I do know on older vehicles the generator doesn't start putting out sufficient voltage at idle. You have to increase RPM's to 3 or 4 thousand or so before the voltage regulator opens the circuit to the battery. Then you can throttle back down. At least that's how I understand it from the manuals.
     
  9. ghrit

    ghrit Ambulatory anachronism Administrator Founding Member

    There's a good chance you'll want a larger battery in parallel with the on board the tractor for starting if you are using the tools heavily. As far as output goes, put a voltmeter on the generator output and run the rpm up until the output voltage gets to about 14 volts, and set the throttle to hold that rpm when you are running the tools.
     
  10. TnAndy

    TnAndy Senior Member Founding Member


    Better have it very close to the batteries and a fairly large cable ( and fusing ), because you're pulling close to 200 amps at 2500watts out.....and surge to 400 amps at 5,000w.


    You aren't going to be able to do that very long, either. An older generator simply doesn't put out enough to keep the batteries up with that kind of draw...you're probably looking at 30-50amp on the generator @ 12v....that's 600 watts....maybe a 1,000 on a really GOOD generator.

    You take 2,000w out, and you won't last more than a couple hours before you drain the batteries to the point the inverter shuts down.
     
  11. Nadja

    Nadja RIP 3-11-2013 Forum Leader

    Are you sure about those amps ? Most invertors in that size range would be lucky to pull out anything much over 25 amps. Heck, my prof. skill saws only pull 15 amps.
     
  12. BTPost

    BTPost Old Fart Snow Monkey Moderator

    Inverters of the size being discussed here (2Kw and above) really need to be powered by 24 Vdc rather than 12 Vdc. and if your going for 4Kw and above, you should be 48 Vdc. 2Kw at 120 Volts is only about 17 Amps, but at 12 Volts it is 170 Amps. for that kind of current, your looking at WELDING Cable, even on short runs. That kind of amp draw can kill a battery right quick. As far as the OP's question goes, it is simple "Goes In'ta" and "Goes Out'ta"... If the "Goes In'ta" is less than the "Goes Out'ta", the difference comes from the battery. You can do that for a while, but eventually the battery will go dead, if you don't have more "Goes In'ta", than "Goes Out'ta". The battery doesn't care where the "Goes In'ta" comes from. Generator, Alternator, Charger, Solar Panel, all the same but it is the AMPS that matters, as well as the Charging Losses in the battery itself.

    Bruce in alaska <BTPost>
     
  13. melbo

    melbo Hunter Gatherer Administrator Founding Member

    Is he talking about the amps draw from the batteries (upstream)? 2500w/12V = 208 amps

    The downstream side is what you saw pulls from. 15 amps x 120 V = 1800 watts.
     
  14. Joseph Thomas

    Joseph Thomas Monkey+

    Had to update this. I'll never use the inverter at it's rated amps/watts. It would only be for power tools and in an emergency for the refrigerator and deep freeze in Summer. I read the manual and followed their recommendations. Got #2 AWG cable for the connections with an inline fuse circuit that I had to order from W.W. Grainger (Along with plenty of spare fuses) . As some of you guys said the inverter needs to be very close to the battery so that the cable runs are short. I'm thinking of building a second battery tray to hold a second in parrallel battery as well as the inverter right on the side of the tractor. If I do that I think I'll be good to guy for what I would use it for. Thanks for all the input.
     
  15. TnAndy

    TnAndy Senior Member Founding Member

    I'm wondering how you can live off grid and not understand there are two sides to an inverter......
     
  16. Nadja

    Nadja RIP 3-11-2013 Forum Leader

    Its really quite simple TnAndy. I am NOT a techie , but a user friendly kinda guy. If it ain't broke, don't fix it. I don't know how many amps are going IN to the invertor, but can assure you I do know how many are coming out. You don't need to be an electrical genius to live off the grid, only a knowledge of how to hook up and maintain everything. I also can't wind a motor , but sure do know how to turn it on.
     
  17. BTPost

    BTPost Old Fart Snow Monkey Moderator

    There is a BIG difference between an Operator type, and a Techie type, when it comes to hardware. Most folks have NO real hard understanding the technology they use every day. I suspect that most folks have "No Clue" how their Cellphone works, or the technology required. All they care about is that it "DOES Work" when they have one. The same is true for most Technical things. How many people understand how the Grid works with all it's PowerPlants, Control Centers, Transmitting SUBs, Receiving SUBs, InterLinks, ect? Naw, they just care that the Wires that feed their outfit, has power on them. This is one reason we talk about these things here on the Monkey, so that those that choose to, can get the basic education on the technology that they want to use, when TSHTF. So they can maintain, and repair, what they have to use, to maintain the gear they have. So, give the Operator types a break, and just help the Techies, educate the Operators, as they ask for information.
     
  18. sdsharp61

    sdsharp61 Monkey++

  19. Nadja

    Nadja RIP 3-11-2013 Forum Leader

    Thanks Bruce. I do have a basic understanding of how it all works, and even more so, of finding problems when they do arrise , which they do. However, if my invertor quits putting out power, or doesn't accept my generator charge etc then I have a friend who can fix them and quick. I really don't find the time to learn the techie type of approach, and really don't care to, as I am an old f...t like you also. I can barely turn my cell phone on , much less understand it. Heck, I don't even know how my car runs anymore. But bet I could still fix an old one. Like something from the 40's or 50's. These electrical formulas that people put up are great if your a techie, but then if you are ,you probably already know them. Or at least should. I would like to see this site really try and stick to the basics of off grid living in all aspects, but basics and sound advise. I intend to learn a great deal here and people like you have been on solar etc even longer then me. So, lets educate the people living under assumptions, which are usually wrong anyway. Again, thanks for your imput and good luck in the coming winter, soon to be on you. Nadja
     
  20. melbo

    melbo Hunter Gatherer Administrator Founding Member

    Should this be moved to a different topic?
     
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