Could someone explain how a isolated dc to dc converter works?

Discussion in 'General Electronics Chat' started by Twisted, Aug 8, 2011.

  1. Twisted

    Thread Starter New Member

    Aug 7, 2011
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    So like the title asks. Im planning a project, im trying to build a hybrid car (out of a conventional car). The only realisticly afordable route I know is to use a small 25hp engine in the rear of the car with multiple alternators to charge the batteries. Here in lies the problem, alternators output from 13.2ish volts to 14.2ish volts give or take .2volts.

    Amperage is a non issue, but voltage is. The batteries in series will supply 144volts which is fine, but in order to charge them at a reasonable rate the voltage from the alternators needs to be stepped up. Theres no easy way to do this from what I have found with the kind of current / demand that would have to pass though a single converter.

    So someone mentioned to me to use an isolated dc to dc converter. Problem is, I didn't fully understand the guys explination so im going to copy and paste it in here. (doesn't help that I don't understand how an isolated dc to dc converter works)

    "What might be easier to find is an ISOLATED dc/dc converter with an input voltage that will work with your 13.5-14v alternator and puts out the voltage you want to charge to (14.4v for example) at the current you are looking for. (high current here might be hard to find as well) but basically you tie all the inputs together and put the output of one converter on each battery, since they are isolated it doesn't matter that the cells are in series."


    Can anyone help me out? All the reading I have been doing isn't really helping explain this.
     
  2. Robert.Adams

    Active Member

    Feb 16, 2010
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    I believe he is referring to a DC-DC converter with isolated grounds. In series, the low end of one converter will necessarily be connected to the positive terminal of a battery. If the converter isn't isolated, you just short the battery to gnd and probably burn stuff.
     
  3. Twisted

    Thread Starter New Member

    Aug 7, 2011
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    Forgive me, but would you be willing to toss up a quick drawing / schmatic? I am normally real good at visualizing concepts in my mind, but theres something about this I just don't think im seeing right. A visual aid I think would make all the difference in this instance.
     
  4. Robert.Adams

    Active Member

    Feb 16, 2010
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    Sure, please excuse my miserable paint skills.

    In the attached image, 2 of the converters are 'isolated' while the converter charging V1 is not. This converter being not isolated shorts V2 and V3 out through ground (which I've drawn in the converter).

    EDIT: Just for clarity, I forgot to draw the inputs to the converters, I hope that didn't confuse you.
     
  5. Twisted

    Thread Starter New Member

    Aug 7, 2011
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    I think I get it now. Thanks!
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    The difficulty you're facing is that trying to build an isolated DC-DC converter for the amount of power you'll need is quite challenging. To supply 20A at 144V to charge the batteries while supplying the current to run the vehicle, that's 2,880 Watts output - and you'll have probably around 80% efficiency for a decently-designed converter, so that's 3,600 Watts required for input, which translates to ~257A @ 14v.

    The absolute minimum you'd need to generate that much power would be two GM 130A alternators, which are the heftiest generators I know of - and then figuring a way to have just one regulator control the output voltage/current for both of them. But, trying to build a converter that has ~260 amps current on the primary side won't be something for a novice to attempt.
     
  7. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    Are u sure you want to go that way? 144V nominal battery voltage gives you 12 x 12V batteries... That means also 12 isolated DCDC converter. Doesn't look like a affordable project :D

    Do you have an idea of the charging current? C10? What's the battery capacity (Ah)?
     
  8. Twisted

    Thread Starter New Member

    Aug 7, 2011
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    Just in the planning stages right now. And although I have not totally given up on this I am starting to realize that this is not as likely or easy to do as I originally thought.

    Just ****'s me off, because mounting a small engine in the back of the car, and a few 130amp alternators is all to easy! And I could do that and I suppose hook them to the bank of batteries in series (going with old school flooded lead acid). But im thinking due to the batteries in series bumping voltage up to 144 (12 batteries) or 180volts (15) a realistic 13-14volt charge from the alternators isn't enough. The 12-15 batteries can provides plenty of power the K11 motor to near it's peak output (it's rated at 192volts peak hp rating is 162hp) steady (as in 100duty cycle) is like 52hp I forget the voltage thats rated at.


    Making an EV is to easy, it's just the damn hybrid charging solution thats the hard part. Like I was saying above and maybe got off track, I could do it like I planned but it wouldn't be able to KEEP the batteries fully charged, it would just slow down the discharge (I would think) and extend the distance of the car before the batteries got to a crictically low point. :( The amperage is there! Just need the voltage!! *bangs head on desk* *sigh*...hell if I could even just step it up to 40volts with 5 130amp alternators if I recall my calculations correctly that gives me like 29KW of juice, which would be enough to meet the motors requirements to run at like 70mph without draining the batteries. They could maintain a 100% charge.


    Idea was to then replace the OEM fuel tank with a larger 20gal unit...hell at least if you run out of feel...some theoretical 800-1000 miles later you still got 50-60 miles on the batteries to find a gas station lol! ah but without a way to step up the voltage SOME then this project isn't going to get past the planning stages. :(
     
  9. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    So is this only thought as a backup solution?
    Will this be used as regenerative braking solution?

    If not regenerative braking, then this system makes only sense if you charge the batteries at home or somewhere else, but not charging them while running on fuel. The alternators are a load to your explosion engine and the energy you put in to your batteries you will have to take out of your engine, i.e. you need more fuel, thus decreasing mileage on fuel. However if this is only a backup/emergency solution and if you want to charge the batteries while running on your car's engine and you only want to KEEP them charged and don't need to charge them from zero, you may need a converter with much lower current output (and a smaller alternator)...

    Building a power converter as Wookie described from scratch is not easy as there are several issues which need to be addressed, it takes months to develop it and it's not unlikely that you will burn a few (expensive) components building it.

    please have a look at this link, maybe it helps:
    http://www.diyelectriccar.com/forums/blog.php?b=427
     
  10. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    How about instead of a engine running car alternators, use a 'emergency generator' of the type for power outages at home? From that generator rectify the output to keep the batteries charged. The 'emergency generator' could be one of a few thousand or more amps, which is a pretty common size.

    The rectifiers would be the heavy duty 'stud mount' type often used in welders and industrial applications.

    The generators a pretty fuel efficient, they will run several hours on a couple gallons of gas.
     
    Twisted likes this.
  11. praondevou

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    That could work, but I still don't see the use of carrying around batteries if I charge them with a generator or the car's engine. I will use gas to charge batteries, so I could use the gas instead to run the car's motor...

    The only way this system makes sense is if I use regenerative braking.
     
  12. shortbus

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    The only bad thing about regen braking from what I've read is, that the batteries can't be charged fast enough to use it. Most regen braking is used to charge capacitors that then discharge to the motor when accelerating from the stop.
     
  13. Twisted

    Thread Starter New Member

    Aug 7, 2011
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    Well think of it this way. If a normal lets say 3.0L V6 engine burns 1 galon of gas, to travel 30 miles. BUT my solution thanks to the high energy efficiency of electric motors only requires the generator to burn say 1/4 -1/2 a gallon depending mostly on if the engine is gasoline powered or diesel. As diesel has a higher energy content in the fuel per cubic centimeter and thus needs less fuel to run the engine at X load. Then a lot of distance can be gained by this type of hybrid solution. The whole point to the batteries is to act as more of a buffer anyway. The generator is whats really supplying the power (ultimately) The batteries are just there so if I half to pull out into traffic with more than just a little throttle, theirs plenty of juice on tap. Then the generator just restores the batteries charge over a short period of time while powering the car.

    Ideally I'd want to use a diesel generator. Thanks to short bus's idea im going to start looking into this approach. vs the small engine with multiple alternators OR even with multiple automotive generators. Which was more practical to implement but WAY more expensive last I recall.


    Remember internal combustion engines are only roughly a little more than 30% efficient. Electric motors are on average around what like 80%...that's a big difference. Also keep in mind the whole idea was to use a small engine to turn the generator or previously alternators (or automotive generators) than your standard car uses. Or really could be driven with LOL (safely anyway). No if I can locate a emergency style generator that is up to the task. And with a high load on it wont burn but fraction of the fuel the normal ICE would to sustain a practical highway speed is yet to be seen and at this point...the big and seemingly determining IF. Which I will actually be surprised if I can't.

    Exactly, add in it's not practical for a DIY to build on a budget either!





    Just when you thought this thread was dead...This will just be a very long term project. Until im confident I think I have all the real problems solved and parts sourced, funds ready etc. I wont actually be building anything.
     
    Last edited: Aug 31, 2011
  14. praondevou

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    Well, good luck then. ;) I've seen some DIY projects on the internet, but they were all completely electric solutions, not hybrid....
     
  15. shortbus

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    @Twisted - you might want to include a planetary gear type transmission between the motor and final drive. This will allow torque multiplication to get the car started from a stop. A single stage of a Lenco transmission or even as a DIY version a gear set out of a normal automatic transmission will work. If not familiar with Lenco's here is a link; http://www.lencoracing.com/TheLENCOfamilyoftransmissions.html The great thing about them is no clutch is needed to shift gears.

    Using a three phase motor instead of a DC motor is to me a better cheaper solution too. Fro throttle control a VFD without the front end(AC input) would be a fairly cheap way to go. If not familiar with a VFD, it basically takes AC in, converts it to DC, and then outputs three phase AC that can vary the frequency to either speed up or slow down the motor RPM. By eliminating the AC input and using the DC from the batteries you have an easy dependable motor control. All with out any real design work, it's available off the shelf at a reasonable price. And three phase motors are usually less expensive and more easily available than a DC motor of a HP to power a car. Just my thoughts and ramblings on this.
     
    Last edited: Sep 1, 2011
  16. Twisted

    Thread Starter New Member

    Aug 7, 2011
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    ^ I find myself thanking you once again shortbus. I think I might have located a small (250lb) gas powered generator that will just be up the task, and since the frame and some other crap is not needed weight could be dropped from it once mounted in the car. Going to crunch a few numbers in a min and make sure it looks like it's up to the task. If so then I think I got my main power source located.
     
    Last edited: Sep 10, 2011
  17. shortbus

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    Twisted, glad to be of help. Pleas keep us updated on the project. Like to here about results.
     
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