solar inverter - 100v to 12v

Discussion in 'The Projects Forum' started by tom_s, Oct 6, 2014.

  1. tom_s

    Thread Starter Member

    Jun 27, 2014
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    scenario –

    have got solar panels rating VOC 92V, PMAX 60W ISC 1.19A (kaneka G-EAO60) as the panels are new they idle closer to 94-96vdc

    what I have to charge , a quantity of 90AH SLA batteries. cycle 14.4 – 15.0v, standby 13.5 – 13.8v

    there's nothing really commercially available to fit at that voltage. have built linear chargers and could put something together to do the job, just trying to bring myself up to date with something a bit more current.

    objective –

    have 4 panels up already (individually) been wasting sunshine for a few months now but working wonderfully as an awning :) got another 4 going up shortly. Would like to have the charging circuit(s) running off the batteries as they are being charged. Current draw for circuit there would be negligible ~ 100ma worst case

    thoughts -

    obviously pwm / smps circuit is best option, from the majority of reading i've done, most people seem to prefer to use high side pchannel mosfets. Do have irf9540's, by229's on hand and its nchannel companion (irf540's). mosfet drivers down(under) seem to be a bit of a rarity unless massive quantities are involved. opto couplers and photo interrupters are available. current thoughts, 555 for pwm - opto - mosfet

    the odd issue :)

    just to clarify myself, due to memory issues, I do sometimes have troubles understanding concepts. (couple major car accidents in the teen years) putting that aside, so many options available, would like some thoughts on people preferences.

    have sufficient tools to do the job (cro/iron/bits and pieces/coffee) also plenty of time :)

    If anyone can point me in the right direction, greatly appreciated. will also be building this device for someone else which will have to be id-10-t proofed, but can worry about that after working prototype.

    edit: page formatting
     
  2. Lestraveled

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    May 19, 2014
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    Last edited: Oct 6, 2014
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  3. Kermit2

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  4. tom_s

    Thread Starter Member

    Jun 27, 2014
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    not sure if that particular one is available downunder, got a quote not long back (4 months?) from a 3rd party supplier. TSMPPT45 and a $550aud price to go with it was all that i was given.
    will dig a bit further. a few solar companies i've called deny anything over 60v (or not interested in selling, they make more $ on installs).

    don't mind building something myself, its more for memory stimulation, will add a (hand drawn) circuit of what i had in mind (if i can get it to stay still long enough) shortly :)

    edit: typo
     
  5. Lestraveled

    Well-Known Member

    May 19, 2014
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    Here is a app note for a Switch mode battery charger with a MPPT function. It uses the BQ2031 chip. I have used this chip in several battery chargers. The app note is based on a lower voltage panel but could easily be modified to work with your panel voltage. At least it could give you some food for thought.
     
  6. alfacliff

    Well-Known Member

    Dec 13, 2013
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    switchmode power supplies will work on dc input, even those rated as ac input. since the input of the switcher is filtered then rectified, dc of either polarity may work.
     
  7. tom_s

    Thread Starter Member

    Jun 27, 2014
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    thanks guys, actually did buy a 240v smps (switchable voltages) which i'm keeping the batteries topped for the time being. don't want to pull it apart and have a peak at it until the warranty runs out :)

    do have 555 clock, driving pchannel fet and its adjusting volts fine with 12v in and a pretty good idea on how to use opto coupler to drive the fet for higher volts in.

    got lots of questions, but one that sticks out and might be in your department Lestraveled, would i need an inductor? wouldn't the pulse itself act as de-sulphering the battery as its charging? batteries i'm using are out of a firehouse pumpstation and replaced every 2 years, so they should be good for at least another 2.

    and going to be busy work wise for a few days and an unplanned day trip into hospital (colonoscopy). one of pre tests came back positive :/ the last one was a false alarm, maybe i shouldn't be eating so many rare steaks...

    edit: typo. about time this keyboard got replaced that drops characters now and then
     
  8. Lestraveled

    Well-Known Member

    May 19, 2014
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    Adding an inductor and diode would basically create a "buck" power converter which is an efficient way to convert the 48 - 90 volts down to your charging voltage. This is what most solar converter/controller are.

    As far as desulfating, I am guessing that your batteries are pretty beefy. A low current desulfator won't do much with big batteries. I suggest that you concentrate on charging them correctly, per the manufacturer specs if you can get them. The voltages you stated in your first post are in the ball park. Keep them as close to fully charged as you can and when you draw them down, recharge them as soon as you can. Also you need to charge them at about 10% of their amp hour rating. Example, a 100 AH battery should be charged at 10 amps rate. A buck power converter is an excellent way to get that kind of current from your panels.
     
  9. tom_s

    Thread Starter Member

    Jun 27, 2014
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    yup, plenty beef in batteries and going to be using 8 panels (got 22 more but rest are going to a friend on a farm)

    what me was thinking -

    [​IMG]

    tis rough but i'll get around to a little protection shortly.

    from my 'understanding' and the reason for the opto, basically want the 555 to run off the battery
    555's controls output volts (ton/toff);
    12v zener to protect mosfet gate;
    10k in series to pull high (start in off state);
    same 10k and 33k as voltage divider on opto to float ground reference (have done calcs 60 to 90v).
    under 60v from the panels, there's negligible current generated, hence the 60v cut off point.

    thoughts?
     
  10. Lestraveled

    Well-Known Member

    May 19, 2014
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    Tom
    Here is a suggestion. When dealing with a buck converter that has a high voltage input, a "low side buck converter" might be a better configuration. This is where the mosfet is referenced to ground instead of floating at a high voltage.

    [​IMG]
    On the left (Q1 L1 D2) is the normal buck converter with the parts switched around so the mosfet is at ground. Much better for higher voltage applications. Your regulator/controller chip is now at the same voltage potential as your switch mosfet. Driving your Mosfet is a lot easier this way.

    There are two drawbacks to this configuration:
    One, the negative terminal of your panels and the negative of your battery can not be connected. One of them must float. (Generally it is the panels that will float.)
    Two, the battery is floating at a voltage above ground. To get feedback back to your regulator/controller (at ground potential) you have to something special. That is where the circuit (Q5 R5 R6 R7 R8 R9) on the right comes in.

    Q5 R5,6,7 form a voltage controlled current source. The current is applied across R8/9. The transistor is rated at 300 volt. (oops, Q5 should be a MPSA92, the number on the schematic is wrong.) The values shown are for a 48 volt battery and a 7.5 volt feed back voltage. LTspice will help you change it for 12 volts.

    This configuration works well. I used it in a tough situation and it was bullet proof.
     
  11. tom_s

    Thread Starter Member

    Jun 27, 2014
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    thanks, i understand most of that. was tossing up whether to do high or low side. will breadboard it up and have a play on the weekend.
    pretty limited on component outlets here (currently it is the sunny side of melbourne). all we have is jaycar and i doubt they'd have anything related to the mpsa92, though probably have something in one of the transistor trays at home resembling a 300v pnp. /me will find something :)

    did install ltspice but currently at the stage of playing a concert piece on a piano.

    one of the things i really wanted to do in the younger days was learn to play piano. took lessons, bought a keyboard (yahama dx9). been told i have perfect hands for playing keyboard. sadly there was something missing between the brain and the fingers. 3 years later gave it to the ex's brother that actually had a talent to be able to play any instrument.

    i'll get the better of spice eventually, it don't need me to play it a rhythm ;)
     
  12. Lestraveled

    Well-Known Member

    May 19, 2014
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    The MPSA92 is a pretty inexpensive transistor. You might check EBay.

    There is an op-amp circuit that can do the same function. If that might work better for you I'll sketch it up.
     
  13. tom_s

    Thread Starter Member

    Jun 27, 2014
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    i hate ebay :) there's a few 30-40a 100-150v mppt controllers showing up available at a very good price (read as cost me less than making it/them) around $230AUD ex sydney/china free delivery...

    doesn't mean i'm going to stop building my own, just putting on the side for a little while. thanks to all for the help.

    edit: added link - http://www.ebay.com.au/itm/121093861938
     
    Last edited: Oct 25, 2014
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