PV charge controller - please help

Discussion in 'General Electronics Chat' started by Voltaire, Aug 19, 2010.

  1. Voltaire

    Thread Starter New Member

    Aug 3, 2010
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    Gentlemen/Ladies
    Your previous (5) threads on this subject have been enlightening and thanks. My question relates to a simple charge controller that uses only a) zener diodes b) diodes c) relays d) 50W 12V light bulbs and e) resistors. The panel generates about 15A @ 20V. There are 2 x 105Ah batteries connected in parallel.
    It is recommended that the maximum charge rate must not exceed 20% of 210 A or 40A, so it is not necessary to control the amps. The bulbs are intended to burn off excess charge. Literature suggests that the batteries should be charged to 14.4V

    Specific questions:
    1. Can it be done? :D
    2. If so, how does one switch the lights on so that they step i.e. so that all 6 lights do not switch on/off every few seconds?
    3. How is this achieved using only the above, considering that there are no 14.4V zeners? ;)

    Thank you in advance to all you out of the box thinkers

    Live in peace
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    After looking at your prerequisites, I won't touch this project with a 10-foot pole.

    Come back when you achieve some flexibility.
     
  3. Voltaire

    Thread Starter New Member

    Aug 3, 2010
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    How would you do it Wookie? I mean just the basics along these lines. What else would you need?
     
  4. timrobbins

    Active Member

    Aug 29, 2009
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    16
    Would you like to put the 5 thread links up for those that haven't seen them.

    Would you like to explain why you have gone out of your way to identify particular parts (a) - (e).

    Why have you asked question 2.
     
  5. Voltaire

    Thread Starter New Member

    Aug 3, 2010
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    Hi Tim
    If you type 'charge controller' into a new thread then you will get the list. Just follow the links :)
    RE Q2: One would have say 6 x 50W bulbs in a bank. If the voltage is just slightly too much then one would switch one light and if the voltage still rises a second and so on. If all 6 came on at once then they would flicker on and off all the time.
    a) to e) because that is the stuff that I have
     
  6. Voltaire

    Thread Starter New Member

    Aug 3, 2010
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    Newbie mistake :eek:
    Links are at bottom of page
     
  7. tom66

    Senior Member

    May 9, 2009
    2,613
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    Without some way to efficiently step down voltage (you have apparently ruled out active devices) it would be very difficult to do this. You could use resistors, but you'd be wasting a TON of heat (like 100's of watts if not more) and getting rid of this is not trivial to say the least.
     
  8. Voltaire

    Thread Starter New Member

    Aug 3, 2010
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    Hi Tom
    You are, of course, absolutely right. However, if we assume that 'wasted' energy is not a problem; can it be done?
     
  9. timrobbins

    Active Member

    Aug 29, 2009
    318
    16
    Why don't you have a relay coil in series with some zeners such that coil turns on when voltage gets to equalising level as per manufacturers spec. Relay contact brings in one light bulb. But use a relay with two contacts - such that second contact brings in the next coil/zener circuit (which turns on a second light bulb) - iterate to however many light bulbs you have.

    Unfortunately coils are likely to have large hysteresis - so may need to use a relay with 3 contacts - and use the third contact to bring in another zener in series with coil to increase the coil drop out voltage.

    Ciao, Tim
     
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  10. marshallf3

    Well-Known Member

    Jul 26, 2010
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    I was born confused and now I think I'm even more so. :)

    A picture is worth 1,000 words, a picture with 100 words is worth even more.

    I cas assume it's a battery charger with some sort of cutoff circuitry when the battery is fully charged? I guess it's the light bulbs that are throwing me off.
     
  11. timrobbins

    Active Member

    Aug 29, 2009
    318
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    This was a thought.[​IMG]
     
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  12. Potato Pudding

    Well-Known Member

    Jun 11, 2010
    684
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    If you don't mind wearing out lots of relays and and they are normally closed or double poles then it is easy enough to create a relay oscillator if that is what you want.

    It is on and it has a zener and a bulb in series, but it has its coil wired through its own contacts so it is on but turns itself off. That turns it back on again and it turns itself off again. Oscillator.

    The Zeners would need to be very large high wattage shunt regulator types! The bulbs will not limit the current much under the flickering power.

    I think it would probably work badly, and dangerously, and not for long but it might work. A few dollars worth of transistors, resistors, and capacitors wouls make it much better. Literally 2 dollars worth more would make it safer and better and able to last for years insead of days. You also want a fuse at least and a few other parts so call it $5.
    How much are these batteries worth? More than $5 I expect.
     
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  13. Voltaire

    Thread Starter New Member

    Aug 3, 2010
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    Hi Marshall
    In a nutshell:
    I want to charge the batteries to about 14V using a solar panel. When the voltage goes above 14V then I want to burn off the excess voltage using light bulbs. The bulbs must be switched on sequentially depending on how much power needs to be dissipated for example: 14.1V = 1 bulb then the next one comes on at 14.2V etc. When the voltage is down at 14V then everything should be off again.

    Is this feasible or is there a simpler way without using complicated electronics?
     
  14. Potato Pudding

    Well-Known Member

    Jun 11, 2010
    684
    92
    What is your level where electronics get complicated?

    Do you have any old electronics around that you can scrap and recycle?

    You really would do better to try and make hay while the sun shines instead of wasting any extra power.

    You want a switching current control power supply.


    There are several that are uncomplicated to wire up. They start under $2 a chip.
    You can use some chips to drive some cheap Mosfets if you want a ton of current.

    You use switching mode for the transformer action that means you can turn around 60% Pulse Width of the Solar panel 20Volts output and get a 100% duty cycle current of the battery voltage. That means power input will be closer to equal the power stored in the battery.

    Say you were using the batteries to run the sattelite TV in your camper after dark.

    If you are in the middle of the big game going to overtime, you would want to have a 5 hour charge instead of a 3 hours use charge. That is the kind of power you could waste with your simpler solution.
     
    Last edited: Aug 22, 2010
  15. timrobbins

    Active Member

    Aug 29, 2009
    318
    16
    Voltaire - do you appreciate how the relay circuit works? It achieves all your requirements - you trim the on-voltage of each sequential relay to turn on a bulb as battery voltage rises - you can choose your voltage set points to suit your needs.
     
  16. marshallf3

    Well-Known Member

    Jul 26, 2010
    2,358
    201
    You can't really "burn off voltage" so to speak. You simply want a charger circuit that shuts off at a preset voltage level or when the current flowing into the battery drops to a point that indicates it has fully charged.

    Sounds like Potato Pudding has an idea up his sleeve, he usually does and it will usually be an easy solution.
     
  17. Voltaire

    Thread Starter New Member

    Aug 3, 2010
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    Hi Tim
    My understanding is that small current creates an electromagnetic field which latches a contact closed. The larger current then flows through the contacts.
    While I am sure that an electrical eng would understand your diagram perfectly, could you please explain some aspects to me?
    1. how does your numbering convention work?
    2. what is the purpose of the switch at R4/3?
    3. is array = solar panels/s or batteries?
    4. what is the purpose of the switch at R4/1? I cannot see the current being regulated at that point
    5. do you achieve voltage set point by putting zeners in series? If so, which value is first? high or low?
    6. where do the lines from RL1/3 and RL2/3 go to?
     
  18. Voltaire

    Thread Starter New Member

    Aug 3, 2010
    25
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    Ok Marshall
    I'm open to suggestions
     
  19. timrobbins

    Active Member

    Aug 29, 2009
    318
    16
    Voltaire,

    The array - solar panel. The solar panel is connected to battery (not shown in schematic). The relay/bulb circuit acts as a shunt controller - best you read up on shunt controllers/chargers for PV panels - they are effectively just a resistor in parallel with panel and battery to shunt array current away from battery - and hence to regulate battery voltage - vey common charger technique in the early days.

    Zeners in series add their voltages - when the total summed voltage is exceeded then significant current passes through the relay coil - you will need to learn more abou relay coils and contacts to understand this a bit better A relay coil RL1/3 indicates the relay is RL1 and that it has three contacts. Each contact on RL1 is shown as RL1/1, RL1/2, RL1/3 and has a contact which may be normally closed or open (my schematic has a error in logic for the relay contacts - but no matter for understanding.)

    The schmeatic is a single line diagram - it doesn't show the full circuit (ie. it is missing the 'negative' return path).

    R4/3 is actually RL1/3 - my poor sketch! same with R4/1 = RL1/1.

    The bulb symbol is your 'load' resistor - it turns on when the relay turns on.

    I'll do another schem at some time.

    Ciao, Tim
     
  20. marshallf3

    Well-Known Member

    Jul 26, 2010
    2,358
    201
    I like to keep things practical and spend as little as I need to when building projects. Since MOSFETs have improved to the point where they can replace most any relay the cost of relays has gone up. They're also more prone to failure and don't forget you'd want a rectifier across the coil of each one. It just seems to be a far more complicated way to go about this than need be and I think you'd get better results with a true charger circuit, the batteries would probably last longer to boot.

    I have one of those panels but it isn't anywhere near as good in output capability. It sits in my van that I rarely use and keeps the battery topped off. Has it been overcharging it after weeks of sitting in the bright sun when the van hasn't been used? I don't have a clue but chances are good that it has been.
     
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