How to connect a solar panel and a battery, but not charge the battery?

Discussion in 'General Electronics Chat' started by DarkNova, Aug 7, 2012.

  1. DarkNova

    Thread Starter New Member

    Dec 18, 2011
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    I'm working on a creating a project where my end goal is to create a stable 12V DC power source, primarily using a solar panel, but also having a battery as a "backup", since the sun may go behind a cloud and the solar panel will stop producing power for a few minutes.

    This is commonly done by doing:
    Solar panel -> Charge Controller -> 12V Battery -> 12V Load

    but for my particular application, I don't want to be charging the battery, I want the battery to only be used as a source, and I also don't want the battery to be drawn upon if the solar panel is supplying enough power.

    So basically my goal is something like:

    If it is currently sunny:
    Solar Panel (at around 17 Volts) -> Voltage Converter (Buck?) -> 12V Load

    If it is not currently sunny (Solar Panel isn't producing enough power for the load):
    Battery (I can be flexible with the battery design, but it could probably be 12+V for the same voltage converter to function) -> Voltage Converter (Buck?) -> 12V Load

    So what I'm trying to figure out is how to design the circuit that can basically switch between the two current sources (solar panel and battery) based on if the solar panel is supplying enough current to meet the load's needs or not. Thanks.
     
  2. sparkyuiop

    New Member

    Aug 19, 2011
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    I'm no expert but it sounds like a relay and a diode might be needed. Would a light photocell be any use, like the type that allows lighting to power on at night?
    It may also be an idea to allow the solar panel to deliver any charge it is producing to the battery if the circuit is running off of the battery at the time.
     
    Last edited: Aug 7, 2012
  3. #12

    Expert

    Nov 30, 2010
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    Kinda depends on how powerful the solar panel is. What range of amps can it deliver?
     
  4. DarkNova

    Thread Starter New Member

    Dec 18, 2011
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    Thanks for the quick responses. For my particular application, I'm not using a lead acid battery so I don't want this circuit to be able to do any battery charging, just draw from it. I have not purchased the solar panel yet (in planning phase) so I can be flexible, but I'm hoping to be able to do 5 or 6A if that's feasible. I may end up making two versions, one may be with a small panel that's maybe 1A or less. Thanks.
     
  5. #12

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  6. DarkNova

    Thread Starter New Member

    Dec 18, 2011
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    Thanks for this. So, if I'm understanding this correctly, when the solar panel is in the sun and has a higher voltage than the battery, the power to the load will be entirely supplied by the solar panel, but when the solar panel is shaded and is not producing enough current, its voltage will drop to below that of the battery, and then the circuit will switch to draw from the battery instead. Is this correct?

    So in my situation, I would want the battery voltage lower than what I would expect the solar panel to normally output, but higher than what it will produce if there is little light falling on it, right?

    This seems pretty straightforward in the cases where it is fully sunny or totally dark, as if it is fully sunny most of these panels produce 17+V and if it is dark they will be 0V. I am still somewhat confused how this will actually work in practice while the panel is partially shaded.

    For example, here is the datasheet of one "12V" 85W panel. If you scroll to the bottom you can see its I-V curves.

    Say I have a load that is trying to draw 2 amps. If it is quite sunny there might be 800W/m^2 of sunlight falling on the panel so the panel will be at around 21V, even with a 2 amp draw, so no problem there, it will just run off the panel.

    But what if there are a lot of clouds for the time being, and there is only 200W/m^2 of sunlight falling on the panel? According to the I-V curve, this panel can only produce a little over 1A of current at 200W/m^2 so it would not be able to produce enough for the load. If the load tries to pull 2A off it, the panel's voltage will drop to 0V, so with this diode-OR network, the batteries would take over and be able to supply the load. But as soon as the battery takes over, there would be no current draw off the panels so their voltage (open circuit) would go up to 20+V and the circuit would attempt to switch from the battery to the panel (assuming the battery voltage is like 14V or something), and the cycle would repeat.

    I'm not sure, but I'm guessing this oscillation is bad? I'm thinking this may be able to be improved with a capacitor between the output terminals of the solar panel? But that would probably just serve to reduce the oscillation, not eliminate it?
     
    Last edited: Aug 8, 2012
  7. #12

    Expert

    Nov 30, 2010
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    You're over thinking this. The diodes do not communicate with each other. One does not switch the other "off" when it starts to conduct.

    As the solar input fades, the solar panel will suffer lower and lower voltage while trying to keep the amps flowing. As soon as that failing voltage gets below 11.something, the battery/diode combination will begin to contribute current. The voltage on the load will not decrease below (battery voltage minus the diode voltage loss). The solar cell will not suddenly drop to zero volts just because the battery is contributing. They will share like nice playmates.

    Of course, when the battery is empty, that voltage is drek, but you didn't ask me about recharging the battery.
     
  8. JMW

    Member

    Nov 21, 2011
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    I understand what you WANT to do, but I think you are going about it the wrong way. If you want a "stable" voltage PV's, wind, are not the way to go. Use the battery as the primary with PV to assist the battery. Having the battery most reliable backing up the least reliable doesn't make sense.
     
  9. Austin Clark

    Member

    Dec 28, 2011
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    There's really no reason to switch back and forth, use #12s general idea, but don't use the diode coming from the battery (It's not really needed as far as I can tell, and only serves to prevent the panel from charging the battery,which I don't see why you would want). It's the easiest and is efficient enough to be effective. If you're charging the battery whilst using the battery to power your load, then the panel will already be contributing to the load, if the panel has enough current to fully power the load, it'll do that as well, and can even charge the battery and power your load at the same time, if there's enough current coming from the panels.

    The only thing I can see that could be an issue is over-charging. If the panels can reach voltages greater than the maximum of your batteries (minus the voltage drop across the diodes), then you could overcharge and even explode them.
     
  10. DarkNova

    Thread Starter New Member

    Dec 18, 2011
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    Ah, OK, that is helpful, I'm starting to understand diodes better, thank you. I have enough info now that I should be able to buy some parts and give it a try.

    I appreciate the device, and I know how to rig up a charge controller and do it the "traditional" way (where the battery is the primary draw) but for my particular application I'd like to do it with the panel as the primary and the battery as the secondary because:

    - I'd like to mainly use this when it is sunny out
    - I'd like it to possibly be portable (a small version at least) without having to bring a charge controller along -- I might use LiFePO4 cells that need a more complicated charging scheme than lead acid, or NiMH or I could possibly use non-rechargeable batteries even, the point is it wouldn't matter what kind of batteries they are because they wouldn't be ever charged (and they hopefully will be drawn from little in sunny conditions)
    - I couldn't find much information on a design like this -- what better reason is there to try something :)

    Thanks everyone for the advice.
     
  11. DarkNova

    Thread Starter New Member

    Dec 18, 2011
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    And one other thing I'm thinking I'd try is, if you don't need it to be portable, you could connect the circuit in the same way but instead of a battery, use a power supply. Then the load would normally run off the solar panel but only use power from the supply if it isn't sunny enough. No battery needed in this case.
     
  12. Austin Clark

    Member

    Dec 28, 2011
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    Whatever floats your boat really. I kinda think it's a shame not to use the panels more fully, but it's really up to you. Just know that panels can actually get pretty hot if you don't have them powering something useful. In essence, the power that they would be using powering something useful is generating heat. It will act more or less like a sheet of black tar.
     
  13. crutschow

    Expert

    Mar 14, 2008
    13,009
    3,233
    Since typical solar panels are only 10-15% efficient, their temperature will not vary that much whether they are providing power or not.

    For best efficiency you can use large Schottky type rectifiers. They have about half the forward voltage drop of a standard junction diode.
     
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  14. DarkNova

    Thread Starter New Member

    Dec 18, 2011
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    Thanks, I will have to look into this more to try to pick out the best one to buy.
     
  15. #12

    Expert

    Nov 30, 2010
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    Cheapest shottky at Mouser of at least 6 amps that is not a surface mount device.
     
  16. DarkNova

    Thread Starter New Member

    Dec 18, 2011
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    Thanks again for the help. I'm starting to understand these diode datasheets a little more, hopefully :) So when I look at this one, it says the "Working peak reverse voltage" is 21V. According to the datasheets of some of the solar panels I've looked at, with little load some of them deliver close to 22V, so that would be over the reverse voltage of the diode and there could possibly be a situation where it would reverse feed current back into the battery, right? Maybe I should get a diode with a higher reverse voltage so it's not so close to the margin?

    I was searching around Mouser and saw this one. Does that seem suitable? Thanks!
     
  17. #12

    Expert

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    Solar panels don't make "reverse voltage". If the sun quits and the battery provides 12 volts, the "reverse voltage" is 12 volts, but you're welcome to buy diodes with a safety factor.
     
  18. DarkNova

    Thread Starter New Member

    Dec 18, 2011
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    Sorry, I meant in the scenario where the solar panel is in the sun, and the batteries are dead for some reason (and no load is connected). Normally wouldn't happen, but thinking of possibilities. Then would the diode in front of the battery possibly reverse and the panel start feeding a small amount of current into the battery?
     
  19. #12

    Expert

    Nov 30, 2010
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    Not likely with a "maximum DC blocking voltage of 30". If the sun oscillates, or stripes of clouds move quickly in front of the solar panel, that could cause a peak reverse voltage, but I don't think it's likely. Still, buying diodes that have a safety factor is not a sin.
     
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  20. DarkNova

    Thread Starter New Member

    Dec 18, 2011
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    OK, I wasn't sure how the maximum DC blocking voltage related to the "working" reverse voltage. Some diodes looked like it was the same, some were different like this one. Thanks again #12, you've been a big help.
     
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