Shorting a solar panel

Quick question - Is it damaging to a solar panel to be short-circuited in full sun?
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  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Shorting a solar panel

    Welcome to the forum.

    Quick answer: nope. :D

    (One of the numbers you'll see in panel specs is "Isc": Intensity (Amperage) at short circuit. A panel can produce this current and not suffer any ill effects. It will not produce any greater current than that.)
  • mmmalmbergmmmalmberg Registered Users Posts: 13
    Re: Shorting a solar panel

    Nothing better than a quick answer to a quick question - thanks! Now I just need to figure out how to safely crowbar a 58F capacitor with a minimum of parts...
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Shorting a solar panel

    I think I'll let someone else ask the inevitable question: "What on Earth are you trying to do?"

    I really do not want to know.
  • mmmalmbergmmmalmberg Registered Users Posts: 13
    Re: Shorting a solar panel
    I think I'll let someone else ask the inevitable question: "What on Earth are you trying to do?"

    I really do not want to know.

    Ha! OK I'll wait then for someone else to ask:)
  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    Re: Shorting a solar panel
    mmmalmberg wrote: »
    Ha! OK I'll wait then for someone else to ask:)

    I'm someone else. please tell... --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • mmmalmbergmmmalmberg Registered Users Posts: 13
    Re: Shorting a solar panel
    vtmaps wrote: »
    I'm someone else. please tell... --vtMaps

    Thank you:)
    I make solar-powered robotic sculptures; the project I'm working on is for ISEA2012 in Albuquerque.

    I currently have a buck-boost regulator between the panel and the capacitor, but I'm finding it slow to charge the cap all the way up to 15V with a 15V supply - the rate of charge drops dramatically as the voltage in the cap rises. I'm hoping that a direct connect from the panels, which have an open circuit voltage of about 23VDC, will top the cap off more quickly.

    I'm looking at using a crowbar-type circuit modified by using a mosfet instead of an scr so that it doesn't latch. I'm not an engineer so I kind of hack my way through this stuff 'til it starts working:)
  • BB.BB. Super Moderators, Administrators Posts: 29,351 admin
    Re: Shorting a solar panel

    Energy in a capacitor is stored with the square of the voltage.

    E=1/2 x C x V^2

    So the more voltage you store, the longer it is going to take to charge.


    Sorry, this is from my phone. Typos and stuff.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mmmalmbergmmmalmberg Registered Users Posts: 13
    Re: Shorting a solar panel

    That's hugely interesting and relevant. Is the same true with battery storage?
    BB. wrote: »
    Energy in a capacitor is stored with the square of the voltage.

    E=1/2 x C x V^2

    So the more voltage you store, the longer it is going to take to charge.


    Sorry, this is from my phone. Typos and stuff.

    -Bill
  • BB.BB. Super Moderators, Administrators Posts: 29,351 admin
    Re: Shorting a solar panel

    No, batteries store energy via chemical reactions. Which is usually much more dense (watt hits per cubic inch/lbs, etc.).

    It is much more complex to use capacitor energy efficiently because storage is change of voltage vs batteries which are closer to constant voltage output.

    Using switching regulators is the right direction, but still not easy to do easily.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Shorting a solar panel
    mmmalmberg wrote: »
    Thank you:)
    I make solar-powered robotic sculptures; the project I'm working on is for ISEA2012 in Albuquerque.

    I currently have a buck-boost regulator between the panel and the capacitor, but I'm finding it slow to charge the cap all the way up to 15V with a 15V supply - the rate of charge drops dramatically as the voltage in the cap rises. I'm hoping that a direct connect from the panels, which have an open circuit voltage of about 23VDC, will top the cap off more quickly.

    I'm looking at using a crowbar-type circuit modified by using a mosfet instead of an scr so that it doesn't latch. I'm not an engineer so I kind of hack my way through this stuff 'til it starts working:)

    I think a fundamental problem is that you are starting with a crowbar as a model. A crowbar is not meant to regulate, it is meant to protect. In essence it shuts down a defective power source by jamming a crowbar into the works! It is not meant to keep the voltage at a particular level, it is meant to crash the system totally if the voltage goes higher.

    The only reason that a crowbar approach is relevant to your problem is that short-circuiting the panel will not damage the panels or blow any fuses or breakers. But by the same token it means that you need additional circuitry beyond what is in the basic crowbar design to make it do what you want. I understand that you would like to use as few components as possible, but there is a minimum that will be required.
    SMA SB 3000, old BP panels.
  • fix_it_guyfix_it_guy Solar Expert Posts: 26
    Re: Shorting a solar panel

    Hi mmmalmberg, I like your work. Looks like you and I would get along nicely, my shop is filled with pieces and parts and and all kinds of stuff you would like to use. If I can't fix something I take all the parts out of it so I can fix the next one.
  • bill von novakbill von novak Solar Expert Posts: 849 ✭✭✭✭
    Re: Shorting a solar panel
    mmmalmberg wrote: »
    That's hugely interesting and relevant. Is the same true with battery storage?

    Yes, that equation always holds. However it is usually not an issue because batteries operate over a narrow voltage range. About the only time you see it is when you have a battery bank that's getting low and you get less and less total power out of it at the same current i.e. if your bank is at 12V at 100 amps you're getting 1200 watts delivered to your inverter, but if it's at 10.5 volts at 100 amps you're only getting 1050 watts delivered.
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Shorting a solar panel
    Yes, that equation always holds. However it is usually not an issue because batteries operate over a narrow voltage range. About the only time you see it is when you have a battery bank that's getting low and you get less and less total power out of it at the same current i.e. if your bank is at 12V at 100 amps you're getting 1200 watts delivered to your inverter, but if it's at 10.5 volts at 100 amps you're only getting 1050 watts delivered.

    The underlying situation is not quite that simple. For both battery and capacitor the current flowing in for a given fixed voltage drops toward zero but never gets there. If you have a fixed amount of power (rate or energy transfer) then you will be able to deliver all of it to the battery while it is in a low State Of Charge (SOC). And then as you get closer and closer to the Float voltage, the current becomes constant at a rate which just offsets the self-discharge rate. But in a capacitor, if you were able to deliver a fixed amount of power, the initial current would have to be infinite. Not a practical scenario.

    But a solar panel actually delivers a nearly constant current as a function of voltage as long as the panel Voc is enough greater than the load voltage. So if that were the only factor, the voltage on the capacitor would increase linearly as charge it. But since the voltage is very low in the early stages, less power is going in and at the end, the power is at its greatest.

    But the real problem that you are facing is that you (theoretically, of course) can never charge a capacitor to 15 volts using a 15 volt power source. The internal resistance will cause the current to be proportional to the difference between the supply voltage and the capacitor voltage, and that will go toward zero but never actually reach it!

    So to charge the capacitor to exactly 15 volts in a reasonable time, you have to start with a voltage source of more than 15 volts and NOT reduce the current coming into the capacitor until you actually reach 15 volts. That is what your circuit needs to do. You do not need a buck/boost regulator at all if the panel voltage is reasonably matched to your target voltage. You just want to send the full current from the panel to the capacitor until it reaches your target, then stop. For that purpose, a series switching element to cut off the current is much more reasonable than trying to short out the panel.
    SMA SB 3000, old BP panels.
  • BB.BB. Super Moderators, Administrators Posts: 29,351 admin
    Re: Shorting a solar panel

    Also note that the equation E=1/2 * CV2 is in "Watt Seconds"... I.e., a 14.5 volt to 10.5 volt 58 Farad capacitor stores, in battery terms:
    • E (watt*hours) = 1/2 * 58 F (14.5v2 - 10.5v2) * 1 Hour / 3,600 Seconds = 0.81 Watt*Hours

    A similarly rated 12 volt battery would be rated:
    • 0.81 WH / 12.5 volts = 0.07 Amp*Hour @ 12 volts

    A single AA battery also stores around 1 Watt*Hour of power... (I think I got all the units right--but is is possible I messed something up--been a long time).

    Big/super caps are not really all that they are cracked up to be a battery replacement in most applications.

    If you can work with low voltages--A pair of NiCad batteries in series (2-3 volts) might be a good answer. They can be run down to zero volts, and two batteries in series is low enough voltage that they cannot damage each other if one is of lesser capacity.

    For example any multi-cell rechargeable battery--say 6-24 cells for 12-48 volt battery bank, if one cell is "weaker" than the rest, the once cell can go to zero volts and actually be "reverse charged" by the other cells).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • bill von novakbill von novak Solar Expert Posts: 849 ✭✭✭✭
    Re: Shorting a solar panel
    inetdog wrote: »
    But a solar panel actually delivers a nearly constant current as a function of voltage as long as the panel Voc is enough greater than the load voltage. So if that were the only factor, the voltage on the capacitor would increase linearly as charge it. But since the voltage is very low in the early stages, less power is going in and at the end, the power is at its greatest.

    Agreed, and that's true of battery systems as well with PWM controllers. (Which is one reason MPPT controllers can do a lot for you; they can match a cold panel to deeply discharged batteries and not waste as much power.)
    That is what your circuit needs to do. You do not need a buck/boost regulator at all if the panel voltage is reasonably matched to your target voltage. You just want to send the full current from the panel to the capacitor until it reaches your target, then stop. For that purpose, a series switching element to cut off the current is much more reasonable than trying to short out the panel.

    Also agreed. At best a buck/boost will improve your charge times, but it needs at least a simple MPPT function to accomplish that. AFAIK no such device is available off the shelf.
  • mmmalmbergmmmalmberg Registered Users Posts: 13
    Re: Shorting a solar panel
    inetdog wrote: »
    But the real problem that you are facing is that you (theoretically, of course) can never charge a capacitor to 15 volts using a 15 volt power source. The internal resistance will cause the current to be proportional to the difference between the supply voltage and the capacitor voltage, and that will go toward zero but never actually reach it!
    THIS is exactly what I'm trying to fix. I've got theoretically 22 or 23 V open circuit from the panel, so it seems to me that if I feed that directly to the panel, that would be best and I just need a way to stop feeding the cap when it gets to 15V.
    inetdog wrote: »
    So to charge the capacitor to exactly 15 volts in a reasonable time, you have to start with a voltage source of more than 15 volts and NOT reduce the current coming into the capacitor until you actually reach 15 volts. That is what your circuit needs to do. You do not need a buck/boost regulator at all if the panel voltage is reasonably matched to your target voltage. You just want to send the full current from the panel to the capacitor until it reaches your target, then stop. For that purpose, a series switching element to cut off the current is much more reasonable than trying to short out the panel.

    I'm right with you on that. I just don't have a really simple way to do it (the shunt circuit seems like it can be quite simple) and I also like that the shunt circuit does not put anything between the panel and the cap until the target voltage is reached.

    Two questions:
    Do you know of a super simple way to open the circuit rather than short it?
    What is the real problem that a number of people seem to have with shorting the panels? For me it's heat, anything else? Re. heat I was thinking I could use the shunt to divert power to a small cooling fan. But I'd love to have a good, clean and super efficient way to open the circuit.
  • mmmalmbergmmmalmberg Registered Users Posts: 13
    Re: Shorting a solar panel
    BB. wrote: »
    Big/super caps are not really all that they are cracked up to be a battery replacement in most applications.

    If you can work with low voltages--A pair of NiCad batteries in series (2-3 volts) might be a good answer. They can be run down to zero volts, and two batteries in series is low enough voltage that they cannot damage each other if one is of lesser capacity.

    -Bill

    I actually have a 1.2 AH SLA that will be charged by the panel at the same time through a current limiter at 150 mA and a small morningstar charge controller. The cap is intended to take the heavy I/O demands of the motors in my system because in the last iteration of this piece, which had the SLA only, I was killing the SLA's in six months from having to charge and discharge them way beyond their intended limits. This is why I need the cap to run at 15V, so that I can do the heavy lifting with the caps, and then in the evening as the light fades, the caps go down and the battery allows some extended time in the evenings to keep doing things, run some lights etc.
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Shorting a solar panel
    mmmalmberg wrote: »
    THIS is exactly what I'm trying to fix. I've got theoretically 22 or 23 V open circuit from the panel, so it seems to me that if I feed that directly to the panel, that would be best and I just need a way to stop feeding the cap when it gets to 15V.

    I'm right with you on that. I just don't have a really simple way to do it (the shunt circuit seems like it can be quite simple) and I also like that the shunt circuit does not put anything between the panel and the cap until the target voltage is reached.

    Two questions:
    Do you know of a super simple way to open the circuit rather than short it?
    What is the real problem that a number of people seem to have with shorting the panels? For me it's heat, anything else? Re. heat I was thinking I could use the shunt to divert power to a small cooling fan. But I'd love to have a good, clean and super efficient way to open the circuit.

    Well, the problem most people have with shorting the panels is just that you are wasting energy. There is no point in diverting that energy to a fan or to heating unless you NEED that fan or heating. The panel will not get any hotter delivering current into a short circuit than it does just sitting in sun with an open circuit.

    Shorting the panels as a way of regulating charge to battery causes big current spikes that generate RF noise, can damage a partially shaded string of panels without bypass diodes (or can overstress underrated bypass diodes used in some cheap panels), and is just inelegant. The shorting device in the controller has to be able to handle the Isc current continuously.

    A simple series FET or bipolar transistor with the right drive circuitry will provide a good cutoff. Or you could go "solid-state" and use a relay. :-)
    SMA SB 3000, old BP panels.
  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    Re: Shorting a solar panel
    inetdog wrote: »
    The panel will not get any hotter delivering current into a short circuit than it does just sitting in sun with an open circuit.

    That is certainly true. In fact, the shorted panel will run cooler than the unloaded panel. --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Shorting a solar panel
    vtmaps wrote: »
    That is certainly true. In fact, the shorted panel will run cooler than the unloaded panel. --vtMaps

    Can you explain why that happens or point to a reference? The short circuit current is not carrying any energy away from the panels (as it would at MPP or anywhere but open or short circuit.)
    SMA SB 3000, old BP panels.
  • BB.BB. Super Moderators, Administrators Posts: 29,351 admin
    Re: Shorting a solar panel

    At least in terms of charging your capacitor to 15 volts--That would be just a standard solar charge controller. The charge controller opens the circuit to the solar array--no current flow. Solar panels are perfectly OK with no current flow (personally, I think that is better than shorting the panel--You get I2R heating of the wiring and other current paths--which is never a good thing).

    Super caps also (typically) have a limited lifetime of a few years under voltage.

    Do you know why you are killing the lead acid battery? Over charging and discharging to zero State of Charge are both common reasons they get killed. NiCad and (to a lesser degree) NiMH batteries might be a more rugged tech for short/high current surge. Also, if you can run low voltage (two cells in series), you will avoid the over discharging problem where a weak cell reverses voltage and actually begins to reverse charge (a guaranteed cell killer for most rechargeable battery chemistries).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    Re: Shorting a solar panel
    inetdog wrote: »
    Can you explain why that happens or point to a reference? The short circuit current is not carrying any energy away from the panels (as it would at MPP or anywhere but open or short circuit.)

    As I understand it, the sunlight striking a panel is either transmitted through it, reflected by it, or absorbed by it. With no load all the absorbed energy is converted to heat in the panel. If there is any load (a short circuit is a load) then some of the absorbed energy is exported from the panel to make its heat elsewhere.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • BB.BB. Super Moderators, Administrators Posts: 29,351 admin
    Re: Shorting a solar panel

    A short circuit does not have anywhere for the energy to go (nothing external to the panel gets hot, nothing is moved)--So, from what I can see, no energy is removed from the panel. There may be some sort of effect at the PN junction with current flow vs not--But I have not read anything one way or the other. In decade or so past, a shunted solar panel as a safety measure was suggested by one of the posters here (boB), but manufacturers did not want to support such a function--They believed it would shorten the life of the panels.

    In any case, you are only, at best, removing ~10-15% of the heat as electricity from the panel--Generally, that would have very little cooling effect.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    Re: Shorting a solar panel
    BB. wrote: »
    A short circuit does not have any where for the energy to go (nothing external to the panel gets hot, nothing is moved)--So, from what I can see, no energy is removed from the panel.

    In any case, you are only, at best, removing ~10-15% of the heat as electricity from the panel--Generally, that would have very little cooling effect.

    Unless your short circuit is superconducting there is some heat produced. I do agree with you that the effect is minimal and of little or no practical consequence. Sort of like turning off the radio in the car to get better gas mileage.
    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • mmmalmbergmmmalmberg Registered Users Posts: 13
    Re: Shorting a solar panel
    inetdog wrote: »
    Well, the problem most people have with shorting the panels is just that you are wasting energy. There is no point in diverting that energy to a fan or to heating unless you NEED that fan or heating. The panel will not get any hotter delivering current into a short circuit than it does just sitting in sun with an open circuit.

    Shorting the panels as a way of regulating charge to battery causes big current spikes that generate RF noise, can damage a partially shaded string of panels without bypass diodes (or can overstress underrated bypass diodes used in some cheap panels), and is just inelegant. The shorting device in the controller has to be able to handle the Isc current continuously.

    A simple series FET or bipolar transistor with the right drive circuitry will provide a good cutoff. Or you could go "solid-state" and use a relay. :-)

    Once the caps at 15V, the energy available from the cells is going to be wasted one way or another, either by opening the circuit or shorting it.

    Spikes make sense to me as something to worry about.

    The fan would be used to cool other electronics in the power supply. On a day that's generating enough power from the panels to get ahead of the devices ability to use it, there's going to be lots of motor usage, lots of charging/discharging, current limiting etc. going on so it would be an appropriate time for a fan as my enclosures are otherwise only convection cooled.
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Shorting a solar panel
    vtmaps wrote: »
    Sort of like turning off the radio in the car to get better gas mileage.
    --vtMaps

    Unless your radio is attached to a 1000 watt audio amplifier. :-)
    SMA SB 3000, old BP panels.
  • mmmalmbergmmmalmberg Registered Users Posts: 13
    Re: Shorting a solar panel
    BB. wrote: »
    At least in terms of charging your capacitor to 15 volts--That would be just a standard solar charge controller. The charge controller opens the circuit to the solar array--no current flow. Solar panels are perfectly OK with no current flow (personally, I think that is better than shorting the panel--You get I2R heating of the wiring and other current paths--which is never a good thing).

    Super caps also (typically) have a limited lifetime of a few years under voltage.

    Do you know why you are killing the lead acid battery? Over charging and discharging to zero State of Charge are both common reasons they get killed. NiCad and (to a lesser degree) NiMH batteries might be a more rugged tech for short/high current surge. Also, if you can run low voltage (two cells in series), you will avoid the over discharging problem where a weak cell reverses voltage and actually begins to reverse charge (a guaranteed cell killer for most rechargeable battery chemistries).

    -Bill

    Hi Bill. I haven't seen any small solar charge controllers for 15V. I'm using a small 12V solar controller for the SLA's, but the SLA's are 1.2AH (the biggest I can fit on these sculptures) and they need to be able to charge with the full output of the panels which is on the lines of 1.2A. So they get hammered on both charge and discharge, plus spend every night in a state of partial discharge. Caps seem to hold up to that sort of high charge/discharge just fine which is why the supercaps. These are the supercaps I'm using.
  • bill von novakbill von novak Solar Expert Posts: 849 ✭✭✭✭
    Re: Shorting a solar panel
    mmmalmberg wrote: »
    Hi Bill. I haven't seen any small solar charge controllers for 15V.

    Sunsaver 10. Charges to 14.4 volts, around $40.

    Xantrex C35. Charges to any voltage you like (settable) and it's around $80.
  • mmmalmbergmmmalmberg Registered Users Posts: 13
    Re: Shorting a solar panel
    Sunsaver 10. Charges to 14.4 volts, around $40.

    Xantrex C35. Charges to any voltage you like (settable) and it's around $80.

    I'm actually using a smaller Morningstar controller (SunGuard) for the SLAs. They're spec'd for 12V systems. Now that you mention it, it does sound familiar that it might charge to 14.4 'though I can't find that spec now anywhere on the website. Do you know if it's designed to trickle up to 14.4, i.e. some battery chargers I believe start backing off charge rate as the battery gets close to full charge.

    My original concept was to have the cap running a couple of volts above the battery most of the time so that it takes the brunt of (and makes the most of) the high charge rate the panels offer as well as the high discharge rate the motors can request. I might look though at re-topologizing my system and use the SunGuard to charge both if I can confirm that it charges full-rate to 14.4. The good thing being it's a 4 or 5 amp unit designed for a much bigger battery than the 1.2AH SLA's I have so its concept of slow topping-off charge might be enough.

    I've also used the Sunsaver 10 in another project but it's too big for this one. The Xantrax also is way too big (and $$$).
  • bill von novakbill von novak Solar Expert Posts: 849 ✭✭✭✭
    Re: Shorting a solar panel
    mmmalmberg wrote: »
    I'm actually using a smaller Morningstar controller (SunGuard) for the SLAs. They're spec'd for 12V systems. Now that you mention it, it does sound familiar that it might charge to 14.4 'though I can't find that spec now anywhere on the website. Do you know if it's designed to trickle up to 14.4, i.e. some battery chargers I believe start backing off charge rate as the battery gets close to full charge.

    Cheap ones don't. Better ones back off to float after a while (hours.) This is referred to as three stage or four stage charging.
    My original concept was to have the cap running a couple of volts above the battery most of the time so that it takes the brunt of (and makes the most of) the high charge rate the panels offer as well as the high discharge rate the motors can request.

    How will you down-regulate the 15 volts to the 13-14 volts the batteries need? Capacitors can help with transient loads, but once you add batteries you have to be a lot more careful with voltages. Also when you compare actual charge rates the caps aren't going to reduce max charge rates on your batteries by much. I'd recommend just getting some very good (small) batteries that can handle the higher charge/discharge currents. As someone else suggested, nicads are a good choice. There are also some good AGM's out there.
    I've also used the Sunsaver 10 in another project but it's too big for this one. The Xantrax also is way too big (and $$$).

    With all the time you've spent thinking and posting about this you should probably just build one; it sounds like you don't like any of the commercially available solutions. Plans for doing that are readily available on the net.
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