Shorting a solar panel

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  • mmmalmberg
    mmmalmberg Registered Users Posts: 13
    Re: Shorting a solar panel
    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.

    I'll measure the output on one of my SunGuard and see what kind of voltage it's outputting...
    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.

    The panels have an open circuit voltage of something like 22-23V. The batteries get charged through the Morningstars, and I have a small constant-current circuit in front of that so that the battery will only see 150 mA instead of the full output of the panels. This will also divert the rest of the panels output to the capacitors. In parallel, in the current configuration, the panels feed a 15V switching buck-boost regulator to charge the cap, and with which I'm having trouble and am going to replace with something simpler.

    This is the second time around for this project. The first time I was killing the SLA's and decided to replace them with big capacitors. My previous pieces have all run on capacitors only and have lasted six and seven years now, but they're smaller lower-power things. A bit later I decided to keep the batteries as a hybrid source along with the caps and get the best of both. By keeping the cap at a higher voltage than the battery, the battery can charge at an easy rate through the day while the cap does most or all of the work. Later as light fades and the cap voltage drops, the batteries can kick in and extend the awake time of the sculptures, and when the battery gets to its cut-out voltage, the cap has a lower cutout V. so that it can do a little bit of early evening lighting stuff.
    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.

    The amount of time I've put into this whole project so far is definitely insane - this little issue is the least of it to be honest but it's stopping me until I fix it... I'm out of time and need to solve this quickly and simply. I'd love to use something commercial if I can find something that will charge the caps quickly to 15V, are affordable (I'm over $2K into this and pretty much tapped out) and guaranteed to work. The buck-boost regulators I bought aren't working either because they're junk or because they're a bit too feature-rich to do only what I want them to do. I'm not sure what the best solution is so I can't really start building whatever circuits I find on the net and test to see what works. I'm stealing time from my day job to try to research this so that I can make the most of the evenings and two weekends I have left which includes finishing mechanical fabrication, programming etc. I've built several boards for other parts of the power supply, as well as a carrier board for the Arduino including an XBee tranceiver and an audio amp, but I don't have time for more board layout/printing/assembly, so I'll have to wire this point-to-point which is dissappointing:) In fact I still haven't thrown out the idea of one big fat zener:)
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Shorting a solar panel
    mmmalmberg wrote: »
    The panels have an open circuit voltage of something like 22-23V.

    Here's some info you may find helpful: open circuit Voltage of a solar panel is almost utterly meaningless. They are a current source, and the Voltage can be anywhere from zero to Vmp while outputting Imp. This makes them great for charging batteries. Not so great for charging capacitors (without help from some extra circuitry).

    Capacitors do not need to be charged to the rated Voltage, btw; they work fine below that level. Above it they tend to explode like blasting caps.
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Shorting a solar panel

    coot,
    as an aside to what you said of panel voc, this is important in figuring that nothing is going to be damaged by a pv's highest voltage at 25c so unimportant it isn't.
  • techntrek
    techntrek Solar Expert Posts: 1,372 ✭✭✭
    Re: Shorting a solar panel
    ...Capacitors do not need to be charged to the rated Voltage, btw; they work fine below that level. Above it they tend to explode like blasting caps.

    We used to have fun with this in electronics class way (way) back in high school with spare caps we didn't need for projects. :-)
    4.5 kw APC UPS powered by a Prius, 12 kw Generac, Honda EU3000is
  • mmmalmberg
    mmmalmberg Registered Users Posts: 13
    Re: Shorting a solar panel
    techntrek wrote: »
    We used to have fun with this in electronics class way (way) back in high school with spare caps we didn't need for projects. :-)

    Yes, definitely not my plan:) They're rated 16.2V - I feel like I'm pushing it at 15, as I think they have a longer life if de-rated a bit.

    Tested the the Morningstar SunGuard last night, was able to charge the cap to 14.5V using a pair of SLA's in series as a source. Moving forward with testing new plan of charging both the cap and the battery to 14.5V via the Morningstar, for the time being ditching the original plan, shipping something that works well enough is better than not shipping something that will eventually work great...

    Will move constant current circuit from before the charge controller to after, that being what most needs tested. Hoping the charge controller is not too smart else it might be confused.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Shorting a solar panel
    mmmalmberg wrote: »
    Yes, definitely not my plan:) They're rated 16.2V - I feel like I'm pushing it at 15, as I think they have a longer life if de-rated a bit.

    Tested the the Morningstar SunGuard last night, was able to charge the cap to 14.5V using a pair of SLA's in series as a source. Moving forward with testing new plan of charging both the cap and the battery to 14.5V via the Morningstar, for the time being ditching the original plan, shipping something that works well enough is better than not shipping something that will eventually work great...

    Will move constant current circuit from before the charge controller to after, that being what most needs tested. Hoping the charge controller is not too smart else it might be confused.

    Basically the SunGuard is going to try to push the Voltage on its output to 14.1 and keep it there. It's not fancy. You'd probably have more trouble with a more elaborate controller.
  • mmmalmberg
    mmmalmberg Registered Users Posts: 13
    Re: Shorting a solar panel
    Basically the SunGuard is going to try to push the Voltage on its output to 14.1 and keep it there. It's not fancy. You'd probably have more trouble with a more elaborate controller.

    Perfect. Here's the new topology I'll be testing. Hoping for the best... Finally found the spec; panel output is 1.7 amps, 23.8V open. Attachment not found.