Very small system: does it need a controller?

[DamonHD]

Hi,

I'm putting together a very small (off-grid 12V solar PV) system to be used to show young school-children and as an exhibit.

I have a ~3Ah SLA and a 1.5W (amorphous) solar panel.  I can't work out whether the panel is big enough relative to the battery to need a controller.  The whole thing is likely to be neglected most of the time, so undercharge is as big a risk or bigger than over-charge.

Can anyone on here muster an opinion, knowing how shy we all are?  B^>

Rgds

Damon

[DanG]

Crying and throwing tantrums in the controller candy aisle would simply reflect many of the posts here.

When Kiddo tells Mum about Ghurd Controllers at the dinner table, and smart children demand PWM or the cleverest MPPT, and Schottky diodes are discussed on the classes' facebook page you've accomplished your mission. Expect to be harangued over that SLA shortcut when they've cornered you upon the gel-cell minimal self-discharge bonuses. And then there is the omni-present temperature compensation issue you'd be neglectful of (again) if there was no controller costing more than the battery and panel combined.

Seriously, DamonHD - what can you tag a 1.5W panel and 3AH battery as 'doing' that would be a clear and present benefit?  Why not dissect a variety of garden path lamps and floodlights to keep the utility aspect closer to home? The Solar powered bird-bath fountains can be a delightful introduction too. They have a small controller circuit present AND are scaled more kid-sized. Then perhaps the larger panel & battery to show portability of the stored power - you'll have them organizing RE-Raves in the woods behind the school in no time!

[SparWeb]

1.5 Watt / 12volt = 0.13 Amps

0.13A / 3A-h = 0.043  or   "C/23"  charge ratio.   Sounds fine to me.  Skip the charge regulator.

-But-

For demonstration purposes, most people expect immediate, visible results.  It's more likely that you will need to put a Load in the system, like a motor, a light, or whatnot, that DOES something when the sun shines.  Once there's a load on the battery, you suddenly need a low-voltage disconnect.  Otherwise the battery will spend 90% of its life run flat.

This turns your question about panel-vs-battery charging controller into a question on the other side:  battery-vs-load control.

If you build a Ghurd kit, set up for load control, and mount it into a little clear perspex box, you will have something nifty to look at in the demo, and it will operate a little lamp or other "interesting" machine.

[DamonHD]

Yes, SparWeb, I was thinking that LVD might be more important, either using the ghurd-megacorp-inc design or one of my own that cheats by using a nice 8-pin DIL chip driving a high-side FET:

http://www.earth.org.uk/low-voltage-drop-out-circuit-design.html

I have a nice bright reflector LED and I intend to get/incorporate a little cooling fan too, since people will see the value of that in hot weather.  I don't expect both to be left on continuously, and indeed may make up a smaller load that could be (like a pretty LED fairy-light display).

This thing should be robust enough to be left out in the playground in the sun much of the time, and will probably be in the form of a wooden handcart so we can take it round several places to see.  (That would make the bird-bath thing tricky.)  If it proves very popular then I'll upgrade the panel and battery and include a grown-up controller with LVD.

My kids' school just has had a huge PV array plonked on its roof, so the kids can compare and contrast the micro off-grid with the chunky on-grid technology.

Rgds

Damon

[OperaHouse]

Why not use one of these LM2596 regulators.  Less than $3 shipped to you.  Just looking for a reason to post this. Add a diode o the output to prevent back flow.

You can add multiple ones of these for quite a bit of current.  Lifting one pin you can also turn it into a power point controller.

[DamonHD]

Hmmmm that looks nice.  Can it be persuaded to do the LVD part?

Looks like an up-market version of the the reg that I currently use for my SheevaPlug.

Rgds

Damon

[ghurd]

A decent 1.5W PV will overvoltage a ~3AH SLA.

A VW PV is rated 3.2W?  (most put out a lot more than 3.2W)
But they will get a 7AH SLA overvoltage in a very short time.
A VW PV and 7AH SLA is what I did all the first round of testing on all the circuits on the road to the ghurd controller.

Might look at PB137ACV.  Small solar controller... TO-220 and 2 caps... thats all.
http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00001634.pdf

G-

[DamonHD]

Ah, very interesting, thanks.  Happily, RS stocks PB137ACV at very nearly $0, so time to order one and a Raspbery PI or three!

Given all that I might just throw in the cheap no-name controller that came with one of my Maplin purchases, which probably has charge and load control maybe possibly.

So I now want a 4-terminal complete PWN solar controller and LVD, and all for about $0.

Rgds,

Damon

[independent]

Which pin?

[SparWeb]

Damon,

If you do get a Rhaspberry pi and try it out, please lete know how it goes!

[DamonHD]

Roger Wilco.

None in stock yet (they're brewing up a big batch), so no order placed yet.

Rgds

Damon