How to run some lights from a solar panel?
March 30, 2011 11:44 PM   Subscribe

What is a simple way of connecting a PV panel to a battery to a set of lights?

Hi. I’d like to hook up a small (40-50 watt) solar panel to a battery (like a car battery) to a few LED lights (less than 10 watts). I’m not interested in powering my own home at this point, just making a sort of set-up that allows me to experiment. I’m looking for advice on how to best do this, but I don’t want to buy a kit. I think I know how to calculate my watt-hours, but I’m fuzzy on AC/DC power, and whether to buy a PV panel or make my own from some solar cells. Are a charge controller and inverter necessary? I’d like to do this as cheaply as possible (under 200 USD)- any suggestions from your experiences? It’s complex, so web page links are fine; ones with pictures and diagrams would be useful since I’m a visual learner. Thanks in advance.
posted by eggrollover to Technology (9 answers total) 7 users marked this as a favorite
Best answer: A charge controlled is desirable because it will handle everything and make everything simple and reliable and effective. I think it will be worth it to you as a simple ilver bullet that solves a lot of issues you would otherwise have to deal with.
It will use the solar panel to charge the battery, but it won't let the battery overcharge. Likewise, it will allow the lamps to run off the battery, but won't let them run it so flat that they ruin it.

You have no use for converting anything to AC power. Solar panels are DC, batteries are DC, and LEDs are DC. The only reason some people add AC inverters is so they can have a "wall socket" outlet from which to run regular mains-power devices. Ignore AC for now. If you want mains AC, it's trivial to get a car inverter and use that.

40W panel - $100 on ebay
Charge controller - $50 (I think I've seen them for as little as $15)
If you start with an old/used car or motorcycle battery, I think you can do the rest for under $50
posted by -harlequin- at 12:00 AM on March 31, 2011

Oh, I've also made several panels from cells, but looking at the prices that some panels are going for now, it doesn't seem worth it any more. The price has dropped so much over the last few years that I think once you take into account the materials you'll need in addition to the cells, you aren't saving any money, and you're certainly putting in a lot of time, especially if you want to match the cells to each other for best performance (which in turn requires buying more cells than you'll use).

I'd recommend just buying a panel, unless you need the panel to fit custom dimensions.

One other solar note/rant - avoid the garden pathway solar lamps. They are terrible disgusting things. They use toxic NiCd batteries instead of NiMH because they don't have proper charge controllers, and NiCd is more resistant to the resulting over-discharge. Since they over-discharge the battery every single day, the battery only lasts a few months or a year, and then the lamp stops working properly, at which point not only is it likely to become toxic landfill, it's likely to be thrown out with a solar panel which would still be good for another 20+ years for power generation. And to top it all off, the lights don't even work - they don't store enough power to make a useful amount of light.
They make the dirtiest coal power look environmentally responsible in comparison, and yet are marketed as green. Pathway lights should use a few cents-worth of wire instead of solar panels and batteries and other assorted crap.

If you're interested in playing around with solar, you could consider finding the garden lamps that people throw out because they've stopped working. Recycle the NiCd, and hey - free solar cells!

posted by -harlequin- at 12:15 AM on March 31, 2011 [2 favorites]

Search for info on "MPPT controllers" for better efficiency. Might be OTT for your application though?
posted by gallagho at 2:11 AM on March 31, 2011

All you need is panel, charge controller, and battery. Without the charge controller it'll be a nightmare to operate the system. If possible, run your LEDs or whatever directly off the battery voltage (12V) and stay away from both DC/DC converters and inverters as long as possible, since every time you convert the power into another form you take a (sometimes major) efficiency hit.

If you can afford it you really want a deep-cycle (marine) battery for this application, not a car battery. Car batteries are designed to slam out a huge amount of current reliably but suck for trickling it out for long periods (eg, they will fail after 50-100 trickle charge-discharge cycles). Deep cycle batteries are designed for this sort of application and last a lot longer.
posted by range at 7:10 AM on March 31, 2011

Hi. I looked into doing a similar project a couple of years ago. I found this site, How to Make a Solar Power Generator for less than $300, very useful. My plan is to build a system simlar to that but with a charge controller.

I agree with -harlequin-, buy a panel versus making a panel from individual cells. I use these 15W Sunforce panels which you can find at Amazon or Northern Tool.

I would definitely recommend a charge controller because they prevent overcharging the battery. CCs aren't that expensive, here is one on Amazon for $17.

I agree with range that you should use a deep cycle battery if possible for the reasons he mentioned. My dream batteries are $130 each, so I use $50 marine batteries I found at Wal-Mart.

Buld It Solar has some interesting solar project ideas.
posted by Rob Rockets at 9:59 AM on March 31, 2011

Yup. You need a charge controller so you don't overcharge your battery, but don't need an inverter. With a system this small, you really don't even *want* an inverter, you're much better off only running things on 12V. In addition to 12V LED's, you can also get 12V incandescent or florescent light bulbs with standard Edison (your household screw-in) bases if you want. Obviously the incandescents draw by far the most power and the LED's the least.

You didn't say where you're using this, but a car stereo is another fun item that's designed to run on 12V.

A deep cycle battery is desirable, but not necessary unless you're discharging it considerably. You won't manage to do that if you just run a few LED's. You will be amazed how long you can power LED's with a garbage car battery. You can always go to a better battery later, but then you'll want to avoid leaving it too discharged for too long as you'll reduce its maximum capacity that way.

One other thing you'll want is a digital volt meter (about $10), unless there's one built into your charge controller. Then you can tell how charged your battery is, though it'll only be accurate when there's no sun on your panels. 12.6V is fully charged, anything much higher than that is a surface charge. 12.25V is about 50%, and you really want to avoid going below that if possible.

The 15W Sunforce panels referenced above work well, and it's super easy to start with one and add more later.
posted by LowellLarson at 12:26 PM on March 31, 2011

Car batteries are designed to slam out a huge amount of current reliably but suck for trickling it out for long periods

Actually the thing you shouldn't do with car batteries is deep discharge them. As long as you don't over-discharge, they will work quite well.

One other point, just in case you're tempted to go the ultra-simple route, do NOT connect your panel directly to the battery unless you have a blocking diode in the circuit [the panel may include one].
posted by HiroProtagonist at 7:05 PM on March 31, 2011

Response by poster: Thanks so much, everyone. Now for wiring: I'm assuming most PV panels come with two little +/- wires that stick out the back. Is it safe to connect the wires of both the panel and the lights to the battery? And what prevents the lights from turning on automatically? Should I wire in a switch between the light and the battery?
posted by eggrollover at 1:34 AM on April 1, 2011

Yes, the panels have +/- wires off from them. Those go to your charge controller, and the wires from the charge controller go to the battery. Your lights also connect to the battery. If you just wire them directly to the battery, they'll be on all the time except for when the battery is dead. You could wire a switch into the line for them, or if you want to keep it simple just put clamps on the end of the wire for the lights and clamp them on/off.
posted by LowellLarson at 6:41 AM on April 1, 2011

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