Outdoor lighting project

Ok, so as the title describes, I'm looking to build some outdoor lights. Now for some elaboration, I want to make some deck lights using LED's and some rechargeable batteries.

So far my project is in the R&D state.

What I'm thinking is using 4 rechargeable batteries in a parallel circuit to keep the voltage at about 1.5V

Now I know basics of electronics, and this project shouldn't be too complex with a little help and the internet. But what I'm thinking is putting a 1 Watt solar panel at the beginning of the circuit to constantly, or charge the batteries under control with the help of a switch.

Basically I want to know if there's an easy way to trickle charge, or automatically turn off the charge when the batteries are full? That's my first problem.

Now my second problem is that the solar panel I have fluctuates from 0 (nigh time) to about 20 Volts. So I'm wondering if I need to make a small voltage regulator circuit to lower it to 5V when I'm charging the batteries? Or maybe even regulate it, then lower it to 2V or 1.5V to make sure the batteries don't over heat.

Basically what I have right now (I don't have a schematic program, so please excuse the written version) I have the Solar panel as the main power source going to a 200 µF electrolytic capacitor in parallel, then to a 7805, followed by a second electrolytic capacitor in parallel, then after that I can throw in a diode or a resistor to lower the voltage in series, then put the batteries next in the circuit in parallel so they keep their voltage. After that I have my lights and that's all easy after that.

Sorry for the long message, but if you're still reading, then great, I'll sum up. What I need to know, is will that circuit work properly? Also is there a transistor, or a micro-controller I can add in to see when the batteries are fully charged, and make the solar panel stop charging, or trickle charge?

Any help would be very appreciated, thank you

Log2 said:

What I'm thinking is using 4 rechargeable batteries in a parallel circuit to keep the voltage at about 1.5V

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1.5V is too low to run an LED without a converter.

Log2 said:

Basically I want to know if there's an easy way to trickle charge, or automatically turn off the charge when the batteries are full? That's my first problem.

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You can use a NiMH/NiCd charge controller IC. It's best to set it up to work in SMPS mode otherwise you'll be wasting a lot of power trying to charge your 1.5V batteries from 20V -- i.e. 1W 12V panel puts out around 58mA, but 58mA into 1.5V is only 88mW! Using a linear regulator will waste the other 912mW; using a SMPS will be much better.

Log2 said:

Now my second problem is that the solar panel I have fluctuates from 0 (nigh time) to about 20 Volts. So I'm wondering if I need to make a small voltage regulator circuit to lower it to 5V when I'm charging the batteries? Or maybe even regulate it, then lower it to 2V or 1.5V to make sure the batteries don't over heat.

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If you were going to do that (linear regulator), you might as well directly connect the panel to the batteries through a diode (to stop discharge during the night) & you'll actually get more power into the batteries as there's no regulator quiescent current.

I'll have a look for a charge controller IC for you.

well what if I were to put 2 sets of parallel batteries in series, so it would theoretically double the voltage. Also I know a bit about circuitry, but I'm not familiar with a lot of it. So I don't know about this SMPS mode and such

Also 20V is the Max, during peak, it is a 12V solar panel, so it would, most of the time, be running at that on a sunny day as far as I can tell. I haven't tested it all out yet, gonna start tests and readings sometime this week hopefully.

dougy83 said:

If you were going to do that (linear regulator), you might as well directly connect the panel to the batteries through a diode

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By this you mean just use a diode to lower the voltage to 4v or similar? The only reason I decided to use the regulator was because the 7805 drops the voltage to 5V so I could then insert what I needed to drop it another couple volts, knowing that I wont be lowering it too much. For example if I have a diode that drops the voltage 7 volts, if it's a cloudy day and I'm only receiving 7 volts, I wouldn't be charging at all... Unless I've misunderstood how to use the diode, which is completely likely, lol

Sorry, I got sidetracked & didn't look for the IC...

Log2 said:

well what if I were to put 2 sets of parallel batteries in series, so it would theoretically double the voltage.

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Both theoretically & practically What's the forward voltage of the LEDs - your battery voltage needs to be higher than the forward LED voltage or you will need an up-converter.

Log2 said:

Also I know a bit about circuitry, but I'm not familiar with a lot of it. So I don't know about this SMPS mode and such

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You can gloss over Switched-mode power supply - Wikipedia, the free encyclopedia if you wish. Basically switch-mode conversion is more efficient than linear conversion.

Log2 said:

Also 20V is the Max, during peak, it is a 12V solar panel, so it would, most of the time, be running at that on a sunny day as far as I can tell. I haven't tested it all out yet, gonna start tests and readings sometime this week hopefully.

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For a 12V panel, the voltage at which it puts out its rated power is usually more than 12V, e.g. 17V. Measuring the Power of A Solar Panel - Solar

Log2 said:

By this you mean just use a diode to lower the voltage to 4v or similar? The only reason I decided to use the regulator was because the 7805 drops the voltage to 5V so I could then insert what I needed to drop it another couple volts, knowing that I wont be lowering it too much. For example if I have a diode that drops the voltage 7 volts, if it's a cloudy day and I'm only receiving 7 volts, I wouldn't be charging at all...

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If you connect a 1.5V battery across the solar panel, the panel will output 1.5V, the panel acting as a quasi current source. The only problem is that you won't get anywhere near your 1W into your battery if you don't use a switch-mode converter.

Well I'm not worried about losing power at all, because I only need to recharge the batteries. The lights wont be on constantly, and I doubt they would even be on more than 2 nights a week, so that's why I'd like the trickle charge. And the rest of the week, I would only assume the solar panels would be charging the batteries.

As for the forward voltage, I'm not sure what it is, I haven't purchased any parts yet, mainly because I want a full parts list before I go out and buy stuff, but I was it to be a white led, and have 3 or 4 leds in each light, have about 6 - 8 lights.

I could even do 6 batteries total, increasing the voltage another 1.2 - 1.5 volts. As for this quasi current source, would I have to hook up the batteries before I hook up the solar panel, then re-route the solar panel to charge the batteries in some way? I would assume just hooking up the panel in series with the ~4.5 volts going in would force the solar panel to act at 4.5 volts? I'm just guessing this from what you've told me, not sure actually how to put this into practice.

By the way, thank you for all these lessons about solar, and circuitry, and for not thinking (or at least saying) I'm a complete idiot, for what I assume are noob questions, lol

Log2 said:

As for the forward voltage, I'm not sure what it is, I haven't purchased any parts yet, mainly because I want a full parts list before I go out and buy stuff, but I was it to be a white led, and have 3 or 4 leds in each light, have about 6 - 8 lights.

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White LEDs generally have a forward voltage of 3-4 volts each.

Log2 said:

I could even do 6 batteries total, increasing the voltage another 1.2 - 1.5 volts. As for this quasi current source, would I have to hook up the batteries before I hook up the solar panel, then re-route the solar panel to charge the batteries in some way?

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I don't know what you mean. Just connect the batteries to the solar panel (with a series diode); then connect the LEDs and appropriate driver to the batteries (with a series switch).

Log2 said:

I would assume just hooking up the panel in series with the ~4.5 volts going in would force the solar panel to act at 4.5 volts? I'm just guessing this from what you've told me, not sure actually how to put this into practice.

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That's pretty much it.

Log2 said:

, and for not thinking (or at least saying) I'm a complete idiot, ... lol

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lol, indeed!


I take it you want the simplest solution, and not one that'll win any awards for efficiency, etc. If you connect 4 battery cells in series, you have 4.8-6V. If the LED has a forward voltage of 3.4, connecting a 130 ohm resistor in series with it and the battery will allow between 10.8-20mA to flow through the LED. If you do this for each of your 32 LEDs, the total current is 640 mA, and a 2A.Hr battery will die in 3 hours from full charge.

If you wanted to go a step up and use a SMPS to drive the LEDs, the battery life would be around 2.5x the above.

As far as trickle charging the battery goes... you may want to connect a 6.8V 2W zener across the solar panel to try to stop it overcharging the batteries.

Ok, just doing a bit more research here, looks like to get the 6-8 AA batteries, would be around $60 CAD so I might just get a 12 volt (which would have a higher AHr) for $40 CAD but if I were going to go that route, would I hook the 12 Volt solar panel directly into that, or would I have to follow the same solution that we derived, except with a bigger resistor?

And my solar panel states that it has "Built-in overcharge/discharge protection" which I assume is only for the 12 volt application.

What are your thoughts on this?

If you use a small lead acid battery you can just hook the solar panel to it. The problem is that in your first post you say the solar panel is 1 watt. So on a bright sunny day will give you 12 volts but only .08 amps. So if each led takes .02 amps you can only run 4 leds before you start loosing ground. Need a bigger solar panel.

Log2 said:

Ok, just doing a bit more research here, looks like to get the 6-8 AA batteries, would be around $60 CAD so I might just get a 12 volt (which would have a higher AHr) for $40 CAD but if I were going to go that route, would I hook the 12 Volt solar panel directly into that, or would I have to follow the same solution that we derived, except with a bigger resistor?

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It sounds pretty darn expensive.

Anyway, if you get a 12V lead acid battery, you can connect strings of 3 LEDs and a 180 ohm resistor in series for each lamp. If the battery voltage ranges from 13.8V (fully charged) to 11.2V (flat), the LED current will range from 20mA to 6.7mA (or LED Vf of 3.4V).

ronv said:

So on a bright sunny day will give you 12 volts but only .08 amps. So if each led takes .02 amps you can only run 4 leds before you start loosing ground.

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I wont be running it during the day in the first place, therefor, the batteries should be fully charged at night.

dougy83 said:

you can connect strings of 3 LEDs and a 180 ohm resistor in series for each lamp

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So if a put a 180 ohm resistor then my 3 leds in series, then my other lights in parallel all the lights should be the same brightness?

I downloaded a free schematic/circuit tester, and I tried building a simple circuit, with the 180 ohm and the 3 leds and I put the voltmeter across it and it said the voltage dropped from something like 2.5 after the resistor to .75 before the last led, is that what's supposed to happen?

Log2 said:

So if a put a 180 ohm resistor then my 3 leds in series, then my other lights in parallel all the lights should be the same brightness?

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They should be reasonably similar.

Log2 said:

I downloaded a free schematic/circuit tester, and I tried building a simple circuit, with the 180 ohm and the 3 leds and I put the voltmeter across it and it said the voltage dropped from something like 2.5 after the resistor to .75 before the last led, is that what's supposed to happen?

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No. It sounds like you are using diodes (with a voltage drop of 0.75V) not LEDs. For 3x 3.4V white LEDs, the voltage after the resistor should be 10.2V.

ok, well you obviously know a lot more about it than I, so I'll trust you on that, haha.

You've been very helpful, but I have 1 last question, what should the power rating be on the resistors I buy? I see a lot of 1W or 5W, then there are like 100mW and so on, what's the minimum I should get? And what tolerance should I get?

All the above has been for assumed 5mm white LEDs having a forward voltage of 3.4V and a max current of 20mA.

As for your question re resistors: power is I²R, so for 20mA through a 180ohm resistor that's 72mW; anything rated above that is fine. Tolerance of 1% or 5%, it doesn't really matter - it just changes the current by 1%/5%.

awesome, looks like I can get the project off the ground now, thanks to you. You've been very helpful and very informative, rep+

Hopefully I wont be coming back with some blown parts, haha, again, thank you!

You're welcome. All the best with your project.

The way I have chosen will be the cheapest way.

12 V Lead Acid Battery rated for 7AHr - 28.99
50 White Led's - 17.60
30 180ohm Resistors - 0.24
Switch - 5.00
Pot Lights - Free (Tuna Cans)
Plexi Glass - Free (Have a lot laying around)

Total: 51.83

Now if I wanted to use AA batteries, they cost 24.99 for a 4 pack rated at 2400mAHr and I figured I would need at least 6 AA Batteries for them to last 3+ hours, so I would need to buy 2 packs costing 49.98

As for not being able to use the 180 ohm resistor, you didn't provide a reason, The led's are rated for 20mA, and the white takes 3.4V, so please elaborate as to why I wont be able to do that?

Are you buying your extremely expensive parts at RadioShack? Their prices are 3 times higher than normal electronic parts distributors.

Six AA Ni-MH rechargeable cells in series produce only 7.2V, not the 11.2V to 13.8V from a lead-acid battery that the circuit was designed to use.

No, I'm Canadian, everything is already over priced here, don't get me started, but I am ordering online, cheapest method possible, unfortunately shipping is ridiculous if I go to american sites

I'm sure you know all about it looking at your listed location, haha

Here in Canada when I order a bunch of low priced parts from American Digikey or American Newark before 8:00PM then they are delivered to me the next morning. But I wouldn't order heavy rechargeable batteries. I buy Energizer Ni-MH cells locally from whichever grocery store (Loblaws) or department store (Canadian Tire or Zellers) has them on sale.

Yea I found Newark, but they tack on an extra 12 bucks to ship to Canada, then they have all the border taxes and whatnot (Unless I've misunderstood and that's all included in the 12 bucks, haven't actually ordered yet) However, that battery is cheaper than anything I can find locally, minimum 35 dollars