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Solar and Batteries and LED's questions


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Hello, First time poster and new member to this site, please be kind and direct me to further information or correct posting locations if I am incorrect in anything I am posting here.  :) thanks for your assistance.

Ok, Where do I start...

I guess my project outline is I am interested in Solar power.  I recently bought a SolarTek panel.

It's specs are:
Model : ST-20W
Rated Max Power (PMax) : 20Wp
Max Power Voltage (vmp) : 17.5v
Max Power Current (Imp) : 1.14A

I also have some batteries:
4 x RBC6 batteries (out of some APC UPS"s)

I also have a SunSaver 6 (by MorningStar)

I would outline this project as :
Provide LED lighting to the stairs in my house (side lighting to illuminate the stair) via batteries charged by the sun.

I have some LED's which I want to use to provide the light and a want to learn.

I have been testing the current setup and have a "working prototype" which seems to be working but is currently only illuminating one LED and I want to have 56 (or more, to provide more light).

I am using a 12v to 5v Car USB adapter to provide myself with 5v. ( i have placed a Diode in series with the working LED to drop the voltage )

What would you suggest I do to make a circuit which would be capable of handling 56 LED's (3.8V each) which would be safe to run continuously.

Please forgive my question if its very vague, I will respond to provide further information.

Thanks for your assistance again.

Zach

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What voltage and capacity are the batteries?

LEDs require a constant current source not voltage so connecting a diodes in series to power LEDs is not the correct way of doing it.

Your best bet is to connect two or three LEDs in series and run from 12V rather than one run from the USB converter which is probably just an inefficient linear regulator.

What's the current rating of the LEDs?

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The batteries are 12v but I'm not sure of the capacity.  They are gel-cell and are about 1/2 the size of a car battery, like the size of an Odessey battery.

I am also not sure of the current rating of the LED's.  They appear to be just normal white LED's.  I burned one out (kind of on purpose) attempting to find out what voltage they want to run at and it appears to be 3V-4V.

When I had 5V running thru one of them, it got hot and started to dim quickly so I guessed that was too much power.

I am using the 12V-5V usb car adapter because I thought that would be the easiest way of getting the voltage of the batteries down to something useful.

If the voltage of the LED's are supposed to be 3v-4v, how would I determine how many of them I would need to run in series to get the voltage down to what they require?

I am sorry if I am asking simple questions here but I am just getting into hobby electronics.

Thanks again for your assistance.

Zach

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Ok, Ive been doing some more reading and have found this little bit of information.

Is this correct?

"Each LED gives a voltage drop of 3.6V"  Thus using 4 LED's in series, I could effectively run the string without a resister or a voltage drop circuit.

and I would just have to run 14 strings of 4 LED's.

Right?

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The batteries are 12v but I'm not sure of the capacity.  They are gel-cell and are about 1/2 the size of a car battery, like the size of an Odessey battery.

Odyssey make a wide range of batteries so I'm afraid that doesn't mean much.

I am also not sure of the current rating of the LED's.  They appear to be just normal white LED's.  I burned one out (kind of on purpose) attempting to find out what voltage they want to run at and it appears to be 3V-4V.

That's what happens when an LED is connected up to a battery without a resistor.

Each LED gives a voltage drop of 3.6V"  Thus using 4 LED's in series, I could effectively run the string without a resister or a voltage drop circuit.

and I would just have to run 14 strings of 4 LED's.


No, I don't think you understood what I was saying about LEDs needing a constant current source before.

The LEDs should not be connected across a constant voltage source without a current limiting resistor. The LEDs require a constant current source which can be approximated by connecting a resistor in series, Google for LED tutorial and you'll find plenty of advice on calculating the correct value.

You should connect the LEDs in banks of two or three LEDs in series, each with its own series resistor to the 12V battery.

What size are the LEDs? Are they in a standard 5mm package? If so the recommended current will probably be about 20mA.

Suppose you have six LEDs, connecting them as two strings of three will draw less current but the LEDs will dim a lot as the battery discharges and they'll hardly light at voltage below 10.5V. If you connect the LEDs in three strings of two, they'll draw more current but they will dim less as the battery discharges and they will dim less as the battery discharges.
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Odyssey make a wide range of batteries so I'm afraid that doesn't mean much.

You are absolutely correct.  I was attempting to give a reference of size but I know that doesn't mean much when it comes to batteries.

They are 12v 12AH Sealed Lead-Acid batteries.  I have 4 of them connected in parallel.



The LEDs should not be connected across a constant voltage source without a current limiting resistor. The LEDs require a constant current source which can be approximated by connecting a resistor in series, Google for LED tutorial and you'll find plenty of advice on calculating the correct value.

Yes.  I googled LED Tutorials and found many resources.  I believe what I have as a working model right now is correct... The "Sun Saver 6" solar controller has a LOAD connection which limits the voltage to 12.9 V.  Therefore there will never be more than that given to the load.


You should connect the LEDs in banks of two or three LEDs in series, each with its own series resistor to the 12V battery.

Right now I have 3 in series and a diode, which drops the voltage by another .6v.  That makes the voltage right now across each LED 3.95v.  This might be over driving the LED's by a bit so I will have to make sure its correct before I put them in place.

What size are the LEDs? Are they in a standard 5mm package? If so the recommended current will probably be about 20mA.
 
They are 5mm white LED's

I will do more reading to determine exactly what is required to get this system to work as intended.

Thanks for your assistance and I am sorry my answers are vague, I realize that doesn't make it easy to work with.
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Yes.  I googled LED Tutorials and found many resources.  I believe what I have as a working model right now is correct... The "Sun Saver 6" solar controller has a LOAD connection which limits the voltage to 12.9 V.  Therefore there will never be more than that given to the load.

The battery won't fully charge at that voltage, to float charge a lead acid battery the voltage needs to be kept around 13.8V.

Right now I have 3 in series and a diode, which drops the voltage by another .6v.  That makes the voltage right now across each LED 3.95v.  This might be over driving the LED's by a bit so I will have to make sure its correct before I put them in place.

No, you still don't understand that LEDs need a current limited supply not a constant voltage supply, read the Wikipedia article.

Take out the diode and replace it with the correct value resistor.
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The battery won't fully charge at that voltage, to float charge a lead acid battery the voltage needs to be kept around 13.8V.


I was reading the wrong information about the SunSaver 6. It has regulated voltage to the batteries:
Regulation Voltage:
Sealed Battery 14.1 V
Flooded Battery 14.4 V

I think I finally get what you are talking about.  It was hard for me to wrap my head around it before because I didn't understand fundamentally how LED's worked.

I now understand that I must limit the current, lest a runaway situation happen where one LED heats up and starts to draw more current, which then causes it to fail, then the other LED's in the circuit fail too because of the current they take up.

Tell me if this is correct:

The maximum voltage the series of LED's will see is 14.1V (Under conditions where the Solar Panel is charging fully)
Each LED uses 3.2V and ~20MA of current

If I have 4 LED's in series, thats 12.8V at ~20MA

If I then subtract 12.8 from 14.1, that leaves me with 1.3V left over.

Putting that into OHM's Law Triangle I have 1.3V / 20MA = 65Ώ resistor.

Is this correct?

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That's right, the forward voltage of the LEDs is not very well controlled drops as they heat up, good you got there in the end.

The trouble is you haven't accounted for the fact that the battery voltage will drop as it discharges. The LEDs will probably not light if the voltage across them is below 3V each which is 12V in total and even then they'll be very dim which is why I recommend using three LEDs in series and a 220Ω resistor.

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I have an update for my project.  

I have worked out *most* of the issues but I have a question regarding current draw.

I have a SolarTeck panel, MicroStar SunSaver 6 Solar Controller, and 4 x 12V 18ah batteries.

I currently have 3 x 3.2v 20ma White LED's (approx 10,000 mcd) with a 220 OHM resister

I am measuring the current thru my Ostra Digital Meter.

I started the system up with a 500W light shining on the panel and I wanted to see the current drawn by the LED's for one complete circuit.

It started out at 14.21mA and after about 20 minutes its now reading 14.54mA.

My guess is that as the components warm up to working temperature, the resistances will level out and the current  will also level out.

Does this sound correct to all of you?

Solar Panel - > http://flic.kr/p/8KmYnv
SunSaver 6 - > http://flic.kr/p/8KmYri
LED Array - > http://flic.kr/p/8KmYu6
Batteries - > http://flic.kr/p/8KmYyk
Current Reading - > http://flic.kr/p/8Kq2Uu

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Another update.

I am measuring the current drawn by the batteries.

Am I correct in assuming that the Solar Controller is dictating how much current is going into the batteries by how much below full they are?

I am watching the current draw and on a sunny day, with no load on the batteries, the current dropped from 123mA and is now hovering around 45mA.

(I am measuring the current by connecting the meter in the following configuration)
Solar --- + side of meter - side of meter + side of solar controller --- battery

Can I assume that the batteries are now full and the controller is just supplying a float charge? (as in : As the batteries fill up, they will draw less current)

I have measured the batteries and they are all reading full.

Just for clarification.

Thanks
Zach

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It's difficult to say since you don't know what's going on inside the controller.

It sounds like it's float charging but that doesn't mean the batteries will be fully charged because float charging takes a long time to fully charge batteries.

What voltage do you actually measure across the batteries?

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