# Cheapest LED light

Discussion in 'LEDs and Optoelectronics' started by Shine, Jul 31, 2012.

1. ### Shine

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Jul 29, 2012
Hi friends,
I am still new to electronics and have been trying to build a cheapest solar powered LED light using recycled plastic bottles using the following components;
1. 2 LEDs, Vf 3.0~3.2
2. Battery 4V, 4Ah
3. Resistor 3 ohm (Please suggest if it is suitable)
4. Blocking diode (dunno about diode, pls suggest what would be the best)
5. Female jack (to charge the battery)
6. Switch
7. 1 Watt, max output - 8.3 V solar panel
I connect the 2 LEDs in parallel and install the resistor and switch. I don't know what type of blocking diode I should use. I bought many of 10V 5 ah diodes from electronics shop. They are very big and they block some current at first but they do not block the current at all after a few days. Then I put everything in the bottle with the switch and female jack installed on the bottle.
Please, also suggest what other component should I use for the light? This LED lamp is intended to introduce millions of poor households.
Thanks

2. ### Harald KappModeratorModerator

11,268
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Nov 17, 2011
Show us a diagram of your circuit so we can understand better. Without diagram my answer can only be guesswork.

1) Do not connect the LEDs in parallel. Due to small differences in the LED's characteristics, the one with the lower Vf will draw more current and can be destroyed due to overheating. After that, all the current will flow through the remaining LED and this one will be destroyed too..
Use a separate resistor for each LED.

2) o.k.

3) Where is the resistor located? Between panel and battery or between battery and LED? I assume it is between battery and LED where it is required.
The resistor can be calculated as follows:
R= (Vbat-Vled)/Iled = 0.9V/Iled (assumin a mean LED voltage of 3.1V. With R=3 Ohm you get Iled=300mA. Since you didn't state the operating current of the LED, we can't tell you whether that is the correct value for the type of LED you use. Note that with the LEDs having separate series resistors (as suggested in part 1 of my answer) you will need twice that resistance since the current is no longer split between the diodes.

4) Any diode that can carry the current will do. A suitable standard diode could be 1N4001.

5) You will need a male counterpart (or is it part of the solar panel?). Apart from that you should not connect the battery directly to the solar panel. Assume a battery voltage of 4V and a diode voltage of 0.7V. That means that the solar panel sees a voltage of 4.7V at the terminals. The output voltage of the solar panel will break down to match that voltage. You wil have to study the solar panel's characteristics to find out what power it will output inder this condition. You should at least add a current limikting resistor in series with the diode. A value of approx. 33 Ohm would give a charge current of approx. 100mA. Note that a 4Ah battery will take more than 40 hours to fully charge. Even if you can get the max. current of 120mA (1W/8.3V) it will take more than 33 hours.
A better solution is a specialized battery charger IC using switch mode technology. This will allow you to use approx. 0.8W at 4V output voltage (assuming an efficiency of 80%) which translates into a charge current of approx. 200mA. Thus the batttery will be fully charged in a bit more than 20 hours.
Overall, I think the solar panel is too weak or the battery's capacity is to high as you will barely stand a chance to have 20 hours of continued sunshine to fully charge the battery.

6) o.k.

7) See 5

By the way: Why open a new thread when you have already one on this topic https://www.electronicspoint.com/protect-battery-being-over-used-and-overcharged-t250597.html ?

Harald

Last edited: Jul 31, 2012
3. ### donkey

1,293
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Feb 26, 2011
only 1 little issue I find is what are you doing to stop it overcharging? this could ruin the battery.

4. ### Harald KappModeratorModerator

11,268
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Nov 17, 2011
A specialized battery charger IC should have an overcharge protection. One more reason to recommend such a solution.

5. ### Shine

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Jul 29, 2012
Thanks you, Harald and Donkey for your excellent comments and advices. Due to my lack of knowledge in electronics, I hesitate a little about what to do next.

LEDs

Should I connect the LEDs in series? So, please suggest how I can connect the resistors. I can't find the operating current for the LEDs on product package.
Battery

Since the solar panel is weak, I think if it I should use AA rechargeable batteries. Which are better AA rechargeable batteries or lead-acid batteries? What about differences between the lifetime of the AA batteries and lead-acid batteries? We do not expect the battery life to be longer than 2 or 3 months.
Resistor
Since the LEDs require a separate resistor for each as you said, could you please suggest how can I connect them?
Diode
How can I connect the diode with the battery?
Solar Panel
Yes solar panel has a male counterpart. It seems like the power from the solar panel breaks down to match the voltage of the battery. What if we do not install the blocking diode and avoid charging the battery under low sunshine? Since the lamp does not have a circuit, is the IC easy to install? How can I stall it?

PS: How can I attach the pictures? Please enter the URL of your image: What should I type here?

Thanks

Shine

6. ### Shine

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Jul 29, 2012
Attachments

Here comes the pictures!

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7. ### Harald KappModeratorModerator

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Nov 17, 2011
Read this section to get an idea how to connect the LEDs.
With 3 V LEDs and a 4 V battery you cannot connect the LEDs in series (well you can, but there would be no light ).

The batttery you use depends on what average charge you expect. Example:
- Assume the charger can deliver sufficient voltage at a charge current of 200 mA (let us for a moment put the issue of the voltage aside).
- Assume that on the average you have 8 hours of daylight to charge the batteries.
This means that the charger will output typically 0.2A*8h=1.6Ah of charge per day.

- Assume a charging efficiency of 80 %, that is only 80 % of the charge sent to the battery during charging are stored, 20 % are lost as heat.
This means that of the 1.6Ah output of the charger onl 0.8*1.6Ah=1,28AH aer really stored as energy in the battery.

- Since your LEDs require 3.2 V in the max. case, a 4 V battery is a suitable choice. So the parameters of the battery would be 4V/1.3 Ah (or a bit more Ah. e.g. 1.5AH, whatever you can get hold of).
An expected battery life of 2-3 month is very low. Have you considered the environmental impact of all those batteries containing dangerous chemicals being disposed of in such rapid succession? What will be the cost of recycling them - if they are recycled at all? And what will be the cost of new batteries? Will that still be affordable for the audience you want to supply?

- On part 3 of my previous answer I told you already how to calculate the right resistor. Since you still do not give the expected operating conditions (voltage, current) of the LED, I cannot help more.

The design of a solar charger for lead acid batteries is not that simple. See e.g. here: http://www.den-uijl.nl/electronics/solar.html
By no means should you leave out the blocking diode. The anode (plus) of the diode should be connected to "+" of the solar panel, the cathode (minus) of the diode should be connected to the "+"input of the charger circuit.

Harald
1) if the image is already available on an internet server (flickr or similar), you can put in the URL of that image directly into the input mask.
2) if the image is on you computer, upload it as an attachment (as you've done) and copy the URL (link) of the uploaded image from the upload manager's window. Insert this URL into the image-tag.

Harald

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9. ### Shine

16
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Jul 29, 2012
You gave me many things to consider. I'd better follow Donkey's advice that I can replicate the circuit from solar garden lights and other solar lighting products.

I forgot to mention that I have about 200 pics of AA rechargeable which I purchased from China. They are no longer in good condition. That's why the expected lifetime for the batteries I mentioned is very low.

Anyway, I am very grateful to you for your help thus far. Perhaps I should improve my knowledge, and come back to you with more details.

Thanks and best regards

Shine

10. ### donkey

1,293
56
Feb 26, 2011
you can also just hit us up for information too. but the best way to gain knowledge is have something in front of you so you can see what to do.

11. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
See if you can manage to use LiPo. They can be charged to a fixed voltage and not get damaged. I have some solar lights that use a *very* small LiPo battery and a specialised charger IC and they give me no trouble at all.

Unfortunately, finding one of these chips is a totally different problem

12. ### donkey

1,293
56
Feb 26, 2011
what sort of AA batteries did you buy? ni-mh /ni-cd?
if you are using these for this project please tread carefully. whille AA batteries are considered safe they still can get a little acid build up, I have seen a few "POP" as well. and thats the basic batteries, if you have bad LiPo batteries then that needs extreme caution as I have seen car fires, house fires and multiple othe fires from incorrect charging.
if you get a chip for it like steve has and use it correctly then you should have no worries though

13. ### Shine

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Jul 29, 2012
Hi all

I have a 150W solar panel which generates an output DC voltage of 200V and amperage of 18.5 ah.

This panel is to charge my 12V, 90Ah battery.

Please recommend me a suitable type of charging controller/battery charger I should use to match the battery and solar panel.

Since the output voltage is really high, I am afraid the battery and the controller will be damaged.

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Nov 17, 2011
15. ### Shine

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Jul 29, 2012
Yes, Ni-Mh AA rechargeable batteries. Do you mean lead-acid batteries are better than AA Ni-Mh batteries? By the way, I've got one more questions. I've a solar LED bulb. It has a very small built-in battery. I attach the pic of the battery(I am not sure they are LiPo or not) but the bulb light output was excellent. It works up to 6-8 hours. It comes with a 1watt solar panal. I am thinking if I can build a similar one like this. I don't know how to draw a wiring diagram, so I attach the picture with labels. I don't know the components at all and the numbers on each components are so small and illegible! so I just provided you a guesstimate drawing. I would be very grateful if you could help me figure out the components,

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16. ### Shine

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Jul 29, 2012
Thanks for useful links. This was actually a question from a friend of mine. Obviously, I am not going to build the charger myself.

I found a charger in your link which can operate with solar panels up to 200 Volt open circuit, but it only charges batteries between 24V and 100V. Therefore, I don't think it matches my battery's voltage. Another 12V charger controller only support Maximum Solar PV panel open circuit voltage of 26V.

Is there any charge controller that supports 12V DC output and operate with 200V solar panel???

17. ### Harald KappModeratorModerator

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Nov 17, 2011
Sorry, I don't know. I'd have to google that myself.