solarcell charging 6V battery ?

[mettam]

Hi,

I want to charge some rechargeable batteries with a solar cell. Can I just buy the solar cell and attach the rechargeable battery pack to it or do I need to also put something between the cell and the battery pack ?
I have read a little about blocking diods but Im not sure if I would need one. Can someone explain when they are required. It might not be necessary in my case as I am just wanting to charge a 6V battery.

My project is relatively small, eventually I want to power a string of LED garden lights with this 6V battery which turns its self on at night.

Justin.

 

[Oznog]

They're needed because when the solar panel is in the dark and the battery applies 6v across it, it will actually drain the battery at a few mA.

No batteries like to keep taking current once they're fully charged. It slowly damages them. However, the power from a small panel is usually so small that people don't bother with charge controllers.

 

[stevez]

Justin - some comments or thoughts

A. Is the solar cell charger you are considering capable of charging the battery - does the output of the charger exceed the voltage of the battery (in simple terms)?

B. Does the solar charger you are considering have the capacity to replace the power that will be drawn on a day to day basis? This might be in terms of milliamp-hours. An example: a 100 ma current for 5 hours is 500 milliamp-hours or mah.

C. Can the battery hold that much or more, in terms of power or milliamp-hours?

I can't tell if you've worked that out from the data you've provided. It's possible you've already considered all this. There is certainly nothing wrong with just putting your system together and trying it to see if it all works out. The questions I've listed just help to increase the likelihood that you'll be successful. Best of luck to you.

 

[poopeater]

The simplest circuit you can make to do this is just a solar panel with a diode between the panel and the battery. It should work fine, but as stevez points out, there are other things to consider to be sure that you get what you want out of the device.

For the diode, look into using a Schottky diode, as they have a lower forward voltage drop.

 

[mettam]

deciphering the load of my project.

I need some help deciphering the load of my project.
I have 34LEDs in parallel running off 3 AA batteries. The multimeter reading of the 3 AA batteries is 4.74 V and 6.2mA (I assume it is 6.2mA I set the dial on the multimeter to 10A) Is 6.2mA correct ?
When the 34LEDs are all lit by the 3 AA batteries I take the reading from the end of the parallel circuit and I get 3.4 V and 4.25mA

So in summary:
Source is: 4.74V and 6.2mA
Load is: 3.4V and 4.25mA
Difference is 1.1V 1.95mA

So my questions are.
Is the power these 34LEDs are drawing 1.1V and 1.95mA (the difference)
or is it 3.4V and 4.25mA ?

And by setting my multimeter to 10A and getting a reading of 6.2 does this mean 6.2mA ?


Thanks in advance

Justin.

 

[philba]

basically, your observed current makes no sense which makes me wonder how you are measuring the current through the 34 parallel LEDs. In series or parallel with the load? never measure the "current" of a voltage source by just hooking up the leads to the + and - terminals. You may well damage your DMM. current is measured in series with the load.

are you using current limiting resistors for each LED? (you don't say so) Do the LEDs actually light up? 34 LEDs in parallel with current limited (via a resistor) to 20 mA will pull around 680 mA. If you used no current limiting resistors, I'd expect you could see several amps at least. Maybe more. do the batteries get hot? you definitely want to limit the current to 20 mA.

also, to conserve as much battery capacity, I would run 17 parallel strings of 2 LEDs if possible. You would draw half the current.

 

[audioguru]

Little AA battery cells are way too small to power 34 LEDs for more than a few hours. If the LEDs each have 15mA then the voltage of three AA alkaline cells in series will quickly drop to 3.75V which is the same voltage as three Ni-MH rechargable cells, and 2500mA/hr cells will last for about the same amount of time, about only 5 hours.

You could make the LEDs dimmer, then three AA cells would power them all night.

The solar panel will need to be huge and expensive to fully charge a 2500mA/hr battery.

 

[poopeater]

The solar panel will need to be huge and expensive to fully charge a 2500mA/hr battery.

I got some 0.5V, 1A solar cells for $1.50 each. I plan on building a charger that will charge 4 AA batteries using just 8 cells. It should have no problem charging four AA batteries in one day worth of sunlight. Total cost of the materials should be less than $15.

 

[audioguru]

I got some 0.5V, 1A solar cells for $1.50 each. I plan on building a charger that will charge 4 AA batteries using just 8 cells. It should have no problem charging four AA batteries in one day worth of sunlight. Total cost of the materials should be less than $15.

8 x 0.5V= only 4.0V. Ni-Cad or Ni-MH battery cells charge to a voltage of 1.4V so four in series need 1.4V x 4= 5.6V. There must be a diode between to stop the battery from being discharged by the solar panel at night so the total solar panel voltage needs to be at least 6.2V.

You don't have the solar panel at the earth's equator all day so you will be lucky if the battery charges in a few days of full sun or a week of cloudy weather.

 

[mettam]

Little AA battery cells are way too small to power 34 LEDs for more than a few hours. If the LEDs each have 15mA then the voltage of three AA alkaline cells in series will quickly drop to 3.75V which is the same voltage as three Ni-MH rechargable cells, and 2500mA/hr cells will last for about the same amount of time, about only 5 hours.

You could make the LEDs dimmer, then three AA cells would power them all night.

The solar panel will need to be huge and expensive to fully charge a 2500mA/hr battery.

I actually have had no problem running all 34LEDs over night on just 2 AA batteries it's a fairly dim light but still they would work for a soft night light.
So I find it confussing that you say "AA battery cells are way too small to power 34 LEDs for more than a few hours".
I will attach a image of the 34LEDs in parallel running off 3AA.

As you can see in the image acording to the multimeter all 34LEDs are drawing 3.32 V
I did a reading of the current in series and got .26 with the meter set to 10+.
How should I read the current is it .26 of a Amp or 260mA ? and is it telling me that all these LEDs are drawing 260mAh ?

 

[Death By Bass]

260mA, yes....

 

[Russlk]

Those LEDs are quite bright for 7.6mA each, I suspect the current is a lot more when the meter is not in the circuit.

 

[audioguru]

I actually have had no problem running all 34LEDs over night on just 2 AA batteries it's a fairly dim light but still they would work for a soft night light.
So I find it confusing that you say "AA battery cells are way too small to power 34 LEDs for more than a few hours".

My graph shows that AA alkaline or Ni-MH battery cells will power your LEDs for about 5 hrs fairly brightly or all night (nearly 9 hours) dimmly.

Your white LEDs work dimmly from two alkaline battery cells which are about 3.0V, but probably won't light with the 2.5V from two rechargable cells.

 

[poopeater]

8 x 0.5V= only 4.0V. Ni-Cad or Ni-MH battery cells charge to a voltage of 1.4V so four in series need 1.4V x 4= 5.6V. There must be a diode between to stop the battery from being discharged by the solar panel at night so the total solar panel voltage needs to be at least 6.2V.

You don't have the solar panel at the earth's equator all day so you will be lucky if the battery charges in a few days of full sun or a week of cloudy weather.

No, I'm charging them as two banks of 2xAA batteries placed in parallel, so the voltage would only be 2.8V. I'm also using a Schottky diode, so the forward drop is not 0.6V.

Actually, the panel will be right at the earth's equator too, so

I guarantee you that my panel/circuit will charge 4 AA batteries in 1 day of sun or less.

 

[audioguru]

Actually, the panel will be right at the earth's equator too.

How do you turn the solar panel so it follows the sun?
How do you keep the lizards from shading the solar panel?