Need help creating a solar charging circuit for hobby

[Jaron Wiley]

Hi I am fairly new to electronics and I have a project I want to create a circuit for. I have a solar panel rated 6v 50mA. I want to make a rechargeable battery run a series of 2 or 3 bright LEDs.

My question is: What voltage limit will the source need to have in order to take a charge from a 6v solar panel? Will 1.5v batteries in series require greater voltage to charge?

and What components do I need to include in the charging circuit besides the panel. (can I just connect it directly to the battery?)

 

[berkeman]

Hi I am fairly new to electronics and I have a project I want to create a circuit for. I have a solar panel rated 6v 50mA. I want to make a rechargeable battery run a series of 2 or 3 bright LEDs.

My question is: What voltage limit will the source need to have in order to take a charge from a 6v solar panel? Will 1.5v batteries in series require greater voltage to charge?

and What components do I need to include in the charging circuit besides the panel. (can I just connect it directly to the battery?)

Welcome to the PF.

It sounds like a good project. Keep in mind that the output of the solar cell will vary, depending on the amount of sunlight hitting it ("insolation"). Because of this, the solar cell's output voltage may vary from above your battery voltage to below your battery voltage.

So the type of DC-DC converter you would use for this application is the SEPIC topology DC-DC:

www.ti.com/lit/an/snva168d/snva168d.pdf

Or alternately you could use a Boost DC-DC followed by a Buck DC-DC:

http://en.wikipedia.org/wiki/Dc-dc_converter

You will also probably want to put a diode in series with the output of your charger circuit, to keep from discharging the battery when the output of the DC-DC falls below what is needed by the DC-DC to make the charging voltage.

 

[gnurf]

Keep in mind that the output of the solar cell will vary, depending on the amount of sunlight hitting it ("insolation"). Because of this, the solar cell's output voltage may vary from above your battery voltage to below your battery voltage.

So the type of DC-DC converter you would use for this application is the SEPIC topology DC-DC [...]

You will also probably want to put a diode in series with the output of your charger circuit [...]

The solar cell output voltage is linearly dependent on temperature but only (indirectly) logarithmically to the insolation, so large temperature swings are a greater threat to a steady output voltage than varying insolation levels.

Considering his inexperience I think he would be better off following the KISS (keep it stupid simple) principle here and simply clamp his solar cells with e.g a 3 x 1.5V (4.5V) battery that would be in parallel with 2 x 2V(?) LEDs + a resistor. The series (Schottky) diode would still be necessary to avoid discharging through the solar cells. This approach has at least one glaring pitfall though; since the battery would clamp the solar cell, its output power would be at its lowest when the battery is discharged (when you need it the most), so it might be a good idea to throw in a comparator or something to prevent the LEDs from draining the battery too much.

In my mind this should work (albeit rather inefficiently), but my mind doesn't always work. What do you think?

 

[Jaron Wiley]

Welcome to the PF.


Or alternately you could use a Boost DC-DC followed by a Buck DC-DC:

I looked at the schematics for the SEPIC converter and I believe our friend is a correct that it is a little over my head. I understand the basics, I just have trouble with the equations (not knowing the desired values of Vd, etc.) Since this is just one part of a larger project I'm trying to finish by november I will try something more simple. I see the importance of having a DC/DC converter, so I will because I expect to have periods of low Vin. So in a follow up to Berkeman, Will this boost converter work?:

http://www.newark.com/diodes-inc/zxsc300e5ta/ic-led-driver-smd-sot-23-5300/dp/34M9011

It has an output voltage of 8v and output current of 3.6mA. The low current worries me that it would restrict the charge. The solar panel has a max output of 50mA. would charging a 4.5 v circuit with an 8v booster hurt anything? Is this why I need the buck converter to bring the voltage back down to say 5v?

this is a buck converter I found:

http://www.linear.com/product/LT1076-5

Note it says it "only 9.5 mA quiescent current." Does that mean it needs 9.5 mA input to work. Could it work with the boost above.

Im thinking: A series circuit 3*1.5v(4.5v) source to the solar panel to the boost to the buck (I'd like to discard the buck if I can use the 8v output) to a diode. Then I could create another series led and resistor circuit in parallel to the charging circuit. What do you think? I greatly appreciate your help!

 

[Jaron Wiley]

Would this be a better booster to use?

http://www.newark.com/jsp/search/productdetail.jsp?SKU=65J6904&CMP=KNC-GPLA

 

[Windadct]

To effectively charge a battery the Voltage and Current need to be well regulated - so this does depend on how long you expect this system to last. If you do not charge the battery properly it will not last long - and different battery technologies ( chemistry) like to be treated differently.
The LED is much easier however - a LED driver can probably be found that will run off of what ever Battery voltage you use.