Solar Charger project

[lazarus]

I'm semi-new to electronics. And by Semi-new, I mean I've tinkered with hobby kits, took a basic class in high school, and that is about it.

I have always been interested in solar power. I just love it conceptually and wanted to dabble with it and in the process gain a better understanding of electronics.

I have a project in mind of making a solar charger for my phone. I know i could easily make a solar cell attached to a plug and connect to my phone, but I want more then just that.

My idea would consist of using one (or 2 if necessary) ~6volt, 330mA, 2W solar cells to charge some batteries, including overcharge protection, and adding a USB port for interfacing with my phone cord.

My main roadblock currently is, where do I begin in figuring out what parts I need? I understand basically what I need, like a voltage regulator to get the necessary 5 volts for the USB, but im not sure exactly what the set-up would look like. Also, I know I will be needing some sort of charge controller to prevent overcharging, but I see many different plans for some, but I'm not sure what would work for my needs.


So as ive said, I have (well, I ordered) 2, ~6volt, 330mA solar cells. I also have 7 2300mAh rechargable NiMH batteries I could use for energy storage.

Really I am just trying to get everything organized in my head. I just don't know where to begin in designing the necessary circuitry.

Any help in making this idea into a reality would be amazing. Thank's for your time.

 

[(*steve*)]

A shame you have NiMH for storage. Lead Acid or LiIon are actually better suited for this as they can be charged with a current limited voltage source (which you can easily make with a solar panel and a few external components).

And charging a pair of LiIon cells (one from each panel) would be a reasonable arrangement given your panels.

Those batteries in series (giving 6 to 8 volts) could be used to power a small USB adapter like the ones that plug into a car's cigarette lighter. Most of these should operate down to maybe 6.5 volts or so.

If you get "protected" lithium cells, they will automatically turn off before they get dangerously under or over charged (although the limits are wider than you'd normally want)

 

[lazarus]

Well the batteries are not set in stone. They are just ones i found in our garage. I'm not 100% sure on their usability, I was just working from a theoretical standpoint.

What particular battery would you recommend?

 

[lazarus]

Doing more research, Ive been looking into just a straight battery charging method with no risk of overcharging via a trickle charge.

I've been seeing a lot of references to a LM317 regulator which has a 1.2 - 37 volt adjustable output. With resisters, obviously, I can use this as the backbone for charging the batteries.

Now I was digging through some of my old electronic things and came across an old Lego controller for their old train set-ups for adjusting speed. I popped it open and found it had a regulator in it labeled as a KA317. From what I can tell from google searches, the LM317 and the KA317 are one in the same. Their data sheets are word for word identical.

http://www.fairchildsemi.com/ds/LM/LM317.pdf

http://www.fairchildsemi.com/ds/KA/KA317.pdf

Question being, will this regulator work with my planned solar cells? Is there another step I need to consider given my power source is going to be a solar cell and not a stable flow from an outlet?

 

[(*steve*)]

Trickle charging doesn't offer "no risk of overcharging", but it generally offers a lower risk. Again, it depends on the type of the battery. Some will like it, and it can destroy others.

The 317 may work, however you need to consider what will happen when the solar panel is not producing power. The last thing you want is to discharge the battery through the solar panel (and flatten the battery).

You may need a diode to prevent this.

The current limit may be effectively set by the solar panels, and as long as 300mA is acceptable, you need do no more than limit the voltage.

HOWEVER a series regulator (like the LM317) requires about 2V more than the output voltage, and you may not have that. If you have more than that, you will be wasting a lot of power too.

A shunt regulator (essentially a zener diode or something configured to act as one) will work without any additional voltage.

Although more wasteful of power, this waste comes only when you're not drawing power from the solar panels, as you draw more power the waste drops.

The most efficient way to get energy from the panels is with a MPPT (Maximum Power Point Tracking) regulator which does some fancy magic to always ensure it's drawing the greatest amount of power from the panels.

If I were doing this, I'd probably charge a pair of LiIon batteries. Second best would be 4 lead acid cells (8V) and as stated before, I would use a SMPS to produce 5V from this.

It would be useful if you measured the open circuit voltage of these panels in direct sunlight. If it's 8V or more, you could get away (maybe) with using them to charge a single 12V lead acid battery.