Ive recently been looking more and more into solar power. I love everything about it, but ive never actually done anything with it, save for play with hardly realistic renditions of it in games (Yay Minecraft!) but I don't think that really counts, so, Ive set on quest to see what I can do with it.

My planned project is a (seemingly) simple solar charger so I can make use of the hours of sun my car gets every day, and use it to keep my phone topped off, or use it if I ever find myself on the side of the road and a dead phone (Like what happened just last week...).

So the skinny of my plan is to use 1 or 2 solar cells (more on their specs later) to charge up some batteries (NiMH for now, but Lithium Ion for when I have a working prototype) and from there, it would have a USB connector to allow it to interface with my phone cord.

Things I do know:
- I need a diode to prevent backflow into the panels
- I need some sort of way to regulate the voltage from the panels to the batteries.
- Some sort of protection to prevent overcharghing
- A regulator and necessary parts to convert to 5 volts for USB charging

Now for starters, I want to focus on the circuitry between the solar panels and the batteries.

The panels I am going to use are 6v, 330mA, 2W each, with peak unloaded voltage of around 6.5v. I have 2 of these panels. The batteries I have for playing with are 1.2v 2300mAh NiMH AAs and I have 7 of them at my disposal.

Ive seen several designs that make use of the LM317 regulator to manage voltage, and I have a KA317 I can salvage, and from what I can tell, they are identical. MY question for this is, is this a suitable regulator to use?

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

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

My main issue is I'm not sure what I need to use and what would work best. The biggest questionable part for me is how to best deal with the fluctuating voltage and amperage of the solar panels and stabilize it for charging wish. So any help would be amazing. Something to put me on the right path.

Thanks for your time.

 

Sometimes the simple solution is the best solution.

http://www.ebay.com/sch/?_nkw=cell phone charger solar&clk_rvr_id=462731409597

 

But it's not nearly as fun. Plus I get the satisfaction of "I built that".

If I just buy a charger, then I learn nothing.

 

I agree, I'd like to know a little about how this would be done too.

 

With just a 317 you are limited in what you can do, but you can do just enough to trickle charge a NiMH battery. Trickle charge is defined as charging at .1C rate, so 2300mAh batteries should be charged at 230mA or .23 amps.

So just set the 317 up as a constant current source. Include a diode so the battery doesn't discharge into the panel at night(which may also damage the 317).

 

Did you mean 230mA? 2300*0.1=230.

 

Did you mean 230mA? 2300*0.1=230.

Well yes I did. Good catch. Let me edit that post

 

I really don't condone using a linear regulator of any kind to charge any battery. You want your battery charge circuitry to be ACTIVE - not PASSIVE.

The NiMH datasheet that I've read from Energizer says to trickle charge at C/40 - 2.3Ahr/40 = 60mA. A C/10 rate will charge your batteries in 10 hours if you keep your solar array on sun all day! Pathetic if you ask me. C/40 will charge at 40 hours (4 days if you're lucky!) - super pathetic. You really want to have a C/3 or C/4 charge rate until your voltage starts coming down and then dial back to C/40 just to keep things topped off. This way you charge your battery in one day if it's cloudy out and you don't have to adjust your panel location hourly. Then you want a monitor of some sort to verify your battery temp hasn't exceeded 40ish degrees C.

Any less and you risk your batteries venting.

I'll say this again - read the LT1510 datasheet and app note 68 to really get a feel for what is involved. Both are available here: http://www.linear.com/product/LT1510

Maybe I should make a blog entry on this so I can point people there when they ask.

 

It may not be the best but it seems to be what the OP has in mind.

Not all of us can afford $500 MPPT controllers. ;-)

 

I have a set up running on my house that is a hobby but saves me money too.

I have 4x 125 watt at 12vdc nominal UL Solar panels and 8 storage batteries, and some inverters and lots of dc led lights.

If you want to go bigger with your project i will be glad to share what i have learned. Just ask be happy to help you.

 

It may not be the best but it seems to be what the OP has in mind.

Not all of us can afford $500 MPPT controllers. ;-)

I suppose you're right if you can wait on 10-40 hours of sunlight to charge your storage batteries after charging your phone. Also note that charging at those currents will effect the life of NiMH.

I'm really not suggesting a PPT. I'm suggesting charging at a decent charge rate so that the OP actually uses the device. You don't need a $500 PPT to accomplish that.

I guess there is one other way that you could charge your phone with a linear regulator in an emergency situation, and that would be to use a 5.5V regulator with a diode and to charge directly off of the solar array without a backup battery.

SA ---> diode ---> linear reg (5.5V) --> diode --> 5V USB cellphone

This way the phone will use it's own charge control circuitry to maintain the charge control current. It's still not the optimum scenario, but this MIGHT get the job done in an emergency situation - depends on the emergency situation. The OP would need to put both SA's in series to make this work too since his arrays are 6V.

 

Oh my, I actually got a fair amount of replies... I was only expecting a handful before it died.

I guess ill go ahead and give some updates and whatnot.

So as I said, My solar cells are 6V, 330mA, and Ive since ordered 4 3.7V, 1500mAh Li-Ion batteries identical to the ones in my cellphone (Got them on the cheap at under $4 each new)

I do know that the two solar cells i have are no good for charging all 4 batteries at once, so I would rather instead focus on charging one for now. I am simply looking for a method of charging via the solar cells that is safe to leave out indefinatly, for all intents and purposes. I plan on having the solars sit on my car dash, as its in the sun every day, so charging times are not really an issue. Its more of an emergency charger rather then an every day thing.

I have a general understanding of electronics, though I am not 100% familiar with all the terms and lingo.

 

Subject to review, not safe to leave battery on dash, too hot, good fuel.

 

Yes, the batteries would be kept out of direct sunlight. I know it's not a good thing to do.

 

Check instructables web site?
http://www.instructables.com/tag/type-id/featured-true/?sort=none&q=solar+charger

 

None of them seem to deal with overcharge protection. As Ive said I want to be able to leave the solar cells in the sun probably for days on end and not have to worry about destroying the batteries or the location they are in.

 

Are you planning to use the lithium batteries?

If so then you need a temperature cut-out to stop the charge at 55 degrees celsius. The car is likely to get this hot even when not in the sun.

With lithium I would be looking for a charge control chip that monitors both the voltage and current.

You could probably get away with a 4.2V voltage regulator but ideally the charge would be terminated when the current drops off. I am not entirely sure if the charge must be terminated after the current drops off but it is a very good idea.

Lithium can be quite volatile, you MUST NOT charge them when it is hot and you MUST NOT exceed 4.2V per cell.


Our solar charge circuits for NiMH are only trickle chargers (we use a 6V 1W panel on 5 series 2400mAh NiMH batteries, the 1W panel itself regulates the charge at 133 mA max).

 

Yes I do plan on using them, even if my car isnt the best place. I still have many other locations I could put them to be safe.

 

my s12 can be put on the dash and charge my ipad or phone. It's a USB dual port solar charger that doesn't use any batteries to operate, just the sun. It also works on cloudy days. I keep it in the sun to top off my phone and leave it in the glove compartment just incase i get stuck and need to call someone. check it out here:

http://www.suntactics.com/shop/usb-scharger-12/