I have been researching building a solar usb charger. I am using 3 1.2V 2450mAh rechargeable energizer batteries. My question is this: Can I use the solar panel from a solar security light (9V and dont know amps, but the unit used 3 AA also) without any special regulators? And, is there a way to monitor the voltage or auto shut-off when fully charged? Thanks for any help in advance!

 

You really need to test the solar panel output. If you have a multimeter you can attach different resistors to the solar panel, put it in full sun and check the volts and milliamps that it makes.

The 3 energiser cells are probably NiMH, which should be safe enough with such a tiny solar panel. However it would be good if you can provide more info on the batteries, case size and chem type (ie if they are NiMH or NiCd etc).

 

TY RB! The case size is AA and the chem is NiMH. I will go by Shack today and get some resistors to test the output. Any suggestion on what size resistors to use for the testing of the panel? Any ideas on the shut-off or a way to monitor the charge so Im not killing the batts by putting to much charge to them?

 

Solar cells are basically constant current devices and you can get a decent "characterization" with nothing more then a meter by reading the voltage with no load, and then use the ammeter scale to read the current (you short the output with the meter but that is fine).

The voltage tells you if the panel makes enough voltage to charge the batteries, and the current shows how fast the charge happens.

It is not unheard of to charge lead acid batteries with no charger, just connect the cells to the battery. These cells do come with an internal diode, and the batteries maybe overcharged with reduced life, but it is done.

 

It just so happens that I'm doing the exact same thing right now. A few points:

1.
You really need to know the characteristics of the SA. But you don't need a bunch of resistors, just short your SA in full sun and measure the current, this is your short circuit current (Isc), and measure your voltage output - this is your open circuit voltage (Voc). Somewhere in the middle is your max power voltage and current (Vmp, Imp). You can usually guesstimate your max power at about 0.9*Isc and 0.75*Voc. Although using an array of resistors (between 10ohm and 10kohm) will help you determine your peak power point, but this can get expensive because you might need power resistors (the peak power of my SA is 3.5W). Every thing gets expensive at radio shack!

Here is a graph of a SA IV-curve for your reference
http://pveducation.org/pvcdrom/solar-cell-operation/short-circuit-current

I'd bet that 9V is probably enough, but you need to know the current to know if it is enough ENERGY to charge your batteries in your desired amount of time. You may need to put a couple solar cells in parallel to get your current up to acceptable charge levels for your application.

2.
I'll assume you're using NiMH batteries - that's what I'm using. But regardless of battery chemistry, it is imperative that you follow proper charging procedures. It is possible to vent batteries. Improper charging can cause exploded batteries, leaking, or even fire. Please be careful. I'm thinking about buying an ammo box to perform initial charging and discharging tests. The substances in a NiMH battery aren't inherently bad for you, but it isn't really good for you if you get it on your skin - or so my battery guy says.

In my design I have two ways to shut down charging of the batteries, 1) Anytime the battery voltage falls I enter trickle charge, and 2) anytime the battery temperature rises above 38degC then I disable the charger. I ran this by my battery engineer at work and he confirmed that this is a good way to safely charge NiMH batteries.

I'm using the LT1510 for my battery charging. It's a simple little device designed to facilitate charging a battery safely. It's available in a PDIP package and the inductor is small.

The LT1510 datasheet (and related app notes) explains the safety concerns for the most common commercial battery types. This datasheet, along with the battery manufactures datasheet should get you moving in the right direction.

http://www.linear.com/product/LT1510

3.
Since you're designing a USB charger with 3 batteries, you also have to upconvert your 3.6V to 5V to be within the USB charger spec. I chose to use the LT1302-5 for this application. It's also available in a PDIP package and it's designed to change battery voltages to 5V output.

4.
I chose to only use two batteries in my design. They have enough energy (2.3Ahr each) to charge my cellphone (1.5Ahr) three times if it's cloudy out a couple days in a row, assuming I get some charging during the cloudy days. Three seems overkill to me space-wise and weight-wise, but you may have different opinions or requirements.

 

I guess a fifth point is that it will be MUCH cheaper to just buy one of these.

Here's one for $50: http://store.komando.com/p-1236-por...nsider&utm_campaign=2013-01-31&utm_content=18

The one I have has better specs in a few different ways, but my price will be higher. Some of the capacitors alone are $10 a piece (low ESR, high capacity ceramics). I'm hoping I can build these for $100 a pop. I plan on building 4. One for me/development, two for gifts, and one to sell (hopefully).

 

I have many Energizer AA Ni-MH battery cells. Some are maybe 15 years old and others are new (recent Boxing Day sale). The older ones have reduced maximum current and capacity.
I measured them by charging them then shorting each cell with my Fluke multimeter measuring Amps in the test leads.

New ones give 9A. Old ones give 0.15A. A huge difference.

 

Thanks for the advise Tindel. Im looking at the charger specs you sent as well. Good looking curcuit. I am trying to avoid buying one. I am all about making my own. Even if it ends up costing more. Guess its just the satisfaction I get. I built a simple windmill that charges a couple of deep cycle batteries that runs the lights in my work shed. Just trying to figure out a smaller and more portable unit for my solar charger.

 

@Audiguru- The cells that I am using are brand new. Just picked them up the other day. They show a rating of 2450mAh.

Understanding that this is just an advertised rating, and not the actual rating.

 

Energizer 2450mAh Ni-MH cells were sold about 7 to 10 years ago. Their website shows that they are obsolete. They were replaced with 2500mAh, then 2300mAh with a long lasting charge. They have a stupid code instead of a "date". Their alkaline cells have a "best before date" of 10 years.

I asked my wife to buy me an Energizer 9V Ni-MH battery that was on sale. It was an old 150mAh made in Japan. The new ones are 175mAh and are made in Germany.
I returned the old one and replaced it with a new one hidden behind the old ones.

I trust Energizer's battery ratings to be true. Cheap Chinese battery ratings are lies.

 

I am trying to avoid buying one. I am all about making my own. Even if it ends up costing more. Guess its just the satisfaction I get.

That's what it's all about! You'll learn a ton on this project too... I know I am!

 

Very true. I am about to start school (again!) for Avionics. Enjoy these little projects to help me better understand electronics. I have the mechanical background in aviation, but new to this side of the page.
I am really trying to figure out how to build a control circuit that will work automatically with my charger. Anyone have any ideas?

 

Most semiconductor manufacurers make battery charger ICs for all the different types of batteries.

 

Very true. I am about to start school (again!) for Avionics. Enjoy these little projects to help me better understand electronics. I have the mechanical background in aviation, but new to this side of the page.
I am really trying to figure out how to build a control circuit that will work automatically with my charger. Anyone have any ideas?

Hm, I thought I showed you where to get started - any specific questions?