Instructables

Trouble with homemade Solar Charger for AA AAA Batteries. Parallel issue?

So I decided to make my own homemade solar charger to charge Double AA and Triple AAA batteries and I have been having issues and don't know what's wrong since I have made one before. Maybe this one is too complex?

I had bought 3 cells online for just under $4. Each one generates approximately 2.3Volts at 150Mah. Together in parallel they should generate about 450Mah in full sunlight, correct? Well, I had already tested them and they did work fine, I gave a small charge to a single double AA and success.

I put it all together with the cells, wires and battery compartments and the one way diode. (Use photograph for reference)

Once complete, I had put 2 AAA batteries in. Put together they only require 400mah total to charge or just less then an hour.

But when I took them out, they didn't take a charge at all....if anything they drained right dead. I thought maybe the diode was backwards but that wasn't the case either.

Finally I took a multimeter to it and although it was getting over 2 volts easily, the amps were only showing 60Mah total. Not even half of what one of the cells are capable of. I did have it in full sun too, so there must be something whacky wrong with my wiring. Can anyone spot a possibility of a mistake or do I just have bad luck? I have worked with full size solar panels (100watts worth) and never had an issue like this before.

Any ideas? Thanks!

P.S. Is it possible that the positive and negative have to come out of the same cell? I am not sure if this matters since this is in parallel. I don't have a lot of time on my hands lately and I was hoping to get an opinion before changing something unnecessary. Thanks again!

Picture of Trouble with homemade Solar Charger for AA AAA Batteries. Parallel issue?
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I see the trouble you should have a diode on each of the solar cells.

I series you only need a diode on the output.

In parallel you need a diode on all the outputs.

like in this set of 9 solar cells in series parallel.
3 P Cells.bmp
Smizz94 (author)  Josehf Murchison9 months ago

Hey Josehf, you can read my main post, but adding the diodes to each set or cell shown in your diagram fixed the problem.

The milli-amps zipped up to expectations and the batteries took a charge quickly, thank you very much for your information, I appreciate your help greatly.

Smizz94 (author)  Josehf Murchison10 months ago
I was not aware of this, I will definitely pick up some more next time I visit my local shop and add it on. Thank you for the diagram.
Smizz94 (author) 9 months ago

Hey folks, finally got a nice sunny day and a free day off for this time of year to try it out. I hope everyone had an excellent Christmas and New Year!

Adding diodes to each set made the big difference. In just adequate sunlight, the Milli-Amps zipped right up to 230. It started to reach 300 but it started to cloud, but the reading was definitely what was expected for the weather.

it is working properly now, thank you to everyone that helped!

petercd10 months ago
just guessing, but I think the diode gives too much of a voltage drop. eg 1.2v voltage drop across the diode would only leave you with 1.1v to charge a 1.2v battery, which usually reaches 1.38v on full charge. Your options are either a different diode with a lower voltage drop or more cells in series.
If he used a 1N5817 the voltage drop is 0.45 volts. My concern is that he didn't isolate the solar cells, without diodes on all the cells in parallel one cell can become a load instead of a supply.
Only if he has a shadow on the cells, which given their footprint seems unlikely
They are 8 cell solar arrays from garden lights, about as reliable as throwing darts blind folded.

Even the cells inside the epoxy of the small arrays don't line up, and off angle is as good as a shadow with them.

They are very clear and that is good.
0.25 W each ?
I think his numbers are off a little but each cell should store 1.2 volts 600 mah In 8 to 10 hours, in one AAA battery, that is pretty much standard. They actually generate more than that to run the circuit board he removed to make his modifications.
That's more what I would have thought. 60mA @1.2V is a LOT less than he was expecting.
More like 75 ma 2 volts, however size can be deceiving you get that from 1 square inch to 9 square inch cells they use for these lights. About 150 mw each for a total of .45 Watt.
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iceng10 months ago
This creative circuit  Out Performs even the 1N5817 diode as a low forward drop !

This means  more power  to the battery  and less leakback.
293955-Use_a_self_powered_op_amp_to_create_a_low_leakage_rectifier_figure_1.jpg
steveastrouk iceng10 months ago
The classic synchronous rectifier - Linear Technology do some packaged silicon for that.
iceng steveastrouk10 months ago
Linear Technology is my idea of a Top - Apex - Acme semiconductor design manufactuter.
Interesting circuit icing about $5.00 in parts not to complicated .75 A max.
"There ain't no free lunch"

Is what they say at the lower latitudes here in Reno..
Gaming parlors would offer low cost meals to draw extra clients
hoping they would play the one armed Bandits..
steveastrouk10 months ago
Those look small for 0.25W cells to me. Are you sure ?
Smizz94 (author)  steveastrouk10 months ago
Yes, they are actually slightly larger than the solar light cells. I have a couple of 50 and 60 Mah cells laying around and they don't consist of as many cells as this one does. I checked with a multimeter before starting this project and it was giving me the readings that the seller had guaranteed.
What reading was guaranteed ? Yes, if you measure them with a voltmeter, effectively open circuit, you will probably read 2.3V, and into an ammeter, effectively short circuit, they'll probably deliver 150mA. THAT DOES NOT MEAN they can deliver 2.5V AND 150mA at the same time - try them, in full sun, into a resistor of 16 or so Ohms.
MrRedBeard10 months ago
Your cell wiring is a bit different than what I have done in the past. I've always done it like the image below.

I'm not sure I would use an assortment of batteries from different stages of use & sizes. Maybe you could isolate them using isolation diodes so that they have their own charging leads. In your current config one bad battery could mess everything up.

But just to be sure
You tested each cell prior to wiring them?
Are you getting the correct voltage after wiring?
Have you tested the batteries?

solarcellspar.jpg
Smizz94 (author)  MrRedBeard10 months ago
I will check my wiring and try it the way you suggested and see if I get any difference. As far as the batteries are concerned, I only have them as options, I would never mix up the batteries that I was charging of different size and capacity, I know better, if that's what you meant? I do Double AAs and Triple AAAs separately as well as capacities, just to be safe.
steveastrouk10 months ago
What are the cell specs ?