Solar cells test (from calculators)

[Silvio Klaic]

During years I collected several solar cells from malfunction calculators and now I'm thinking to combine and use them to make solar charger for single AA or AAA NiMh batteries.
I have two AM-1417 and one SA-3515 cells from Sanyo, one unknown marked KSC08B and 3 newish HP-3012DS.
Only data that I was able to find are for AM and HP cells: at 200 lux they produce 1.5V and 5-20 uA.
Now 200 lux is common lighting at desk level from indoor light source, but I planning to use them outside where light levels are 10 to 1000 times greater.

For charging NiMh battery, produced voltage of 1.5V is sufficient, but the question is whether these cells can produce enough power for reasonably fast charging. My term of reasonably fast charging is up to 8 sunlight hours to charge single AA 2500 mAh battery.
That is constant current of at least 312 mA per hour.

Test preparation

As you can see on image above old cells was electrically connected with conductive tape, which I didn't have. So I connected it with conductive glue. Process is simple; place wires and hold it fixed with some type (top center on image).
Then apply 2-3 thin layers of conductive glue in 20-30 min interval (bottom center on image). After 12 hours of drying apply hot glue on top of wires to fixate and isolate them (top right).
After cleaning up excessive glue, all is set to do testing (bottom right).

Test setup and testing
I was using 22W fluorescent lamp on mechanical arm as light source. With this I was able to precisely adjust height to testing surface. I measure wanted luminosity with lux meter LX-101.
Testing cell was done by connected it to voltmeter and precise trimmer potentiometer in parallel with voltmeter. Then I adjust trimmer to get exactly 1.5V and disconnect cell to measure resistance of trimmer. Measured resistance is then compensated for voltmeter impedance and by ohms law calculated to current at that luminosity level.
Description of my precise trimmer potentiometer which I use in this measuring can be found here.

Results

	10	25	50	75	100	250	500	750	1k	2.5k	5k	7.5k	10k	Lux
SA-3515-4	0,00	0,72	2,26	3,81	5,17	13,65	27,47	42,28	56,12	143,55	285,32	411,11	488,75	uA
AM-1417	0,26	1,46	3,01	4,44	5,99	15,18	29,68	44,53	59,67	155,11	304,41	446,58	539,72	uA
KSC08B-981	0,00	0,44	1,49	2,41	3,51	9,46	19,11	29,85	39,73	105,93	211,42	317,27	411,11	uA
HP-3012DS	0,39	1,20	2,20	3,10	4,04	8,62	14,45	19,78	24,19	44,66	66,52	83,62	94,49	uA

As you can see from test results, data given from datasheet matches my results. However I notice some difference, mainly between two AM-1417 cells. Second cell (not shown in table) have almost identical response as SA-3515 cell.
This means that older cells have bigger differences than new ones. Not sure if this is to fact that they are used more or/and aging influences.

You may notice that new cells (HP-3012) have better sensitivity at lower light levels, but are weaker at high luminosity. Which is shame because technology of solar cells is much improved? Or there is some other reason for making these bad cells...
I was measure up to 10k lux which is max for my 22W fluorescent lamp. 10k lux is luminosity outside at cloudy day and on sunny day without clouds can reach over 100k lux.

To compare efficiency of these cells to standard outdoor solar panels, this data need to be converted to standard test irradiance (normally at 1000W/m²).
I don't have data how exactly my fluorescent lamp have luminous efficacy, so I assume to be about 70 lux/W, with this irradiance of 7 klux is 100W/m². To convert data, I'm dividing lux measurements points with 70 to get W/m².
As you can see from graph current is almost linear and I presume that test at 75 klux will give about 10 times more current than on 7.5 klux.
So using max readings of 10 klux and multiplying calculated power with 7 will get me approximately results for irradiance of 1kW/m².
Then dividing these results with surface is close enough data for comparing power per cm2 with standard solar panels.

Cells/panels	dimension	Surface	Power per surface @ Irradiance 1 kW/m2
SA-3515-4	15,5x35mm	5,425cm2	0,946mW/cm2
AM-1417	14x35mm	4,9cm2	1,157mW/cm2
KSC08B-981	11,5x30mm	3,45cm2	1,251mW/cm2
HP-3012DS	9,5x29,5mm	2,8025cm2	0,354mW/cm2
SES 440J	655x537mm	3517,35cm2	11,372mW/cm2
Sole SL-40P	669x500mm	3345cm2	11,958mW/cm2
SEM-45W-M	670x540mm	3618cm2	12,438mW/cm2
SEM-180W-M	1580x808mm	12766,4cm2	14,100mW/cm2

In this table I compared result with some standard solar panels.

Conclusion
Anyhow, making charger from 7 cells which I have is not good idea. These cells are too weak, about 10 times worse compared to standard solar panels at same surface levels.
To build charger I'll need to have 100 times more of these cells. There is no way I could find that amount old cells and with new I'll need minimum 500 of them.
Calculation for charging single AA NiMh 2.5 Ah battery at 1.5V with irradiance at 1kW/m²:
1922 cells of HP-3012DS: 5386.4 cm², 1.272 A/h = 1.907 W/h, 1.967 hours.
For same price solar panel Maxcell 40 Poly: 3345cm², 3.555A/h = 5.33W/h, 33.8 minutes.

So these cells are no go.
I'll use them as light sensors instead LDR in my project for tracking sun movement across sky.
Another good use is irradiance meter, however I need to find precise and know light source to calibrate meter. The best I can think of it now is summer sun at noon without clouds, but this is only calibration for range above 100W/m2.
If you know other method for calibrating irradiance meter, let me know.

I'll conduct further testing in summer to confirm these calculations.
Another testing at that time would be with standard solar panels.
I found that they work only at high luminosity (no, indoor lighting can't be used for power source).
Another test is efficiency at different angles and with solar tracker.
There is also thermal test to see if they really lose power during higher temperatures.

[Lattin]

I'm really glad I found this test as I was planning to go with these cells. You saved me a lot of headache Silvio. What's the best alternative, though?

[Silvio Klaic]

I find that best alternative is to buy standard (Polycrystalline) solar cells and create your own solar panel.
That way you have control of size, power and some other strange requirements.
One problem can be low production of partially shaded individual cells, in that case you can bypass load on them with diodes for balance.
Can increase production by adding aluminum passive cooler (or active water cooling), sun tracking, etc.