1Charger

888w/m2 - it's all about the angle ;-)

http://solarelectricityhandbook.com/solar-angle-calculator.html

0.85A - panel area 3.2 dm2

Parabolic mirror 8cm

The RT8272 looks very promising ( especially with a 22uF cap at Vin ), so far it has no problem charging with my smallest 0.2A to my largest 1.5 A solar panel ( with clouds or clear blue sky ).

http://tw.richtek.com/assets/product_file/RT8272/DS8272-02.pdf

Veery cheap

http://www.ebay.com/itm/10PCS-Mini-DC-DC-Buck-Converter-Step-Down-Module-Power-Supply-For-aeromodelling-/201234570492?pt=LH_DefaultDomain_0&hash=item2eda83dcfc

With a big cap at Vin I think it is regulating around under-voltage lockout ( 4.2V ) when it's cloudy

http://www.fieldlines.com/index.php/topic,144628.msg977808.html#msg977808

This might be taken one step further with the EN pin.....( if it's worth the trouble ). The under voltage lockout is close to the Mppt point for smaller "5V" panels.

But I think it's use should be limited to one panel ( <= 1.5 A )...

Putting several RT8272 in parallel is probably not a good idea since Vref "0.92v" (FB) will differ from circuit to circuit, meaning one circuit will probably get more heated than the others.

You can choose different display types by activating #define of your choice

( by using CE you could have multi-screen ... ;-)

#define LX12864G_086_SPI   (3)
//#define SSD1306_SPI      (1)
//#define SSD1306_IIC      (2)
//#define NOKIA_5110_SPI   (0)


// Selected display type
#define DISP_TYPE          LX12864G_086_SPI
#define DISP_WIDTH         128
#define DISP_HEIGHT        64
#define DISP_NCOL_SMALL    21  // 128/6 small font

The JLX12864G-086 is in my opinion a good replacement for the Nokia 5110.

Similar price 5$, better connection but slightly higher power consumption ( like 10 mA ).

The one I work with has a feature of sometimes forgetting "screen orientation", my workaround is repeating this command before writing any text. My guess it's a light sensitive part at the bottom of the display that triggers the effect. Once I've covered it with tape..problem seems to be gone.

spi_write_byte(0xa6, 0 ); /*  General: Positive Display */

/*  bottom 0XC8: reverse scanning order: from bottom to top*/
spi_write_byte(0xc8, 0 );

/* 0xA0: General: column address from left to right */
spi_write_byte(0xa0, 0 );

http://www.electrodragon.com/product/12864min-graphic-lcd/

http://www.mikrocontroller.net/attachment/252515/JLX12864G-086.pdf

The alternative is trying to get a good connector for Nokia

http://www.starlino.com/nokia_lcd_connect.html

Most lux sensors has a limited range ( < 100 000 lux ) and I plan a different approach.

Current source current is directly proportional to the solar radiation

Measuring Solar Radiation, Chuck Wright

In a solar cell, the parameter most affected by an increase in temperature is the open-circuit voltage -2.2 mV/°C

The short-circuit current, Isc, increases slightly with temperature, since the band gap energy, EG, decreases and more photons have enough energy to create e-h pairs. However, this is a small effect and the temperature dependence of the short-circuit current from a silicon solar cell is

A change form 25 to 50 °C will increase Isc current 1.5%.

The impact of increasing temperature is shown in the figure below

http://www.pveducation.org/pvcdrom/solar-cell-operation/short-circuit-current

http://www.pveducation.org/pvcdrom/solar-cell-operation/effect-of-temperature

Pyranometers and reference cells
Pyranometers and Reference Cells, What’s the Difference ?

Often more than 100 000 lux ( resulting in unreliable lux measurements )

A 8h LiFePO4 battery is fully charged in 3.5h compared to 2.5h from an wall outlet.

I have compared the 6.4 dm² panels with my 11.2 dm² a sunny winter day with a clear blue sky. The light intensity peaked at 55000 lux.

The theoretically available power P at 55000 lux and a 11.2 dm² panel with 0.16% efficiency is:

P = 55000*0.000079*11.2*0.16 = 7.8 Watt ( I measured close to 6.6 Watt on the battery side )

Plotly 5 jan, comparison 6.4 dm² vs 11.2 dm²

Plotly 4 jan,clear blue sky 11.2 dm² panel

I have compared the 6.4 dm² panels with my 11.2 dm² under harsh conditions. The light intensity peaked at 4300 lux and it was impossible to distinguish the contour of the sun. I managed to charge close to 0.5Ah. I think at least 22 dm² ( 30W panel ) is necessary to do something meaningful in this kind of light. ( When it's snowing I've measured 170 lux )

The theoretically available power P at 4300 lux and a 11.2 dm² panel with 0.16% efficiency is:

P = 4300*0.000079*11.2*0.16 = 0.6 Watt ( I measured close to 0.4 Watt on the battery side )

Plotly from 30 december , comparison 6.4 dm² vs 11.2 dm²

Plotly from 31 december sun and clouds, 11.2 dm²

A cloudy winter day, but I could see the contour of the sun behind the clouds. I measured around 12000 lux peak. For this test a geared up and tested two of my "new" 7.5W panlels. You can read more about them at http://cottonpickersplace.com/ . I think these panels are very good and they come in many sizes. I prefer panels that are waterproof and has exposed soldering points. I'm evaluating if the slightly smaller panels (1.6x2dm) or this size (2.8x2 dm) is the best.

With just 6.4 dm² panel area in december (winter in Sweden) I could charge a 2Ah LiFePO4 in less than three hours. The light sensor is just a rough estimate ( It's still uncalibrated ). It is however worth to note that with a drop in temperature the efficiency of the panel increases. As an example a cell that produces 0.6A at 25 °C will produce 0.68A at 0 °C.

( December 22 was the shortest day 2014 )

Plotly from 28 december - sun and clouds