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Return to Hardware:Neo1973:Alternate Cases:Solar power.

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Alternate case for the Neo1973, with a solar panel for recharging the battery.

Design options

The panel could be mounted on some or all of the six sides of the phone, depending upon the amount of power needed, the environment and the power produced by the panel.

Trickle PV charger integrated into a back case may be feasible. Chances are it would not significantly lengthen daily operations, unless it was left in the sun for extended periods of time, and the other problem is that while it sits in the sun you can't really easily use it as a phone w/o a headset. In addition, It's unclear how hot the phone might get (from absorbing the other bands IR etc.. while sitting in the sun), and whether that might be a problem for the phone electronics or the battery in some hotter climates.

To fully power and charge the device, plugging in a small PV module to the USB port is an option. If you integrate the PV module with a Li-ion battery, charging circuit, and a USB hub, It could make sense for extended computing operations outdoors or places without power. For people roaming about the wilderness/bush this might be an option. If it weren't for the GPS, I might even ask the person why they would bring the phone with them, particularly if there were no cells/wifi hotspots around.

For the sake of a high end usage calculation, figure 2.5A @ 5V nominal for this application (12.5 Watt nominal) depending on what peripherals you want to power (500mA @ 5V on 2 usb ports, and charging the hub battery, powering the hub itself, and 500mA @ 5V for the phone running and charging).

> arbitrary chosen polycrystal > solar cell 100mm x 100mm: 0.47V, max current (short circuit) 2,6A > (, #112135-99)

So we could figure 1 string of 12 cells in series to give us the 5V and 2.5A. arranging them in a 4x3 pattern for 12 total we have a size of 300mm x 400mm for powering and charging everything directly from the sun. This assumes perfect alignment of the panel with the sun and a clear sky. The panel will only be able to provide this power and charging for a normalized number of solar-hours/day (4 or 5 where I live). Minimally this panel, battery, and hub could probably fold to something around 320mm by 120mm by 38 mm, making its folded footprint still about 5 times as large as the phone in area and over twice the thickness.

Taking into account the rules of thumb for the energy available (out of the original ~1kW/m^2) in various scenarios:.. the available energy is further reduced.

full sun, panel square to sun: 100% full sun, panel at 45 degree angle to sun: 71% light overcast: 60-80% heavy overcast: 20-30% inside double pane window, both window & module square to sun: 84% inside double pane window, both window & module at 45 degree angle to sun: 64% indoor office light at desktop: 00.4% indoor light store display: 01.3% indoor light home: 0.2%

If you want to charge the device by laying it or a panel down on the ground in the sun, understand that you are going to lose about 30% of the available energy just in not having it positioned perfectly. Of course, the part of the earth's surface you are on is rotating away from or towards the sun, changing the angle of the incident radiation. So even if you position the phone or panel perfectly, it won't be "perfect" all through the day, unless the light is already being scattered by cloudy or overcast situations. Indoors is simply a no go situation, even for trickle charging.

After taking the 30% of the available energy away from our panel due to imperfect conditions, it leaves us with 825mA @ 5V.

To just trickle charge the phone at the 100mA @ 5V through the usb in imperfect conditions, we can reduce the area of the cells by about a factor of 8. 120000mm^2 then shrinks to 15000mm^2 or about twice the phone footprint if you assume it is a 62mm x 120mm rectangle. You would still need 12 cells in series, each with an area of 1250mm^2. 30mm 40mm might be a workable size cell, as about 6 would cover the phone, and another six could fold out.. this would definitely add some bulk.

To go even further, we could interface directly with the battery in the case and provide different currents at a slightly lower voltage, meaning fewer cells in series, larger area, and we might cut out some of the in- efficiencies of the onboard charger. The fully charged neo battery has a 4.2 V open circuit voltage, and a nominal voltage of 3.7 volts. This suggests at an absolute minimum 10 cells @ 0.47V to be able to reliably charge anything (4.7V). The footprint of these polycrystalline cells is still larger than the phone footprint.

If we just said, OK, we just have the non-curvy back of the phone portion as area to use, about 50mm x 80mm, and we are going to use 10 cells for 4.7V to directly charge the battery, 4000mm^2 of cell area divided by 10 gives us a 400mm^2, which points to a rectangular cell size of 8mm x 50mm. this gives us 1/25th of the original cell (100mm x 100mm) area and correspondingly about 1/25th of the current. this is about 104mA under perfect conditions. If we derate it by 30% for non perfect conditions, alignment, etc. we are looking at a charge current of just 63mA @ 4.7 V for 10 cells in series.

So if you can get the phone to draw nothing when it is off, and charge for the full length of the normalized solar day (say 6 hrs.) with the trickle charger in the last example you would get (optimistically) on average, somewhere between 378mAh and 624mAh per day, or about 30 to 50 percent of a battery charge a day. In climates with lower normalized solar hours, you get less, In climates with more you'd get more. I should stress again that these normalized solar hours are NOT the number of hours of sunlight you get per day, you need to look them up on a chart for your region, and then leave your phone outside all day!

What is obvious is that -->Power Management is Important!!!<-- and that you probably won't solve power management problems by sticking some PV cells on the back of your phone. However, the better your power management and device efficiency, the farther the cells you could put on the back of your phone go in meeting some sort of auxiliary charging function.

other stuff:

Voltage of a cell is related to the underlying physics of the cell and its manufacturing process,(crystalline, polycrystalline, amorphous, printed etc.., its particular junction type, and dopants used to make the junction). (This is measured Volts open circuit) more cells in series, more voltage. Cells and modules then have different responses to the effect of ambient temperature on the voltage of the cells, depending on their manufacturing process. Usually there is a slight voltage drop at higher temperatures with all types, but is often more pronounced with Amorphous.

Current of a cell is proportional to the surface area of the cell that faces the light source. (This is measured Amps short-circuit current) Larger area, more current. Cells linked together in series string should be of the same or extremely close surface areas. The smallest surface area limits current and can cause overheating and damage within the module.

modules that have to deal with shading should have bypass diodes. modules should be accompanied by some sort of charge control that has a blocking diode to prevent current leakage from the battery in periods of darkness.

Last but not least, the cell and/or module efficiency of terrestrial cells should be between a low of 9% and high of 17% conversion of incident radiation to electrical current.

Hopefully I haven't made any embarrassing mistakes. Feel free to correct me if you find anything, thanks. HTH

Rendered images

3D model





Solar panel


Leave your nickname here if you are interested in having one made. This is not an order form, but is intended to gauge interest before effort is expended designing the case.

No Nick
1. Tetraden
2. ruskie
3. Denis
4. sin
5. Filippo
6. Deedend
7. Wedge
8. Fradeve11
9. Aeshaettr
10. Madoon
11. larstobi
12. KlaymenDK
13. oz1lln
14. Aztlek
15. Kelvan Maybe there is a option to make changeable solar cover.
16. Tommy That would be wonderful. I'd rather buy a first version without the solar panel quick and then also buy the next version, than have to wait even more if you were to include the solar panel on the first mass market version. But if you could hire someone else to do it for you and the decision would only affect the price and not the release date I would want you to include it in the first mass market version.
17. pokazene_maslo Seems interesting, but I'm sceptic about it.
18. gwylim That would be cool, but I'll buy it without anyway.
19. yorick That would be very interesting, if it could generate enough power. Otherwise I might go for a kinetic charger.