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Author Topic: How can I construct and indoor solar phone charger?  (Read 3118 times)

Offline RomigDB

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Hi all,
I'm attempting to build a small solar-powered (or, rather, fluorescent-light-powered) indoor battery-charging system to be used as an occasional phone/tablet charger.  Currently, my setup includes an 8W 12v solar panel (repurposed from a boat battery-maintenance system), a 5000mAh Ni-MH 12v battery and 12v-5v USB output (for phone/tablet charging), all connected to a solar charge controller.  As the ambient light indoors is insufficient to get the battery charging off the panel (I'm reading about 2v output with average indoor light conditions), I'm wondering if I can't include a step-up transformer/voltage booster between the panel and the charge controller to ensure that, whatever the output of the panel, it's charging the battery.  If not, does anyone have any suggestions regarding how to accomplish this with off-the-shelf parts? It probably doesn't need to be said at this point, but I'm just an amateur garage-tinkerer (I'm an attorney by day) with a limited technical background, so if you could more or less respond as you would to a child I would really appreciate it.  Thanks in advance for any help you can provide. Cheers.
« Last Edit: 13/03/2015 10:22:26 by chris »


 

Offline evan_au

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Re: Indoor solar phone charger
« Reply #1 on: 03/03/2015 20:37:15 »
1. Put the solar panel in the Sun (eg in a window that gets good sunlight).
Reasons:
  • The Sun delivers about 1000W per square meter at the distance of the Earth, or about 700W/m2 by the time it passes through the atmosphere.
  • A compact fluorescent light is rated at about 15W electrical consumption, of which only about 10% is turned into light. This illuminates an area of (say) 3m x 3m=9 m2. This is a light intensity of about 0.2 W/m2.
  • So sunlight is about 3000 times more powerful than the light from your indoor fluorescent light.
  • By putting the solar panel immediately adjacent to the compact fluorescent, you will be able to capture much more of the power, reducing the difference to a mere 1000x. But this will block light to half of the room.

2. Use the manufacturer's standard wall-plug charger; this will save electricity compared to a fluorescent+solar cell charger.
Reasons:
  • The compact fluorescent turns about 10% of the electricity into light (the rest gets turned into heat)
  • The solar cell collects less than 1% of this light, for an optical efficiency of about 0.1%.
  • Economical solar cells have an electrical efficiency of about 10%, for an overall efficiency of about 0.01%
  • This will require you to leave the light on all night and all day to collect enough power to charge the phone. Hardly a "Mobile" phone!
  • Smartphones will probably consume more power than this source can generate, so the phone will go flat, despite being charged 24 hours per day.
  • On the other hand, in most countries, there are now regulations requiring wall-plug chargers to be 80-90% efficient when charging. So a wall charger will be almost 10,000 times more efficient than a fluorescent/solar cell combination.
  • The phone will charge much more quickly with a wall charger, allowing you to carry it around with you during the day.
  • Regulations now also limit the power consumption of a charger when it is not charging. But it is easy to turn the charger off at the wall socket when you unplug the phone.

Solar cells are described as "semiconductors". They have a moderately high electrical resistance. The more current you try to draw out of it (eg with some booster circuit), the lower the voltage, and the worse the efficiency of the booster. Put it in the Sun, and it will produce a higher current at a higher voltage =more power.
 

Offline RD

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Re: Indoor solar phone charger
« Reply #2 on: 03/03/2015 21:02:25 »
As the ambient light indoors is insufficient to get the battery charging off the panel (I'm reading about 2v output with average indoor light conditions), I'm wondering if I can't include a step-up transformer/voltage booster between the panel and the charge controller to ensure that, whatever the output of the panel, it's charging the battery.

Circuits are possible to increase voltage * , but no circuit can increase power : energy cannot be created, see ... http://en.wikipedia.org/wiki/First_law_of_thermodynamics

[ * See Joule thief http://en.wikipedia.org/wiki/Joule_thief  ]
« Last Edit: 03/03/2015 21:09:01 by RD »
 

Offline RomigDB

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Re: Indoor solar phone charger
« Reply #3 on: 03/03/2015 22:49:27 »
Thanks for your responses, I really appreciate the help.  I'm afraid I may not have been perfectly clear in my initial description - the goal of this device would be to harvest ambient light, store it in the built-in battery, and charge a phone or tablet from the power in the battery.  I realize that direct sunlight and/or wall charging would be optimal, but I want to discretely build it into a piece of furniture in the middle of a room which may receive occasional direct sunlight, but for only short periods, and cannot be plugged into a wall.

Essentially, I want to build - with off-the-shelf parts - something approximating this:
newbielink:http://www.geek.com/gadgets/panasonics-solar-powered-table-wirelessly-charges-your-gadgets-1381543/ [nonactive]

Thanks again for your enthusiastic responses!
 

The Naked Scientists Forum

Re: Indoor solar phone charger
« Reply #3 on: 03/03/2015 22:49:27 »

 

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