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Picture of Build your own DIY solar powered mobile phone
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This Instructables describes in detail how to build your own solar powered mobile phone.

Linear Technology article discussing solar charging, using maximum power tracking - see extracts
http://cds.linear.com/docs/LT%20Journal/LTMag-V19N4-04-LT3652-JimDrew.pdf


The idea dates back to 2002 when I was working on my dissertation on methods of energy scavenging that could be integrated on silicon. On a footnote, I added simple calculations demonstrating that if the surface area of a 'candy bar' mobile phone is covered with solar cell of 5% efficiency (which is pretty rudimentary), including a DC/DC converter, the overall output power is sufficient to augment the battery, providing power for stand-by indefinitely.

And here it is! Or rather a simplified version that is equally effective if not better!

Specifically what you need:

The Motorola F3 - a very cheap (I suspect the cheapest mobile phone there is) phone that is widely available both in US and UK. It is this affordability and basic ruggedness that I had chosen this model. More on this later...

A solar panel - the one I had acquired is available online (more on this later) but any other suitable make can be used provided it is rated 4.5V and above, having the dimension not larger than 100mm x 40mm.

A Schottky diode, 1N5817 or equivalent - a basic component which is not hard to find, you can even use the SMT version as long as you can solder some wires on both ends.

Some wires, I use enameled wire, more on this later...

Some basic soldering + a sharp pen knife + aluminium tape + epoxy + a hand drill + super glue

That's it! Surprised it is so easy? Why not get your hands dirty and build one!

The finish product is shown below.
 
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Step 1: Introducing the solar powered phone.

So here's the phone, you can get one from Orange (UK network provider) with a pre-paid SIM card for a princely sum of �9.99! (no kidding! Stretch those credit crunched dollars!)

A sign of its popularity , the phone has its own Wikipedia page!

http://en.wikipedia.org/wiki/Motorola_FONE_F3

Some background on this model.

First off, it is VERY basic, you get more ring tones than functions on this phone, supposedly it was designed for developing countries, interestingly, it has an E-INK display that is very readable in bright sunlight and very good reception.

Turn-offs? SMS on this thing is atrocious! Bottom-line, the price far out-weights everything, if it is made of cardboard it would be disposable.

And here's the solar panel, I chance upon this little panel on an web-shop, www.dealextreme.com (I am not affiliated with the site)

see:
http://www.dealextreme.com/details.dx/sku.12988

The dimension is PERFECT. Fits the phone like peas in a pod.

I don't have any data on the stand-by power consumption, just an estimate based on the number of hours it takes to drain the battery (3.7V, 700mAh). It works out to about 22mW (Typical P = 6mA x 3.7V). Of course this figure jumps significantly to a watt when the phone is in used (as in transmitting). The idea is to provide power in excess of the stand-by power consumption to charge the battery. More on this later.

Step 2: Getting the work done

Picture of Getting the work done
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Attaching the panel to the phone battery cover.

The drilling of holes on the cover definitely void your warranty!

Grab some sand paper and roughen the surface of the battery cover, this helps with the adhesion.

Spread the epoxy like butter, I use JB cold weld epoxy, it is supposed to be good, cures in 24 hours, I think super glue might work but it wouldn't be as durable. Also note that you have to attached the solar panel in the correct orientation.

Now as you can see, the holes let the epoxy escape allowing better adhesion, acting as 'grips'

That done let's wrecked the phone! :D

The only place that needs 'trimming' is a gap on the phone's naked frame, this allows a snug fit of the diode.

Step 3: Wiring the doide...

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So what does the diode do?

A basic lithium ion charger is a constant current charger, with an end-of-charge detector, charging ends when the battery reach its full charge at 4.2V (most common) and the charger goes into trickle charge mode.

A solar cell can be modeled as a current source in parallel with a diode. Higher voltage is achieved by stacking these individual cell in series.

http://en.wikipedia.org/wiki/Solar_cell#Equivalent_circuit_of_a_solar_cell

In this case, the charging circuit consist of just a blocking Schottky diode. Advantageous is its low forward biased voltage, which is around 0.3V.

http://en.wikipedia.org/wiki/Schottky_diode

Is it safe to charge the battery in this manner?

The charging current is very small, comparable to a trickle charger, the solar cell act as a weak constant current source. The panel I brought has a open circuit voltage of 5.8V under bright sunlight and a short circuit current of about 80mA, you can measure this with by simply shorting your digital multimeter across the solar panel terminal, this is slightly more than 10% of battery capacity at 700mAh. But remember when charging occurs, it is at a voltage 4.0V and above (3.7V + 0.3V diode drop), the net charging current (minus the current drawn by the phone) is not going to be any more than the measured short circuit current.

In any case, the rule for charging a battery safely is to do it at 0.1C (i.e 70mA) and here, the charging current does not even excess this figure.

A net positive current in excess of the stand-by power will charge the battery. Some experiment I've done shows promising result. It take 3 hours of good sunshine to charge a 'dead' battery with the phone powered off to last 12 hours when the phone is standby without charging.

Again some calculation, working back, given that the phone draws 6mA on standby, 12h x 6mA = 72mAh, each hour of sunshine gets you 24mAh (72mAh/3), on standby the phone consumes 6mAh, thus a net charge of 18mAh will go to the battery. Consider a day with 6 hours of charging (day) and 18 hour without (night), your phone left on standby will run indefinitely.

It is best however, to let solar panel charge the phone ONLY when battery indicator shows one bar below a full charge.

Step 4: Wiring the rest of the gubbins

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This is how I wired the phone battery contact terminals. It is one solution, you can do it some other way, but this allows the battery cover with the solar panel to be taken apart very easily while maintaining good electrical contact.

The hazard is the crafty-bits. Follow the photos and you will get there. About the aluminium tape, these are used for heating ducts, hence likely available in hardware stores. Alternative may be just to stick double sided tape on aluminium foil.

One last warning, the panel does reduce the reception strength, so when you're indoors, signal could be bad. But because it needs sunlight, ideally it would be placed near a window, so it isn't the worst trade-off.

That's it! Done!

Step 5: Last word

Based on very unscientific empirical result, having use this phone for more than a week without ever charging the battery, roughly an extension of 25~50% of normal usage and anticipating every occasion there's sunlight hoping this thing gets a juice. Success!

Although circumstances differ, on reflection it is very liberating and the idea certainly does work, not cost-effective but that's wasn't the point to moot. Certainly useful for a out and about camper.

How can it be improved?

Interestingly Texas Instrument has a boost converter, TPS61200, that works down to 0.3V providing a regulated output up to 5V. This particular chip is well-sited solar powered applications, anyone interested may well explore further.

http://focus.ti.com/docs/prod/folders/print/tps61200.html

I would assume since a solar panel is augmenting the battery, its dimension could be halved (e.g. using a lithium polymer battery commonly used by RC hobbyist), doubling up real estate for additional circuitry.

Have fun!

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snotty3 months ago

This is hella awesome!

I think I've found a better work around though. In practice it seems like on-phone solar panels don't go well with being indoors and in pockets.

We like to charge our phones at night but solar panels have a little problem at night when there's no light.

So: Make a solar battery charger and charge your phone off that battery. The ingredients are pretty similar. I like to use solar panels off dead solar garden lights. I also like to use dead car batteries; they won't start a car but they sure will charge a phone (with a 5v regulator!)

profort snotty1 month ago
How would you modify this please?
sindhu750635 months ago
Hi...what is the wattage of the solar panel?

The answer is sort of hidden in step 3: "5.8V under bright sunlight and a short circuit current of about 80mA"

5.8v X 0.080A = 0.464Watts nominally.

BUT I'm pretty sure the operational voltage in this case is dictated by the battery. The battery voltage will vary between 3.6v at empty and 4.3v at full. So the actual wattage may change depending on the battery charge state.

Also solar panels' current (AKA amperage) is very sensitive to the strength of incoming light. So indoors out of direct light you get a lot less electricity.

sindhu750635 months ago

Hi! What exactly is the use of the Schottky diode here???

The diode prevents electricity from back-leaking though the solar panel. When the panel is shaded it stops producing electricity and can start to drain the battery instead of charging the battery. Fancier systems are more complex and efficient but the method in this 'ible is probably the easiest, cheapest and most elegant; I use it all the time.

Macgeekal1 year ago

hello i would like to do this on an iphone 5, any suggestion on parts?? any help will be greatly appreciated... :)

On The 3dge3 years ago
You probably know more about this than me, i found a couple of spare diodes and i was wondering if they would work for this, the only thing i know about them is that the markings on the side read; FR153s - then underneath that it reads - 724
Heh, funny profile pic have you!

If you have a digital multimeter, the diode measurement mode will tell you the forward bias voltage (when conducting), if not so simply set up a circuit, battery and a torch bulb, alternate the terminal of the thing you suspect is a diode, it should block one way.
aitchehtee5 years ago
I want to try this with the F3 but I don't know anything about this type of electronics. would you be able to use a solar panel with a built in diode? Also, would getting a more powerful panel be better and increase charging time, or would it ruin the phone? Also, what is the smallest, or least powerful solar panel you could use for this to still charge at a decent rate? Lastly, now that its been a while what would you do differently in this mod?
Hi,

What I have suggested about the choice of panel is in the instructables...

However your last question pique my interest, frankly I had abandon the phone for another, reason, it is very basic, the screen does not do text proper.

But since I did this, there are quite a number of phones with the new feature of augmented solar charging on the back of the phone, google, you will probably get a few hits.
sddhhanover5 years ago
 assuming my phone has a 3.7V, 950 mAh battery, what range of voltage and current should i use for my solar cell? (i am asking for a range, not a specific amp/volt level). thanks!
 Is your math correct here? [or am i just a fool]

(700mAh @ 3.7V) = 2,590mWh of battery available. 
If the phone consumes 22mW continuously in one hour it will have consumed the equivilent of a 22mWh battery and in 117 hours it will have consumed the whole 2,590mWh battery.... 

According to the F3 spec... the phone is supposed to have 300 hours of standby time... not 117hrs. 

What am I missing here? 

Thanks! 
 Also if it jumped up to a Watt durring "talk time" and you spoke for two and a half hours you would have consumed 2.5 Wh --->  2,500mWhrs. The battery is only rated for 2,590mWh and accordint to themotorolla the battery is supposed to get 8.3 hours of "talk time"


What am I missing here? :)  
Hi,

I do think you are missing something... Mobile phone transmmission power can vary depending on its distance from the base station, I am suggesting it could go up to a watt, as a typical maximum, assuming worst case, why would I want to rely on Motorola happy-clappy specification of 8.3 hr (if what u say is even true)?
 That is really interesting! I had no idea that the transmission power of the cellphones were switchable.  How do you think that their 8.3 hours is calculated? 


Yes the transmission power of mobile phone can vary, which I believe is why there are some limitation in your understanding of what works in practise and what is just a specification.

I don't really need to know how their ideal figure is obtain, I know for sure it is not true or exact in real life.

I have a suggestion, why not you get one of these phone and test it out, IF you don't get that figure, please sue them for false advertisment, and then ask me to correct my instructables.

Is that alright?
Hi,

I prefer to use my actual real life run-down test.
 what is the real life rundown test? Did you run it while on a meter?
How can I power my old cell phone with some double A (AA or AAA) batteries? it is a 3.7v & 750mAh battery.
no need to look anymore as i have found the answer:


http://www.instructables.com/id/37v-emergency-charger-Nokia-cellular-with-9v-batt/
GreenD5 years ago
Sorry for the noob question, but your idea is great so I'm doing it myself - Does the positive wire have the shotky diode? I just can't see it correctly in the picture.
Fr0ZenWolf6 years ago
This is a good Instructable, I'm going to try this with a few tracphones.
DIY Dave6 years ago
cool
mspark4006 years ago
Great 'ible! for my prototype i am using an ok solar cell but was unable to find the correct diode @radioshack. How critical is it to have that exact diode,and i have a IN4001 50v 1A diode. would this possibly work? If anyone can tell me ASAP that would be great, Thanks in advance! Cheers, mspark400
The significance of the diode specified is that it is a Schottky diode, a special type with a particularly low forward voltage (the amount of voltage required before the diode begins to pass current). Other types of diodes, such as the standard 1N4001 and its brethren, should work, albeit at slightly reduced efficiency.
flio191 themike6 years ago
Thanks guys for the question and answer, I was going to ask about my 1N4004 but now I totally know that won't be possible, err. really that efficient.
BrianKT6 years ago
I was thinking of getting a Motofone F3 for a spare phone. Is it nice? Anyway, cool Instructable! Great way to save energy!
crait BrianKT6 years ago
This is a late reply but it's an amazing phone!
so u mean that combination of a 4.5v solar panel and a diode charges the 3.7 v battery? IF yes than why are the wall unit chargers rated to 5 volts?
Whether the wall charger is rated 5V bears no relation to how this modification work.
can u post the schematic of your dc booster
If you follow through with the steps, you will know that there's no DC-boost circuit, that's the point, keep it simple, it doesn't always have to be more complicated then it should. The joule-thief is a neat circuit, I had build one myself to the size of a AAA battery, but the way that particular boost circuit works, it rely on the light source being a LED (it could be blue green red, IR, basically a diode with forward biased voltage higher than the depleted battery). The simplest lithium-ion battery charger would be a 'linear charger', there are many jelly bean part to choose from (Try OnSemi, TI, Linear Tech, Maxim etc...). Typically these ICs, has an internal reference and a simple time out maybe a LED pin out for indicator. An application circuit would usually be supplied with the data-sheet of the ICs.
where do you get one in the us?
find itt on eeebaayyyyyyy ;)
i think t-mobile
can a cell phone be charged by using a joule thief?
flio1916 years ago
Nice! I use this phone already, I will look into getting a solar panel for it. Can you still charge by power cord?
Corrugator Supercilii (author)  flio1916 years ago
Yes you can, the modification does not in anyway affect normal charging with the wall plug.
thats great news. thanks!
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