Solar powered Kindle Touch

[geekmaster]

Yes, we ARE free to spend $100USD in supplies and 100 hours of development effort to replace something we can buy from a Hong Kong ebay store for MUCH cheaper (including shipping). But it helps to know what other options are available so you can spend your time developing something new and original.

P.S. I still respect this as a clever hack and interesting educational experience. But some readers may prefer to just buy a ready-made solution.

[geekmaster]

For those who want a cheap solution for a kindle-compatible USB solar charger, you can buy USB solar chargers from Chinese (or Hong Kong) ebay stores for about $3USD (including shipping). This one is rated at 5.1v 0.4w (about the same as charging from a host PC), but for a little more money, you can get one with more charging (or operating) current:
http://www.ebay.com/itm/USB-Solar-Ba...item27c87453f8

And with a USB cord, it lets the kindle sit in the shade while the charger is in full sunlight, so no need to bake your kindle while charging it. And while not charging your kindle, it can be charging your cellphone or camera or other USB chargeable devices.

P.S. I love the DIY method. But sometimes I want a cheap and simple solution too.

[nick-tech]

Please guys lets not argue about what you can buy and what not.

I know that you can buy ready made solutions (like this one I found shortly after I finished my charger: http://www.amazon.com/Solar-Lighted-.../dp/B006E95V8M)

But I wanted it to be one single piece, a solar powered internet device (the first in the world!)


Now back to topic:

If you plan to charge your kindle directly via the usb port, you need a higher charge rate than you can supply with a manageable size solar cell. The built in charge controller (software) needs significantly more energy than the charge controller i am using (a couple of uA).

So if you want to charge your kindle via a solar usb solution you first have to charge an external battery (like the one built in in the device geekmaster showed us). Once this battery is full you can load the kindle battery (now that works because your charging current is higher)

But with such a construction your overal efficiency is about 1-2%

I tried once to load my cellphone (old Nokia) with a (bigger) solar cell directly attached to usb. It discharged my phone.

With that project I basicly wanted to show that it is possible. Perhaps some day amazon will sell solar powered kindles, wouldn't that be amazing?!

[knc1]

The K3 (the 1750MaH battery) battery management includes a 500ma charge curve. About a 4 hour rate.

So anything up to a 5v / 2.5w rated solar panel would be useful.
Anything else of more capacity, the battery management wouldn't make any use of but maybe some of your other devices would.

From another view point, a 5v / 0.5w rated solar panel would closely match the battery management's 100ma charge curve.
Good for keeping your Kindle "topped up" when not using it.

Any of the Kindle models that offer a 3G option will have a similar capacity battery (even if they don't have 3G installed) since that is about the smallest capacity battery that will run the 3G transmitter.

Edit:
That probably explains GM's observation that in some Kindles, in some modes, the battery seems like it will never re-charge.
The battery management chip must default to the 100ma (20 hour) curve unless enough firmware is running to switch it to the higher curve rate.

Note: The above information is taken from the K3's built-in battery management (factory) diagnostics.

[geekmaster]

In 100ma charge rate (when connected to a computer and low battery), it powers down the SDRAM and only uses the smaller RAM in the SoC. You can read about it in the u-boot source code. It functions mostly as a battery charger in that mode. My experience with monitoring the serial port during charging is that fastboot mode seems to charge fastest, and periodically outputs the battery voltage on the serial port.

I really LIKE a built in solar cell, if we can eliminate the extra "solar oven" feature that cooks the kindle while charging.

[knc1]

Moved to: https://www.mobileread.com/forums/sho...d.php?t=184028

[knc1]

[nick-tech]

Hello Again!

I just wanted to give you a short update on my solar charged kindle.

Short version: I haven't charged my kindle since 6-26-12 (other than by its own solar power)

Longer version: The little circut I presented above is doing a pretty good job. I haven't changed anything and it does exactly what I wanted. The summer wasn't a problem at all, since we had a lot of sun. The heat problem turned out to be not that big. It was just during this extremely hot week with no wind and on a tin roof that I couldn't turn my kindle on when it had been sitting in the sun for a couple of hours. As soon as there is a little wind, it always will turn on.

Now with the winter begining, I was afraid that there wouldn't be enough sunlight, but even with two weeks without direct sunlight (heavy clouds) I had no problems.

It turned out that "random sunlight" (that hits your kindle if you just put it somewhere randomly, the solar cell facing upwards) is not enough. Windows seem to block a significant amount of energy, so in-house solar charging just does not work. Now I just put it outside on sunny afternoons about once a week and that is enough...

[nick-tech]

Hello together,

after one and a half years with my solar powered Kindle, I decided to build a new Version of it.. (Version 1 is now powering my mothers Kindle)

It turned out that especially in the winter season with short days and high air mass factors the thin-film solar cell I used is just to weak.

So I looked for another solar cell and found this one here:
http://www.ebay.de/itm/1x-6V-150mA-0...item3cc4c111d0

It should provide twice as much current as the one I used before. And beeing a poly solar cell I also hope for better perfomance with indirect sunlight...
The only problem: The cell is about 3mm thick, so I had to mill a hole in the back cover so it doesn stand out that much.

I also adapted the circuit slightly: The capacitor that smothens the voltage from the solar cell is now 10 uF instead of 1 uF. I hope that will bring me higher charging rates as the voltage won't drop that fast in low-light conditions. The idea is that the solar cell fills up the capacitor, after the threshold voltage for the circuit to start is reached, power is transferred to the battery..

Right now it is sitting outside in the winter sun, even under these conditions I get charge rates of 115 mA with WiFi enabled!

[knc1]

see (for example) :
http://www.solarszled.com/en/product/solarcharger/

I just happen to have about 6 different models in stock.

[zxczxc]

nick-tech, that is really cool what you have done. upset i did not see it sooner!
and the new and updated solar panel you found is super cheap, i would love to put one in my paperwhite, although i use a cover for mine and wouldn't want to go without it. hopefully in the future solar panels will be built into all screens, and not noticeable, haha.

although, if you are using this intermittently, 30 minutes every day etc, is it bad for the battery at all, to charge/discharge a lot without reaching the top or reaching the bottom, i imagine so...? (i'm not talking about battery memory etc, which does not happen with new batteries anymore, but, just generally.)

[nick-tech]

Hi zxczxc,

no, LiIon batteries don't have that problem. It doesn't matter if you charge them every day by a few percent or completely ever other week or whatever. For LiIons its even better not to use the complete capacity, for example charge it to 80% and discharge to 60 % only.



Just to put it clear: I'm not using the internal charge logic. I use a special charge IC (Maxim 1551/1555) that is directly soldered to the +/- connectors of the mainboard (and thus connected to the battery)
This IC brings an own charge curve with 30mA for completely discharged batteries (which should never happen because the kindle software will turn the device of before), 280mA for normal charge (here the power from the solar cell is the limit), a reduced charge rate for nearly full batteries (i.e. cell voltage above 3.9V) and a charging stop at 4.18V

[zxczxc]

very interesting, thanks for the information! i don't know much about circuits/electronics unfortunately, i have never needed to learn any. maybe one day!
is it possible to charge the kindle via usb and via solar panel at the same time, would this cause a problem (in your current set up?)

[azw409]

Here's my solar powered Touch. It charges quite well but the panel is heavy. I couldn't fit the charging circuit inside the case so I had to cut a hole in the case and then just filled over it which is a bit messy. I used black silicone sealant but I might peel it off and try epoxy putty instead for a smoother and harder finish.

[thatworkshop]

Nick, I'm wondering how is your Kindle doing? Anything up?
You still recommend this hack?

How can you make a switch to enable/disable charging through solar? I know you can always protect it from sun but asking for a subtler way to disabling it.