Solar powered Kindle Touch

[geekmaster]

Originally Posted by nick-tech

...
Has anyone the ability to monitor the power consumption of his kindle in "standby mode" (just displaying a page, wireless off).

Does anyone know how exactly these values are being measured?

Using the kindle to measure itself could change the measurements.

You can more accurately measure the (normal use) current with a cheap digital meter. We can get cheap chinese multimeters in the USA at Harbor Freight tool store for $3USD on sale (often) or about $10 normally. Or get a cheapy off ebay. Put the meter in series with the solar cells to measure current (in mA DC mode). To measure voltage, put another meter across the battery (in Volts DC mode).

But then, being handy with a soldering iron pretty much means you know that already. The above text is meant for lurking noobs who stumble upon this post.

[nick-tech]

my kindle is now 100% solar charged.

The only problem that occured so far is that I couldnt turn the kindle on after a while in full sunlight. After a couple of minutes to cool down it starts as usual.

[knc1]

Originally Posted by nick-tech

my kindle is now 100% solar charged.

The only problem that occurred so far is that I couldn't turn the kindle on after a while in full sunlight. After a couple of minutes to cool down it starts as usual.

Time to make yourself an insulated cover.
Perhaps using some 1/8 inch structural foam as the stiff insulation with the solar cells on the outside of the cover.

[geekmaster]

Originally Posted by nick-tech

my kindle is now 100% solar charged.

The only problem that occured so far is that I couldnt turn the kindle on after a while in full sunlight. After a couple of minutes to cool down it starts as usual.

The eink display and the battery each have temperature sensors. In the case of the eink, it adapts the eink waveform to the temperature, and it may have been too high for the firmware to draw to it. But more likely, the firmware decided that the battery was outside safe operating limits, and prevented operation until it cooled down.

It may help to have a fan blowing on the kindle while it is solar charging, but that would consume more electricity than using a USB charger. Perhaps a thin insulating layer (like a mylar emergency blanket from a first-aid kit) may help keep heat from the solar cell from radiating into the kindle PCB. You could wrap the backside of the solar cell like they do with electronics installed in satellites.

[knc1]

Originally Posted by geekmaster

The eink display and the battery each have temperature sensors. In the case of the eink, it adapts the eink waveform to the temperature, and it may have been too high for the firmware to draw to it. But more likely, the firmware decided that the battery was outside safe operating limits, and prevented operation until it cooled down.

It may help to have a fan blowing on the kindle while it is solar charging, but that would consume more electricity than using a USB charger. Perhaps a thin insulating layer (like a mylar emergency blanket from a first-aid kit) may help keep heat from the solar cell from radiating into the kindle PCB. You could wrap the backside of the solar cell like they do with electronics installed in satellites.

Although you could probably skip the gold foil mylar (satellite) type, the aluminumized mylar (first-aid) type should be good enough.
Of course, you will lose a number of bragging rights if you skip the gold foil.

[geekmaster]

Originally Posted by knc1

Although you could probably skip the gold foil mylar (satellite) type, the aluminumized mylar (first-aid) type should be good enough.
Of course, you will lose a number of bragging rights if you skip the gold foil.

My mylar emergency blankets ALL have a silver side and a gold side. I think the gold side absorbs infrared so you need to keep the silver side toward your body to keep warm. I am not sure of the physics and do not have time to look it up (if somebody finds I have it backwards, please post a correction). For extra geek factor, be sure to keep the gold side visible in your disassembly photos.

[nick-tech]

Well, yesterday was probably the hottest day this year, normally we don't have temperatures like that here ;-)

The emergency blanket idea is not bad, but the reflective side would have to be on the surface, so the sunlight that is not captured by the solar cell is reflected. For that I would have to disasemble my entire coating which probably would destroy the solar cell...

Here is a question for the software geeks: Is there a way to get the gasgauge-information on the kindle itself (for example through a ";711" like page) or even as a symbol next to the battery symbol (e.g. Current Charge rate is 11 mA, Capacity is 1400 mAh)

I'm not to familiar with linux, so I have absolutely no Idea how to do this, but I think it could also be interesting for non solar users to have more detailed information about power consumption.

[NiLuJe]

You could add a debug command script, that parses gasgauge and outputs relevant info via eips. Not at home right now, so I don't have the exact path on hand, but it's somewhere in the pillow search bar stuff in /opt

EDIT: Or not, it's actually in /usr/share/webkit-1.0/pillow/debug_cmds.json .

If no one has a little gasgauge-info parser on hand (or a proper thing based on procfs, if the info is there, which it probably is), it shouldn't be too hard to implement, I might even take a look at it myself, I'm not a big fan of the 'hey, let's guess how much charge you really have left from this tiny 2x3px icon' ^^.

EDIT²: Or, easier, plug something into the GUI Launcher?

[twobob]

Awesome development.

I'm thinking third world here.

Big stuff. I tip my hat to you.

[geekmaster]

Originally Posted by nick-tech

Well, yesterday was probably the hottest day this year, normally we don't have temperatures like that here ;-)

The emergency blanket idea is not bad, but the reflective side would have to be on the surface, so the sunlight that is not captured by the solar cell is reflected. For that I would have to disasemble my entire coating which probably would destroy the solar cell...

It is okay if the back cover gets hot as long as you keep that away from the circuit board, so the film should go behind the solar cell. But be sure not to obstuct airflow to the electronic components too much (probably not an issue of NOT playing video games or other live video though ). Or you could put a thermal window film OVER the solar cells, to keep out IR but let visible light in for charging.

[nick-tech]

@twobob
I thought about that too. That charging circuit can be so much smaller that it would fit in the kindle even with the umts modem installed. So you wouldnt need Electricity at all. Now let's add more sophisticated Software like an email client to it and: you have your solar powered mobile tablet. This is really what I'm dreaming of and I still hope that some day android will be running on my kindle.

If you equip the kindles with a solar cell right when you buy them at amazon, you can use polycrystalline cells that are more efficient and cheaper. I had to use a flexible cell due to the roundet surface of the kindle. That could be adapted for stock solar kindles. Thus the whole construction would not be that much more expensive than the usb wall charger everyone has to buy extra now.


@ geekmaster
Thermal insulation for the kindle touch is a rather complex problem

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Our sun can be approximated as a so called "black body", that means it emits radiation (energie) according to Plancks law (yellow in the graph). This radiation has its peak somewhere at 600 nm (greenish yellow). If you place an infrared filter over your kindle, you would cut of the amount of energy (=surface in the graph) right from the point where it says "IR". But this energy has to be reflected (e.g. mirror). If you place an absorbing filter on your kindle, it just heats up your filter and the filter heats up your kindle.

Once the sunlight hits your kindle it is being absorbed, that means your kindle heats up. Now the following happens: Your peak wavelength is no longer at ~600nm, but somewhere at ~10,000nm (that is the wavelength of the maximum intensity for a black body at ~300K (your kindle). Look for Wien's displacement law if you are interested). This wavelength is waaaay on the right side (IR-Part) of your spectrum. Besides of that the general shape of the spectrum will still be the same (that little hill that is steeper on the left than on the right and it would be very flat and wide). Here is an example:

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Now if you still have your IR-Filter in place, congratulations, this energy will never escape and heat up your kindle. You built an energy trap

After that point where the solar energy reached your kindle, you can't win on the radiation side (i.e. reflecting covers, ir filters etc.). All you can do is fight against conduction to the battery/circuits and let the cover heat up.

The only (pracical) way to reduce the energy reaching the kindle is to cover the area that is not covered with the solar cell with a reflective foil (for example the emergency blanket that you mentioned) This blanket will reflect almost all energy that hits that area, reducing the amount of energy that heats up your kindle. You can't do anything against the solar cell being heated up and heating up your kindle.

The other (non pracitcal way) would be to place an IR-filter somewhere in front of the kindle (but not attaching it directly to it. That would reduce the amount reaching your kindle by lets say 25% without reducing the efficiency of your solar cell significantly. If your filter is not placed directly on your kindle, some energy still can be emitted as this emission is omnidirectional and your IR-Filter would only cover a specific area. The area covered by the filter (=the filter itself) would just heat up from the energy emitted from the kindle, but you wouldn't care as it is away from your kindle.


I hope this is clear, please let me know if there are any questions.

[geekmaster]

So perhaps IR window film over the solar cell, and aluminum paint on the kindle back cover?

[knc1]

A good way to make hot paint.

Re-reading my original post I realize I probably was less than clear.

The initial design here was to place the photovoltic cells on the back plastic cover.

When I suggested to make a cover from structural foam, I did not mean a replacement for the plastic cover.

I meant: "hinged bookcover" - - I just failed to type the thought onto the page that way.

Similar to those expensive, protective, covers Amazon sells as a Kindle option.

The cover finish material over the inside and outside of the two foam boards that form the bookcover would be "dealer's choice".

The photovoltatic cells would be attached to the outside, top, bookcover leaf.
In use, with the top bookcover leaf prositioned (partially open, except at solar noon) at the best angle to the sun would have an air space between the inside, top, bookcover leaf and the Kindle.
Which would also put the Kindle in the shade of its own, top, bookcover leaf.

- - - -

Drat! A hard post to write without using that multipurpose word: "cover" in a mis-leading way - give me points for trying.

[geekmaster]

You can already buy a cover with solar cells embedded in it.

[knc1]

Originally Posted by geekmaster

You can already buy a cover with solar cells embedded in it.

The fact that a software feature pre-exists does not seem to hinder DIY efforts in software.
Rhetorical: Should the fact that a hardware feature pre-exists hinder DIY efforts in hardware?
Rhetorical: Should the fact that something pre-exists even be relevant in a hobbyist's forum?