Introduction: Solar Charging E-reader.

Picture of Solar Charging E-reader.
Hi, I made this instructable to show how to add some solar cells to your e-reader, so that you will never need to charge it again.

Update: I replaced the Schottky diode with a MAX1555 Li-Ion cell charging IC. Thanks go to moldboy and Kohaku for their input on making the charging circuit safer.

What you need :

  • An e-reader,  I use a first gen Kobo, but I'm sure this is also possible with any other.
  • Some solar panels,  like these. Perfect size and voltage and decent power too.
  • A Schottky diode , you can get this at any shop that sells electronic parts. You need a diode to prevent discharge of the battery. The advantage of using a Schottky diode is the lower voltage drop.
  • A MAX1551 or MAX1555 IC. They operate with no external FETs or diodes, and accept operating input voltages up to 7V, so very easy to implement. Here is a datasheet. I ordered a sample from the manufacturer.
  • A SOT23 to DIP breakout board. You need this, because it's quite hard to solder wires directly to the MAX1551/1555, since it's so small. I got this from eBay.
  • A small capacitor, to make life a little easier for our MAX1551/1555. I used a ceramic one I had laying around.
  • Some wires, a soldering iron, solder, a dremel and padded double sided tape. A multi-meter can also come in handy.

Here are some minor technical details:
This e-reader (as most of them are) is powered by a 3.7V li-ion polymer battery. These kinds of batteries need pretty tricky charging curves: fast at first and trickle charge till full, then the current stops, since over charging can be dangerous. For this reason I replaced the Schottky diode I used before with a MAX1555 charging IC. A Schottky diode also works, but it can damage the battery in certain circumstances.
The specifications of these solar panels claim that they can do 80mA, but after some measurements I came to the conclusion that in reality they will only reach about 50mA. Adding the two panels together we come to 100mA. This means that they should be able to charge the battery from nothing to full in about 10 hours. However, the charging current is controlled by an IC and will be lower during the later half of the charging cycle.

Step 1: Taking Apart the E-reader.

Picture of Taking Apart the E-reader.

The first step is to open the e-reader, but since it is not held together with screws, this can get kind of tricky. What I did was to take a knife and pry it in between the two halves of the casing at a corner. After this came apart, I worked it all the way around the entire device until it came apart. Watch out for the double-sided tape that is between the top cover and the screen.

Detach the battery wires from the main board and remove the battery.

Remove the four screws and carefully lift the PCB with attached screen from the rear panel. There is also some clear plastic behind the main board, don't lose this.

Step 2: Cutting a Hole in the Back Cover.

Picture of Cutting a Hole in the Back Cover.

Decide where you want to put the panels and mark an outline. Cut a hole with the dremel and make sure the corners are nice and straight.

Step 3: Wiring the Solar Panels Together.

Picture of Wiring the Solar Panels Together.

Wire the two panels in parallel; this means wire the pad marked '+' to the one marked '+' on the other panel and the same for the '-' pads.

Step 4: Putting in the Solar Panels.

Picture of Putting in the Solar Panels.

Use some strong padded double-sided tape to attach the panels to the metal shielding. Make sure the solder pads you want to connect end up at the top and are not covered in tape. The kobo has a little square spacer in the center of the back cover; cut it off the piece you removed from the casing and glue this back to its proper spot.

Step 5: Soldering the MAX1551/1555 to the Breakout Board.

Picture of Soldering the MAX1551/1555 to the Breakout Board.
The MAX1551/1555 is in a so called SOT-23 package, which means it's really tiny. To make it a bit easier to wire, you can use a breakout board; I used a SOT-23-6 to to DIP adapter I found on eBay. Soldering the IC to the adapter can be tricky and requires a little bit of patience.
You can follow this guide, but I'll summarize here:
  1. Place a small amount of solder on each of the pads (the guide linked above does this a bit differently).
  2. Place the IC with a pair of tweezers or a small clamp on the board and solder one of the legs.
  3. Gently put a little pressure on the IC and heat all the legs so that solder will flow between the board and the legs.
  4. After letting it cool for a second, we can check the connections by measuring the resistance between the IC leg and the soldering holes in the board.
You can also fill the holes with a little bit of solder now to make it easier to attach the wires later.

Step 6: Wiring the Panels to the Battery.

Picture of Wiring the Panels to the Battery.

Solder wires to the positive and negative terminals of the solar panels, with the capacitor in between (I used a ceramic one, so it doesn't matter which way it's connected. If you use a different kind, check the polarity).

Solder another pair of wires to the positive and negative terminals of the battery; you will have to cut away some of the tape. After you are done soldering, put some tape back so it won't short out against the back shielding of the device.

Put some tape beneath where you want to put your IC so that it doesn't short out against the EM shielding.

Now you can solder the wires from the battery and the solar panels to the IC. Follow the nice schematic attached to this step: the negative terminals of both the battery and the solar panels are wired to pin 2 (GND), the positive terminal of the solar panels is wired to pin 1 (USB) and the positive terminal of the battery is wired to pin 5 (BAT).

Place the board somewhere where it's not in the way of anything when you put the e-reader back together and put some more tape over it to fix it in place.

Step 7: Put the E-reader Back Together.

Picture of Put the E-reader Back Together.

Remember that clear piece of plastic you found between the PCB and the EM shield? Put this back to cover the shielding and the wires/IC.

Put the main board with display back in its proper place. Be careful and double check the position of the board you just soldered, so that it's not in the way of anything. Screw the four screws back in.

You can now snap the back and front of the device back together and see if it works. I tested mine out by first checking the battery (it was at about 1/3rd) and then finding a nice bright sunny spot to charge. After an while I checked the battery level again and it was at the next level (about 2/3rd).


DoWatt (author)2012-07-20

At a rough calculation, the 1000mAh battery in the Kobo would take just over 6 hours to recharge using the 2 panels you reference. Do you find this practically?

flapke (author)DoWatt2012-07-23

I'm not sure how long it takes exactly, since it would be a bit hard to practically measure (empty the battery, put the device in homogeneous sunlight for x hours till full).
However, if I leave the e-reader on the windowsill (south side) when I'm not using it, the battery does not run out. I even tried to empty the battery once and run it on just the panels and this does work as long as the panels are facing the sun directly.

bitbangr (author)2011-09-07

I have a thought....Amazon makes a cover for the Kindle 3 that connects to the battery to allow you to power a built-in light. I wonder if you could tap into that and build all the goodness into the cover, avoiding opening the reader at all.

lhospadaruk (author)2011-08-04

Why not just wire the solar panel to the USB power and let the built-in charging hardware handle things?

flapke (author)lhospadaruk2011-08-05

Because if you apply 5V to the USB connector, the Kobo turns on or goes into charging/mass storage mode if it's already on. It would be pretty annoying to get interrupted every time a little light falls on your e-reader, not to mention that it would be less efficient.
It might be possible to do this to other e-readers, though.

lhospadaruk (author)flapke2011-09-07

That's definately true, I thought at first that maybe it only goes to usb mode when it can actually talk to a usb host, not just when it gets usb power, but that's not the case.

ksilverwing (author)flapke2011-09-06

In that case, couldn't you just have some kind of cover on the solar panels so that they're in the dark while you're using the ereader, and then you open the cover when you want to charge it?

ewookie (author)flapke2011-09-06

i don't think he intended to leave it plugged in...just when charging. it is stupidly inefficient that these type devices turn themselves on when you plug them in to charge.

Satka (author)2011-09-07

Thanks for this great instructable, very clear and detailed.

It make me want to solar power more things now :)

clark981 (author)2011-08-05

Ya You're right. Besides, the nook's internals are under a grey panel, and are WAY MORE complicated than Kobo's, and I would go to juvy if B&N found out I totally hacked my nook. Oh Well.

clark981 (author)2011-08-03

Does it work with nook 1st edition, because i dont want to ruin my nook. I save up all my birthday money and allowance to buy it myself.

flapke (author)clark9812011-08-05

It probably would, but I don't think you should mess with stuff you can't afford to replace.

stakahashi (author)2011-07-03

I really like how professional this turned out. The only downside I see is that you have to leave your e-reader out in the hot sun. I could never do that to my beautiful kindle.

jeoncs (author)2011-07-02

Awesome job this is epic for any e-ink based reader since they use so little battery. For people afraid of voiding a warranty it wouldn't be too much effort to get one of the cheap silicone skins and a usb cable to stick to it too.

Kohaku (author)2011-06-15

i would look at replacing the Schottky diode with a MAX1551 IC which is designed to safely charge lithium cells.

flapke (author)Kohaku2011-06-15

Thanks, I'll look into it :)

moldboy (author)flapke2011-06-15

Yeah that would be better than the method I described. The other thing you might look at doing is simply connecting the the power and ground to the USB port as it provides the charging circuit through a MAX1551 or similar. The only problem would be having the USB and solar connected at the same time, so be sure to still include the Schottky and probably only plug in USB with the device on its back.

It would be possible to cut a trace and add an inline diode to the USB as well but that might cause a voltage drop and prevent the charger from working. (Though it could be specified at 4.5V and USB is typicaly 5 so a 0.3 drop diod would be doable as well. I have a kindle not a kobo so I can't say for sure how easy that would be. You may have to use surface mount parts to make everything fit better, but there isn't any challenge there. In fact you can probably find a dual Schottky diode in a small SOT package that would allow you to separate the USB and Solar but still have them both power the charger circuit.

flapke (author)moldboy2011-06-16

Powering the USB port is what I tried before this. The problem with that is that this ereader turns on every time it detects 5V, so that it can go into mass storage mode. So this kind of defeats the purpose if it uses power every time the slightest bit of light hits the panels.
I'll see if I can find a MAX1551/1555.

flapke (author)flapke2011-06-28

Alright, I replaced the diode with a MAX1555 IC and updated the instructable :)

bobgdolly (author)2011-06-16

Could you make this work using normal charging port ? without opening the unit .

flapke (author)bobgdolly2011-06-16

Not really. The ereader turns on when it detects 5V on the USB port and if you're reading, it goes into mass storage mode.
A slightly less invasive method would be to add a charging socket and attach the solar panels on the outside of the casing.

bobgdolly (author)flapke2011-06-17

The Kobo has a separate mini rca charging port, it cannot be charged thru the usb .

flapke (author)bobgdolly2011-06-17

That's not true.

bobgdolly (author)flapke2011-06-17

Your wrong ! I plug the trans. in to the wall the other goes into the charging port NOT THE USB!!!!!!

flapke (author)bobgdolly2011-06-18

All I see is a USB port.

bobgdolly (author)flapke2011-06-18

Thats what you have , I must have a earlier version, mine does have two ports one usb one charging, Would you know if I could do the solar thru the charging port ? and can you let me know how to do it ? Thanks for all your time.

flapke (author)bobgdolly2011-06-19

The version I have is actually the first, maybe you have a Sony. But ok, making a solar charger for that kind of port is even easier, all you have to do is make a plug at the end of the wires and plug it in. You can stick the solar panels to the back of your e-reader, or to a cover. There is no danger of overcharging, since the e-reader will have circuitry protecting the battery connected to the charging socket.

bobgdolly (author)flapke2011-06-20

Thanks for your your input, I thought that might be the solution , the reader is a Kobo Literati, I have a Nook that I use more often but I use the Literati for nighttime reading because it's back lit. I think I'll make a solar charger with batteries to extend the reading time. Thanks again. Bob

Gocho (author)2011-06-17

Nice Job!

I can't wait to see the improved version with the circuit to save the battery.

How many lifetime would have the Solar Cells?
Is not detailed in the DX web. Maybe in the attached docs.

flapke (author)Gocho2011-06-17

No idea about the lifetime, but surely it's longer than the ereader :P

Gocho (author)flapke2011-06-17

Good point :)

dark sponge (author)2011-06-16

Great concept, really similar to my solar DS. After making that, I realized how it was actually bad for the battery and potentially dangerous. You can add a charge protection circuit (I have a link in the materials list of the updated version) for really cheap and save the battery and a potential disaster, especially with your solar cell at 5.5 volts. The updated version is here:

I don't want to seem like a spammer, I just want to prevent a battery disaster :-)

flapke (author)dark sponge2011-06-16

Hey, thanks for the comment :) People already pointed out that this could be bad for the battery, so I ordered parts to save the battery (see below).
Maybe I should apply this same mod to my DS! :)

bojopopo (author)2011-06-16

great design! looks awesome, just got new kindle, so wont be gutting it just yet, but its fun to think about haha!

eandresen (author)2011-06-16

That battery looks like a LiPo

flapke (author)eandresen2011-06-16

You are correct, sir.

illuminatis (author)2011-06-16

Aren't you worried about cracking the LCD by leaving out in the sun?

Anidion (author)illuminatis2011-06-16

Most e-readers have an electronic ink screen, which is ink that is arranged electronically into letters or numbers or whatever.

sqeeek (author)2011-06-16

I've done something similar with an old Palm. The easiest way for me to do it was to wire the power source (solar/batteries/whatever) to the 5v part of a USB cable, then hook a USB female side to the other two, magnet that on the side...

Then when I want to hook up my Palm to my computer, I plug it straight into the computer or USB extension cord. Don't have to worry about discharge, and don't have to remember to bring my USB cord anywhere :)

Cool instructable.

alison201102 (author)2011-06-15

Looks good,but how to use it still not understand...

moldboy (author)2011-06-15

I would advise that no one do this to their ereader if they like it. This is a 100% guaranteed way to destroy the battery.

Li-ion batteries have a maximum cell voltage of 4.1 or 4.2 volts. Anything more than this and you cause serious long term damage to the battery. The proper li-ion charge cycle starts off with a continuous current charge mode, that is at less than 4.2 (or 4.1) volts a constant current is supplied to the battery. Again anything more than 4.2 V and you damage the battery. The current is not too important as long as you don't exceed the battery maximum. After some time the the battery voltage will rise, when the battery voltage rises to 4.2 volts the charge mode switches to constant voltage mode. That is to maintain the 4.2 volts the charge current has to decrease. The charger will continue to charge the battery with a decreasing current until some predetermined cutoff point, usually <10% of the charging current. There is then, in some cases, a trickle charge state that keeps the battery full. Depending on the battery this isn't always advisable.

What this design does is supply a voltage of around 4.5 - 5.3 volts (depending on the panel and diode) at a random current, which is going to destroy the battery. Some batteries like being trickle charged (like car batteries) li-ion batteries do not.

flapke (author)moldboy2011-06-15

Thanks for your comment :)

You are right about the voltage being too high, but I don't think this will destroy the battery, as long as you do not put the solar panels in direct sunlight when the battery is already full. I could be wrong, of course, but this is a very simple circuit and not meant for extreme circumstances.

It is always possible to put in a regular diode with a higher voltage drop or a small Li-ion charge circuit, although that would make the whole thing less efficient.

moldboy (author)flapke2011-06-15

I would say the simplest thing you could do to improve the design would be to add a precision 4.2 V regulator (or even 4.15V to be safe). Look at the TPS7800 from TI. It is essentially a LM371 (an adjustable LM7805) but more efficient.

Like you said the panel doesn't output too much current so current protection isn't really necessary. Then you will have a fill cycle charge and 'trickle charge' system. When the battery is dead it will be charged with 4.2 V at ~50mA until the battery gets close 4.2 V then the battery and source will be the essentially the same, since the difference is small the charge current will be lower I= (Vbat-Vpan) / Rbat.

Once the battery is fully charged the output from the panel should all but stop and no charging will occur. A real charge circuit that shuts itself off is still probably better, but more complex and considerable less efficient. Though to completely cover your bases having a battery heat sensor in the loop wouldn't be too bad either.

flapke (author)moldboy2011-06-15

Alright, that sounds pretty good. I'll see if I can find one of those anywhere :P

zack247 (author)2011-06-14

very simple and neat mod!
i wonder how much battery it uses for the ereader to display "powered off"?
unless it doesnt use any power and the screen is capable of keeping picture displayed for a prolonged period of time.

rivetgeek (author)zack2472011-06-14

E-ink uses zero power to retain a display, it only uses power to change the display.

moldboy (author)rivetgeek2011-06-15

In this case the Kobo probably uses a minimal amount of power to keep a system clock or something like that. But if you remove the battery from the device the display will still show what it did before.

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