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Originally Posted by Ken Maltby
The specs for the example adapter show an output at 5.5v @ 1000mA (1.0Amps) USB2 specs allow for 1000mA and you can usually draw 20% more than that without encountering problems.
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Minor correction: The adapter is rated at 5.0V not 5.5V.
USB2 specs only allow for 500mA not 1000mA. USB3 can provide up to 900mA.
http://en.wikipedia.org/wiki/USB#Power
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Not that the power circuitry for the eReader's builtin battery would allow drawing an excessive amount while charging.
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It's not a question of drawing an excessive amount, it's being able to provide the minimum amount that the eReader wants to allow charging.
The recommended adapter is perfectly suitable for this situation. It's the 1.8W solar panel, which was stated would be sufficient, that's the problem.
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The OP was starting with a $2 kit that had a tiny solar panel with an output of 5.5v @ 80mA. The components that I described offer the closest to his initial kit, in terms of panel size, and price - that would provide reasonable, if longish, charging times.
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The OP's $2 kit included an internal battery that the panel can charge (over a possibly long period of time) and then the Libre can be charged from that battery, which can sustain sufficient current. The problem is that your solution didn't include this "buffer" battery and expects a 1.8W solar panel to provide sufficient charging current immediately and continuously on its own.
Even if the panel could provide its specified 1.8W, the Libre probably wants 2.5W. Wattage is calculated as volts times amps. When the Libre tries to draw the 500mA (0.5A) that it thinks it can get from USB, the 1.8W (in a perfect situation) from the panel could only be converted to 1.8W/0.5A = 3.6V. This voltage is out of spec and probably too low to charge the Libre.
My point is, that without a buffer battery, that can be charged via solar over a long time and then provide sufficient power over a shorter time for device charging, with a 1.8W panel you likely won't get any charging at all.
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If you went to the links I provided and looked around, at all, you could find larger and higher priced panels that also could be used as I described, these might charge the eReader a good deal faster. (Or a larger panel might not make any difference at all depending on other relevant factors.)
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I did go to that link. That's how I knew your recommended 1.8W panel was identical to the panel that I have been experimenting with. I also saw all the other panels there.
With just a panel and a 12V to USB adapter, any of those panels will either charge the device or not. None will charge "a good deal faster". It's the device itself that determines how fast it charges but you have to be able to provide at least as much "juice" as it wants.
Another thing to note. These panels tend to act more as current sources as opposed to voltage sources. They will vary their output voltage (within limits) in order to provide as much current as they can convert. I've measured voltages as high as 24V on the 1.8W panel under no load. When the device is fully charged it will stop drawing current. Without a buffer battery or other voltage regulator, the resulting higher voltage might damage the USB adapter, depending on its design.
The reason for my previous post was to point out that, without a buffer battery you will probably need a panel rated at at least 6W. A 1.8W panel, or even a 3W one probably won't work to charge the Libre directly, along with just a 12V to USB power adapter.