Quote:
Originally Posted by Ken Maltby
The regulated power from the adapter should be able to provide enough charging current for your device even as the solar input fluctuates.
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Not likely.
I have a number of the 1.8W panels mentioned. On a bright, cloudless summer day, with the Sun directly overhead and the panel perfectly aligned with the Sun, the most I've ever seen is about 1.5W. They must rate these panels based on standing on a mountain top at high noon in Arizona.
The Libre charger is rated 1A at 5V, which is 5W. But, USB is only specified to provide 500mA at 5V, which is 2.5W. When charging via USB the Libre probably draws the full 2.5W when the battery is low, so that's what the panel has to provide to maintain the 5V required.
So, given perfect efficiency in the USB adapter, we need the panel to provide 2.5W under normal conditions. By "normal" I mean no clouds but the Sun at about 45 degrees and the panel reasonably but not perfectly aligned.
My experiments show that a 1.8W panel can average about 1W under the above "normal" conditions, so we would need about a 2.5W x 1.8 = 4.5W solar panel to maintain charging the Libre.
However the 12V to USB adapter mentioned won't be 100% efficient in converting to 5V. It will probably more like 80% efficient, or worse, so we have to increase the panel output by that factor to compensate, which brings us to needing about a 5.6W or greater panel.
If you're not going to be around to re-align the panel with the Sun every half hour or so (or build yourself a Sun tracker mechanism, which would need additional power), then even the above "normal" conditions are quite optimistic.
Also, every time the Sun goes behind a cloud the voltage will probably drop such that charging will cease.
To compensate for these problems, it's best to use a "buffer" battery. I.e. use the panel to charge a 12V battery and then charge the Libre using the USB converter attached to that battery. However, depending on the output of your solar panel and type of 12V battery, you may also need a charge controller to prevent frying your 12V battery. This introduces additional inefficiencies, so your solar panel now needs to have even greater output, to charge the Libre with the same amount of "Sun hours".