The recent failure of my SPA has me thinking that it would be a good option to have an AC power adapter unit with battery backup. Many of us have our Sky units installed on the roof or other locations within a reasonable distance of an AC outlet . A snap-in mechanism similar to the solar power array with a lengthy cord could be a solution for us that don’t prefer to rely on batteries or have long periods in the winter where the sun is impeded.
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Or even better, a SPA with an AC power adapter. 
When the sun is high enough (during the summer months), the internal batteries are fully recharged by the solar panel.
When the sun is lower or too low (during the late fall, the winter months and the early spring), the internal batteries are fully recharged by the AC adapter if the solar panel cannot provide enough energy.
The best of all worlds. 
I think one of the issues is that charging LiFePO4 batteries when they are below freezing can cause the Lithium to plate out as Lithium metal thus removing the ion it was part of to no longer be available for charge storage. How many SPAs have died with enough light to charge but never had a temperature above freezing so the batteries could charge?
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I cannot remember what the temperature was (if the temperature was near or below freezing) when the SPA for my SKY unit started showing signs that its battery was not charging anymore - it eventually stopped transmitting completely 8 days ago.
What is odd, thought, is that I also have a SPA on my AIR unit (the AIR unit is located about 6.5 meters below the SKY unit, about 2.5 meters above ground level, the solar panel of the SPA is pointing north, it is in the shadow of the house all days with no direct exposure to the sun) has gone through the same temperatures and has been able to keep its voltage fairly consistent (between 3.32 and 3.34 volts).
Don’t forget how energy-efficient the Air is. I expect the Air to easily last 2 years on a set of ultimate Lithium batteries which is 18Wh of energy. A fully charged SPA holds 3.3Wh. This means that a fully charged SPA can run an Air for over 130 days with no energy from the solar panel.
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Yes, I knew that the AIR unit was more energy efficient than the SKY unit (I had read about it in another thread).
I have a feeling that the SPA on my AIR unit might not be able to make it until the beginning of the summer months when the sun elevation is greater (the sun gets higher over the horizon) and the solar panel get more indirect brighter light…
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Last week the freezing temperatures in Toronto stopped my SKY from charging - a recent warm period had it come back on line with normal 3+V charge. I was adding another camera to my network, and it got me thinking that maybe a POE option for future hardware may help solve the power issue. I have several high wattage POE switches that I need for my remote network APs, cameras, and RPis, but a POE injector could work for those that don’t already have or need a set up like mine.
Tom
One problem with AC power for the AIR device is that installing wiring to the locations where SKY is usually mounted causes a lightning strike hazard. Indirect but nearby lightning strikes can induce current into long outdoor conductors, requiring surge suppression electronics in the SKY and power supply. Closer or direct strikes then cause a fire or electrocution hazard. Also, there may be NEC compliance issues caused by this arrangement.
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What about a battery mounted where it is accessible but not connected to the grid? Is it still a lightning strike hazard but less so?
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