A quick little update on this project.
I left the solar box outside for 3 weeks from January to February 2021, seemingly the coldest part of the winter. Temperatures dipped to -23 Celsius at the lowest without the wind chill.
In order to give the box something to do, I hooked up a USB reading lamp to the 12V USB adapter. It wasn’t pulling a crazy amount of power (only 0.05A). The kit managed to keep going, discharging a bit each night and recharging during the few hours of sunlight it had during the day. One handy thing about the light being hooked up was that I could see if the kit was still on without having to go outside.
It only died once that I’m aware of and the next day it came back to life after a few hours of sunlight hitting the panel. That was 2 days before I brought it back inside.
The battery was not able to fully charge each day due to a combination of the cold, some heavy snowfall and overcast days during those 3 weeks. On a sunny day, the sun was hitting the panel for about 3-4 hours per day, which would normally be enough to fully charge the 7A battery during the warmer months, but not enough in the winter. I could’ve adjusted the charge controller to have a higher float voltage to compensate for the cold temperatures (currently set at 14.1 V). The only problem with this is that I would have to manually adjust it anytime the ambient temperature got too high. Not much of an issue in this situation, but it would be an issue if this was deployed throughout multiple seasons (ie: 6 months from January to June). 14.1 V is a good float voltage for 20ish degree weather. Setting it higher to compensate for the cold winter weather would keep it topped up better in the winter, but would lead to the acid in the battery boiling during the warmer months, thereby killing the battery.
My initial thoughts about increasing the amperage by adding another battery in parallel would possibly prevent the float voltage adjustment from being necessary in such situations.
When I brought the kit back inside, the battery was not fully charged. Charging it with the solar panel in the window for about a month did not do much to bring the battery back up to full capacity. It did get really warm here over the last few weeks and so I set it back up outside. Within a few hours, the battery was fully topped off again.
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