maintenance
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We got a larger, 50 W panel a couple of months ago on Kleinanzeigen.
It's a bit dented in one corner and was able to fully charge the battery pack when we had some sunnier days recently. It's now used for the third time this winter to keep things running. Technically you could say it's the fourth time, because a new problem came up yesterday: Somehow the battery stopped charging before reaching 100%, so now even the battery pack has to be unplugged and replugged sometimes. Maybe compost just really somebody to come looking for it.
Talking about unexpected issues: It gets really cold in the attic, and so the battery cooled down to 6°C yesterday. It may not be possible to keep the phone there for the entire winter, with the coldest months still ahead of us. There are quite a few articles warning you to not let Lithium-Ion batteries get too cold.
For now the window stays closed and the panel is taken inside when not in use. The current battery temperature is 9.5°C, and there's a cozy stream of 420 mA streaming in from the external battery pack.