Battery tender location was killing my 442 until I moved it

I always thought the battery posts were the only spot for a tender, but after finding my 442 dead last February I tried the remote ground on the block and it's been solid for two winters now. The car sits in a detached garage in Cleveland where temps drop to single digits, and the old setup kept showing a full charge that was a lie. Now I check the clamp with a multimeter once a month and it reads 13.2 volts steady. Also, plugging the tender into a smart outlet with a timer makes it cycle better than leaving it live all day. Curious if anyone runs a solar panel in a window for cars stored without power nearby. Does that actually keep a battery topped off or is it just a gimmick?