Some new cars are being shipped with and stored with small solar panels. I happen to get one from the local VW dealer. It does keep a battery topped up, I don't know what its output is, but it was free.

Haasman

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'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)