For what it's worth, mine seems quite happy with a Halfrauds 2.4W/12V solar panel (£25) combined with the (essential in my view) OBDII plug (£10). The 12V sockets front and rear always tend to be slightly hit and miss in my opinion and the crocodile clips won't be viable in mine due to the lack of ski hatch. The OBDII plug just works and the terminal is always live. Reverse polarity protection too.
My recent battery woes detailed here:
http://forum.a8parts.co.uk/showthread.php?t=15074
http://forum.a8parts.co.uk/showthread.php?t=15132 (related thread from Nognar)
There is a larger 6W panel available but that's the size of a house.
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2010 B8 S5 Sportback quattro 3.0 Supercharged in Sprint Blue (current girlfriend)
2000 D2 A8L quattro 4.2 in Audi silver (my ex-girlfriend)
