As mentioned in a
previous thread, I have a particular problem leaving my X3 for prolonged periods in airport carparks.
I now think I have the answer.
I purchased
a 4.3W 240mA solar charger.
It arrived very quickly from the supplier but the
breakout box from China took rather longer so was unable to test it.
In comparison with
Lotus7's useful data, with the solar unit attached, the Time 0:00 voltage (as door is locked) is 12.44 V.
It rises to 12.67 V after 15 minutes and 12.7 V after 30 minutes.
This is on a very dull overcast October day at Latitude 56 with the panel laid horizontally on the dash so by no means the optimal position (I was trying to simulate the valet parking result!)
If stuck to the window in sunlight (even at my Lat. and less than 2 hours before sunset) it was charging the battery at 12.81 V even when at an angle to the sun - see photo below.
That seems to meet the need.
The unit itself is fairly neat and plugs into the ODB socket so is constantly connected. The plug is elongated so is easily connected and disconnected.