On Sun, 12 Feb 2012 21:30:43 +0000, Martin Brown
wrote:

You do need to pay some attention if your PV array is capable of
charging the battery at a decent rate, but for something as pathetic as
this it could stay connected forever and never be able to damage the
main car battery by overcharging. You have to make sure it cannot leak
current back through the PV panel in the dark. The protection diode
should prevent this but do check. It isn't even remotely cost effective.

The OP would be much better off buying a mains powered battery charger
instead. That at least will do the job of making the car start!

The only time I found an off grid application (greenhouse watering
system) where a solar PV could be almost competitive it was still easily
beaten by two lead acid batteries swapped as and when needed.

Yet another know-nothing arsehole.