On Mon, 13 Feb 2012 11:03:17 +0000, wrote:

Yep; I did it for years, as I'd just bought a new battery for my car
but wasn't using it much (at all).
It kept the battery up to snuff until last winter, when -18degC killed
it.

Do you know if it was actually better than not doing anything?

I should clarify it was a polycrystalline 15W panel, mounted inside
the windscreen facing south. Charge rates varied from 30mA to 170mA,
depending on time of year.

The car lay unused for four years and it was in a position where
coupling up a charger would have been impractical. It had to be
started and moved from time to time and the battery was a pita to get
in and out, so the solar panel kept the battery up for all that time.
When the car went back into service the battery was as good as I
expected it to be. Certainly if I'd done nothing it would have been
utterly shagged. A self-discharged battery that's left to lie idle is
a write-off; the solar panel avoids that.
On re-entering service the alternator turned its toes up and I had
difficulty finding another one, so for a couple of months I was
trickle-charging the battery in-situ every night. Now, solar panel or
not, the battery was four years old at this point so I wasn't
expecting miracles. The death blow came during a period of intense
cold, when even with a tubular heater under the battery tray, the
engine bay was cold enough to adversely affect the battery. One night
I forgot to put the heater in - and the battery had definitely lost a
lot of capacity. It became obvious over the next few days that it was
on borrowed time. I replaced the battery in the spring and kept the
old one on an Aldidl charger - it continues to show a healthy charge
even now, but I wouldn't trust it for winter use. It'll do as a
standby/emergency boost battery for now.

The upshot of all this is - I would recommend a decent sized solar
panel for a car that's idle for at least a while; whether anyone wants
to use it for years is up to them. For a few weeks or months I see no
downside.