On 12/02/2012 20:39, The Natural Philosopher wrote:
M.Joshi wrote:
Hello all,

I'm thinking of connecting a 12V 10W solar panel permanently to my
vehicle battery as I am not using it very often.

The battery keeps going flat in the cold weather.

You will have noticed that weather is cold because , by and large the
sun doesn't come out much, or for long, and its a bit weak when it does,
so exactly what do you hope to achieve by this ridiculous exercise in
naïveté?

Increase the profits of people who sell useless solar PV kit.

At the moment up in North Yorkshire I am surrounded by solar powered
"please go round the bend" signs that wreck their batteries at this time
of year. This is exactly the time when they are needed, but alas the
cold dank dark days at sub zero leaves them dead in the water and on
frosty mornings people inexplicably fail to negotiate the bends.

Some people even manage to do it in midsummer too:

http://www.gazettelive.co.uk/news/te...4229-26963106/

The snazzy bend signs are usually working in the summer when there is no
frost and plenty of sun.

What I wanted to know is if others have done this and whether there is a
danger of damage leaving the panel connected when starting the car?

Why not connect it to a micro-ammeter. At least you will be able to see
something happening.

Be fair - in ambient light in winter it might just provide enough to
null out the base load of the car during daytime.

The panel I am thinking of purchasing off eBay has a protection diode
built-in.

Which clearly shows thinking is something best left to people who know
how to do it.

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.

--
Regards,
Martin Brown