On 13/02/2012 10:29, M.Joshi wrote:
Thanks for all your replies so far. I have considered pulse chargers
such as the CTek and others however, I don't have a mains socket outside
near the front driveway. I would have to have a trailing mains lead
from the front door and that wouldn't be ideal for a long-term solution.
The CTek ones are also quite costly too.

I read many positive reviews about using a solar charger to keep a car
battery trickle charged and prevent it from going flat due to long
periods of not being used.

There are some good reviews for the Maplin's solar charger but it is
only a few watts.

Another one I saw on Amazon/eBay looks like a folding briefcase and is
about 4W.

I decided to look at a 10W panel as I assumed this would produce an
adequate output with reduced sunlight in winter.

I happen to have a nominally 8W PV panel to hand. Today on a relatively
overcast afternoon 7/8 cloud it managed a no load terminal voltage of
20v and a much more disappointing short circuit current of under 10mA.
If you are *very* lucky it might deliver 3mA into a 12v load (40mW).

So it will add about 18mAh of charge per 6 hour day at this time of
year. The self discharge rate for a lead acid car battery is likely to
be a lot more than that (especially when you include any continuously
powered car electronics like clocks or alarms). On a perfect sunny
winters day it might give 0.4A for 5 hours so 2Ah on a good day.

Starting from a battery that is fully charged the handful of decent
sunny winters days might be just about enough to replace losses due to
self discharge, but it is never going to recharge a flat battery in
winter unless we have fluke weather or the Earths axis shifts!

In midsummer when it can maybe deliver more like 0.6A and 6Ah per 10
hour day of charge then it stands a sporting chance of working. It would
still take the best part of a week even under the most favourable
conditions to get the battery charged back up to 40Ah.

For comparison the cheapest and nastiest 5A/12volt car battery charger
will recharge a battery from flat in about half a day elapsed time.

After reading some of the replies to my original post, I'm not so sure
now?

At present the battery is completely flat. Is it likely that leaving a
solar panel connected for a few weeks would restore the charge? The
solar panel would be placed inside the windscreen or rear window.

Not a chance at this time of year. Complete waste of money.

I probably intend to also connect a desulphator to the battery with the
solar panel.

You really would be a lot better off with a cheap and cheerful mains
powered battery charger. Amazon has one that looks suitable:

http://www.amazon.co.uk/Ferm-BCM1018...2611&sr =1-13

I expect there are cheaper ones still if you look around.

A solar panel might just possibly stop your unused car battery going
completely flat in winter or at least delay it somewhat, but it is
expensive and no substitute for a mains powered battery charger.

--
Regards,
Martin Brown