On Mon, 23 Feb 2009 22:10:27 +0000, geoff wrote:

http://www.maplin.co.uk/Module.aspx?ModuleNo=98358

at £12 should suffice

I have one of those on the genset battery. Last time I started the genny
(a month or so ago) there was no problem. Panel sits behind a north facing
window so doesn't generate an awful lot of juice and would put 50% more
into the battery if it didn't have 50% duty cycle flashing LED...

Lead Acid batteries have very low self discharge rates. 40%/year according
to: http://www.batteryuniversity.com/partone-6.htm.

So an 80A/hr battery 40% is 32A/hr over a year (8760hrs) = 3mA = 0.04W. So
even a 1.5W panel is more than capable (even taking into account dark and
cloud) of maintaining quite a decent sized battery.

--
Cheers
Dave.