Carl Gibbs wrote:


I wanted to measure the drain on the battery today, but I've just
discovered my mulitmeter is with the SD1, but I guess it'd be a few
hundred milliamps, so I reckon a 15W panel should be enough to
counteract this. Any thoughts? Sound logic or would I just be wasting
my money?

Has enyone else had experience of using these as trickle chargers?

trouble is that the rated output is almost always for full solar
constant; i.e. facing the sun at the equator. If you incline it away
from normal, live somewhere of significant latitude (and thus have a
thicker layer of atmosphere), with notable humidity (and hence
scattering of sunlight) and most significantly cloud cover, then the
real output will be anything from maybe 20% (if you are lucky) to well
under 1% of the rated output.

Putting it behind glass (e.g. inside the car) also removes the UV, which
is a good thing from a skin health point of view, but is also the region
of the spectrum containing the high energy photons from which a god
fraction of the energy you net comes from.

Furthermore, as Steve F mentioned, the silicon panel itself, in the
absence of light, will actually drain the battery slightly (the better
ones have the diodes to prevent this); if the weather is against you,
then it is quite definitely possible for the nighttime drain to exceed
the daytime charge, and leave you worse of than you are without it!

If I were you, I'd fit a battery isolator switch somewhere inconspicuous.