wrote:

On Fri, 27 Feb 2009 11:48:47 -0600, "Pete C."
wrote:


wrote:

On Fri, 27 Feb 2009 11:10:46 -0600, "Pete C."
wrote:


wrote:

On Fri, 27 Feb 2009 09:04:44 -0600, "Pete C."
wrote:


wrote:

On Thu, 26 Feb 2009 17:55:56 -0600, "Pete C."
wrote:


David Nebenzahl wrote:

On 2/26/2009 1:13 PM Pete C. spake thus:

David Nebenzahl wrote:

Told a guy I'd do some research for him into a small simple solar system
for use in maintaining the charge on a 12 volt automotive battery. Found
this at Harbor Freight:

http://www.harborfreight.com/cpi/cta...emnumber=96418

It's a 12 volt, 15 watt panel for cheap ($70).

The 45W three panel package that Harbor Freight sells for ~$200 is the
best value $/W and the panels are decent. You have to replace the junk
charge controller they provide with a decent unit though.

Any suggestions? I'm not familiar with these units (charge controllers),
could use a little guidance here.

45W = 3.75A or so so you don't need a big one.

How do you come up with this Amp figure?

P/V=I

45W/12V=3.75A

It won't be exactly that since the voltage won't be exactly 12V, it
could be more like 3.21A @ 14V. It's also unlikely that it will get
quite up to the 45W rating unless you have a MPPT charge controller on
it.


Just for openers, the output voltage for the panel is more like 18
volts. And it only puts out 45 watts under perfect and optimal
conditions in a Laboratory. In real conditions you may get 30 watts
output from it for a period of a few hours on a sunny day.

The output voltage of the panel varies with the load on it.

The rating used to sell it is the output without any load applied.

No, it isn't as there is no power (power as in Watts) output with no
load applied, only open circuit voltage (closer to 24V).


The advertised ratings are puffery, and are calculated based on
unloaded voltage and theoretical output derived by calculation.


This is
where the MPPT tracking controllers come in, finding and tracking the
sweet spot where the panel can produce the highest power (wattage). At
any rate, the straight P/V=I gets you into the correct charge controller
current rating with some conservatism.

Yeah, if by "conservatism" you mean, "divide the rated output by about
2"! LOL

Most Solar panels sold by bargain outlets have specious specs.

Have you actually used the panels in question? If not hold your babble
until you have.

This is a subject I am VERY familiar with. The only babble I've heard
so far is from someone who believes advertising copy.

I'd do some measurements on the panels, however there is a severe lack
of sun today. Perhaps on the weekend if I get bored.