On 2/26/2009 11:41 AM spake thus:

On Thu, 26 Feb 2009 11:26:00 -0800, 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).

I'm thinking that all he needs to do is wire it up to his battery to
trickle-charge it. I think I'd add a diode (IN200x) in series for
protection. So will this work?

Since they don't list the output voltage for the panel, except to say
"12 volts" in the description, I guess one could opt for this larger one:

http://www.harborfreight.com/cpi/cta...emnumber=40085
(20 watt panel, made for charging auto & RV batteries, $190)

which says it maintains a charging voltage of 17.5 volts, which seems a
little high: perhaps a string of 3 or 4 diodes in series would drop the
voltage just enough to bring it down to the recommended 13.6 volts or so.

A little more information is needed. How many amphours per day will he
need? What type of battery? Is it flooded, gel or AGM? What size is
the battery?

Not sure of current draw. Standard-size automotive battery; I already
stated that.

Solar panels rarely put out more than half the "rated"
wattage.However, most have output voltage around 18 volts, and a
charge controller is used to prevent overcharging. AGM's and
especially GEL batteries will have dramatically shortened lives if
charged at too high a voltage. For an AGM it's 14.7 volts max.

Passive voltage control won't work for this application. If you limit
the maximum voltage enough to prevent overcharging or overvoltage, you
will have no charging at all under average daylight conditions.

So how about an active regulator (say, a 78xx) that would limit the
voltage to the rated max for a lead-acid battery?


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- lifted from sci.electronics.repair