"John" wrote in
news:x5KdnbpZbNocRpbVnZ2dnUVZ_jydnZ2d@caribsurf:

A friend, who is an engineer, informed me that I am wasting my time.
He says I need 12Volts from the solar panel to charge a 12 volt
battery. Thus the 7.2V will not do it.




Your friend would be wrong. Solar panel have quite high internal
resistance. Measure the temperature of unloaded solar panels just
sitting in the sun, then hook them up to a battery and let them charge
the battery until their rising temperature settles down. Note the
intense temperature increase in intense sunlight due to this internal
resistance loading the output voltage.

It is for this reason, that the open circuit voltage of solar panels is
usually many volts ABOVE, not below, the 13.8V (not 12) of a fully
charged lead-acid battery whos float voltage at charge is 14.2VDC.
Solar panels I own are all in the 18-19V open circuit range, allowing
for a 4-5V drop in panel output voltage at rated load.

Look at old panels and you can see where the current came from. Around
the edge of each solar cell, the silicon still has that gun blueing
color. Where the current comes from inside that diameter, heavily
charging solar cells are all brown, cooked by the current IIR drop that
causes the heat. The brown color hides the active part of the cell from
the sun so its output goes to pot, so to speak, as time goes by.

Your house battery needs about 5V higher than float voltage to
overcharge your battery. I say overcharge because the 19V solar panel
will go right on cooking the battery, very slowly converting its water
into hydrogen gas and free oxygen to rust the plates as the ever-
lowering electrolyte becomes much more concentrated sulphuric acid,
eventually eating the bottom right off the plate stacks if you don't
constantly pour more DISTILLED ONLY water into it. Therefore, make SURE
you also buy a CHARGE CONTROLLER, which limits the charging to 14.2V and
prevents the cells from cooking the battery while you are ashore. .1A x
6 hours per day = 18.6AH/month. It's amazing how that sneaks up to a
full 1.280 specific gravity full charge when the boat is shut down with
its loads disconnected.

The 7.2V would make a nice Sellphone charger....(c;