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Larry

wrote in news:1168660902.921941.207290
@a75g2000cwd.googlegroups.com:

I was curious if someone could explain why solar panels that charge a
12V system put out 19V?

Two reasons:

1 - Any charger must put out a voltage that exceeds the charging voltage,
up to 15V on a lead-acid battery, in order to force the current through
it backwards to charge it.

2 - All chargers have internal resistance that is effectively in series
with the charging power source. IN a solar cell, this is quite a high
series resistance. This inherent series resistance manifests itself when
the cells are charging near their design specification by making them so
hot they'll burn your hand and it turns the cells brown over time from
the heat. If they didn't have this, you'd never have to replace them.

So, here we have this circuit:

inherent
power source------series resistance------battery

When you take the battery off, you're measuring the power sources natural
output voltage, 19V. As your meter draws no current through the series
resistance, that resistance drops no voltage and you read 19V. But, if
the power source had no overhead of voltage and was, say, 15V, the
instant the battery started to draw charging current, the voltage drop
across the series resistance would drop the charging voltage below the
battery voltage and there'd be no charging current to speak of. But, at
some design level, if we had a 19V source, we could drop 19-14=5 volts
across the inherent series resistance at the design current output of the
solar panel. 19V is about "normal" for any size panel. The physics just
works out that way. Big panels have less series resistance than small
panels, so put out more current. The bigger the cells, the lower their
inherent resistance.

Keep the voltmeter across the solar panel and hook up a tail light bulb
across the panel. The 19V will drop drastically pulling the current from
the bulb through that resistance in the panel. If you use an ammeter to
measure the bulb current at this measured voltage you can calculate the
actual resistance of the panel, the voltage difference between no bulb
and bulb on, divided by the bulb current = the panel's resistance. If
you track the panel over time, you'll find the panels resistance
INCREASES over time until its useful output current finally drops to
useless when the cells are all cooked brown from the current....self
destructing, planned obsolescence....just like a GM car..(c;

That's why...

Larry
--
Extremely intelligent life exists that is so smart they never called
Earth.