Hi Ian
On (10 Nov 97) Ian Smith wrote to Alec Cameron...
 CH> But you lost the .7 volt drop across the diode in the process...
 AC> That is NO great "loss". Solar panels operate at about 50%
 IS> efficiency, to
 AC> charge a 12v battery you use a panel with an output of
 AC> about 30v. To lose 0.7v from a 30v source, is to "lose" just 3
 IS> percent.
 IS> Um, I'm not sure what sort of efficiency you're talking here, Alec, when
 IS> the very latest experimental solar cells are getting to around 25%, and
 IS> production ones are more like 15-20% tops?  Also, as I mentioned, 12V
 IS> panels that I've measured usually run about 18-22V open circuit?
Thanks. I should have tried harder- we are talking of two different
"Efficiencies"
a. Of the sun radiant energy that impacts on the panel, around 25% gets
converted to elec energy. That's for the experimental cells you cite.
b. I was referring to the efficiency WITHIN the panel- for every watt of
electricity that is converted from sunlight, about a half gets to the 
xternal
load [battery] and the remaining half is lost as heating of the panel thru 
ts
effective resistance.
This half-and-half relationship is a common goal in the design of radio, hifi
and other systems- maximum power flows from a source into a sink, if the
impedances are matched. This leads to the source voltage being halved in the
process: half the volts "lost" inside the source, the remainder half making 
t
to the load. SO the penalty for max power throughput, is to run at 50%
efficiency which is as I imply, a common design criterion.
Something comparable occurs in the design of automobile engine starters- when
the battery is tired and cold, the 12v battery is able to deliver its maximum
power at about half ie 6 volts, when cranking power is most needed.
The extra oomph obtained when the battery is fresher and warmer, allowing it
to crank at say 8 to 10 volts, is a bonus of no great value except! that the
radio may still be heard. Rock on!
 IS> Kali - you remember Kali? - says that on really hot days the o/c voltage
 IS> can drop to where a diode might start to be be a current limitation, but
 IS> I've not struck that much loss with heat myself, and most panels are
 IS> designed with the use of blocking diodes in mind anyway.
I don't recall that. Wasn't aware of a hot day problem. Most semiconductors
work better when hot due to the negative temperature coefficient of
resistance. Bad news for a blocking diode if used against a really high
inverse voltage, but these days just about any diode will withstand voltages
way way above the range of HOMEPOWER circuits.
 IS> Still can't figure this, but knowing engineers, I'm sure there's some
 IS> tricky explanation :)  I mean I get needing headroom from o/c voltage 
or
 IS> regulation into say 13.8V float plus the 0.7V plus wire losses, but this
 IS> 50% figure has rather thrown me .. ?
I hope the 50% makes sweet music now! Best "headroom" seems to apply, when 
he
panel open circuit voltage is twice the battery voltage. This gives max power
[watts] into the battery, and we accept that an equal number of watts goes
down the drain in heating the panel internally from Mr Ohm, as well as from
Ole Man Sol from the outside!
My son says, there is an optional dc dc converter that allows dull day output
[panel voltage o/c below 13.8v] to nevertheless, trickle charge the battery.
Cheers.....ALEC
... ........When I contemplate the moon, my head aches [Galileo]
--- PPoint 1.92
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