[Elecraft] ... and solar power

[Jim Brown]

On 12/1/2013 12:31 PM, Dr. William J. Schmidt, II wrote:
> Which products do you use?  I use their 60 watt panel.
>
> What is needed to prevent overcharge.  Nothing. I don't use a battery.
>
> What does less than full sun do to the capacity.  Full sun for full output
> is a fallacy.  Even 50% sun through overcast gives full output.  Buy a
> bigger panel, bigger than you need and it works almost until dusk.

Let's not oversimplify things. "Works almost until dusk" depends on far 
too many variables, including the load current.

I disagree with virtually all of this, and so does all of the 
scientifically-based work I've seen on the subject.  Full sun on a bare 
solar panel produces fairly high output voltage, but voltage falls as 
the load draws current from the cell. The output of a pair of my 55W 
industrial-grade solar panels varies from about 100 mA near dusk, to 300 
mA in the shade, to 2A in partial sun, to 7A or more in full sun. Output 
current also varies with the angle of incidence (greatest when 
perpendicular) and temperature (greater at lower temperature). These 
charging currents can be increased 20-40% by the use of a good MPPT 
charge controller.  I'm using a Genasun. GV1012, which can be bought 
here http://kansaswindpower.net/charge_controls.htm for $125, in a 
system with four 55W panels and two 210Ah 6V batteries. It is RF-quiet.  
Most charge controllers make noise, including a Blue Skies MPPT unit I 
recently encountered.

A battery IS needed to protect electronics (like a KX3) from 
over-voltage, as might exist in bright sun on a big panel and a load of 
only a few hundred mA (KX3 in RX only), and if you want to operate after 
the sun has gone down.  You don't need a BIG battery to do this -- a 7Ah 
12V battery is small enough to drag up a long trail, and it will let you 
stay on the air much of the night QRP.

I do agree that we can use only our brain as a charge controller if 
we're careful to monitor battery voltage and not overcharge it. All the 
Elecraft rigs I know of include a voltmeter, so that's easy to do, and a 
careful study of the battery manufacturer's data sheet will tell you 
when to stop.

73, Jim K9YC