[Elecraft] [K3] Solar Power

[Robert Beard]

Hi Bill,

The K1/2/3 rigs do very well on solar power. We ran the K3 (at 90W) on solar 
for awhile last Field Day.

You are correct in assuming three components for solar: the PV 
(photo-voltaic) panel, (a constant current source), the storage battery 
(constant voltage source) and an interface between the two - the charge 
controller.

It remains only to determine the "size" for each component by doing a rough 
estimate energy budget.

For example, the K3 draws about 1 A on receive and about 3 A on transmit (at 
the 10W setting).
If you're transmitting 25 percent of the time (a lot....) and a 50 percent 
duty cycle (CW/SSB), figure about 1.5 A continuous at 13V (an average 
battery voltage) or 20 W continuous.

A 15 AH (Ampere-Hour) battery would be good for about 10 hours from a full 
charge. (Theoretically - but more like 7 to 8 hours).

But, how about maintaining the battery charge with a PV? Well, of course, an 
ideal 20W panel would work, but, unfortunately, the panels are not ideal. 
They don't work at night - that's a (again, roughly) 50 percent loss. They 
don't work very well in the shade either, and the sun must be perpendicular 
to the panel for maximum output. Figure another 50 percent loss - we are 
going to need, roughly,  an 80W panel to maintain the battery indefinitely.

Another "but", this assumes the K3 will operate around the clock - not very 
likely - so a 20 W panel  and 15 AH battery would be adequate, after all, 
for casual operating at the 10 W level - without discharging -  the battery. 
This gives us a "feel" for the numbers.

This also specifies the required current for the charge controller - 1.5 A, 
(make that 3 A for cooler running).

So: a 20W panel, a 15 AH battery, and a 3A controller would comprise a 
conservative system for the K3/10.
Of course, these numbers can be scaled down if you don't mind discharging 
the battery over a given number of hours (left as an exercise for the 
student....).

Here at RadioRanch, we are using four 16 AH deep cycle batteries from 
Wal-Mart, in parallel, on an 80 W panel, with a 12 A controller. This seems 
to suffice for casual operation of the K3 at 90 W. The batteries are small 
and light enough to be easily carted about for portable operation.

Good luck with the solar....

Rob, N6VX


----- Original Message ----- 
From: "William Evans" <w4ish.bill at w4ish.net>
To: <Elecraft at mailman.qth.net>
Sent: Friday, September 11, 2009 8:01 AM
Subject: [Elecraft] [K3] Solar Power


> Guys and Gals of the K3 ilk,
> I am interested in setting up my K3 to run off of solar power but know
> very little about what it will take.  I assume a battery and a
> controller/charger but what size battery would work best for operation
> up to 10 watts, and what would work best up to 100 watts
> (occasionally!)  (and I do not need 120 pound flooded lead acid
> batteries -- lighter weight, gelled cells in the 60 pound range are
> preferable -- my back is not what it used to be!)
>
> Charging off the grid is acceptable at first, but the idea is to
> switch over to the solar panel (what size, capacity?) as soon as I
> can.  I am familiar with solar power as I have had, in the past, solar
> panels charging batteries for some of the house current (running
> through inverters), although presently I only send the power back into
> the grid without battery backup. Who out there is doing this for
> emergency or regular use with the K3 and can offer some assistance?
>
> Bill, W4ISH
>
>
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