[Elecraft] Field Day experience with K3

[Fred Townsend]

Thanks Ed for unraveling some of the misunderstanding of solar cells and
batteries. It is pure folly to think that you can apply faulty math to
highly optimistic and sometimes politicized ratings while totally ignoring
system efficiencies and come to any meaningful conclusion.
The standard way of rating audio power was to measure the RMS power at say
3% distortion. When marketing became all about watts the advertising folks
took over and the manufacturers started using peak to peak watts and other
wind at your back measuring systems. The same is true of solar cells where
rebates, based upon power rating and other government intervention, have
made a joke out of rating systems. 
Solar rating problems begin with the sun. It's always moving and therefore
the angle of incidence is always changing. This immensely effects efficiency
of lattice structures. The sun and Ohmic losses cause heating which further
reduces efficiency. The result of all loses is a small fraction of rated
values. 
Let me illustrate with some real test data.  For a 12V/5W rated
(approximately 200 square inches) panel with the American manufacturer's
recommended controller I was able to charge a gel cell at just over 400ma
(13.5v @ 402ma = 5.427W).  That's slightly over the manufacture's rating. To
obtain these values an ammeter was used to adjust the tilt and pan of the
array for maximum. Current was continuously monitored and the time measured
until the value halved (201ma). The controller maintained the 13.5v charging
voltage so the power was halved as well. 
Sounds great but here is the rub. The time to half power averaged 15 minutes
and then only between 10am and 2pm. If the array was laid flat on the ground
and not oriented toward the sun I averaged less than 10 watt hours per day
in June in Southern California high desert. That wouldn't run one K3/10 very
long.

73, Fred, AE6QL

-----Original Message-----
From: elecraft-bounces at mailman.qth.net
[mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Edward R. Cole
Sent: Sunday, July 01, 2012 12:36 AM
To: Elecraft at mailman.qth.net
Subject: Re: [Elecraft] Field Day experience with K3

Dave, ab9ca/4 wrote:

This isn't right. The capacity of the battery in watts is 110x12 or about
1320WHr. If the load is 122w/hr the battery should be totally exhausted
after about 11 hours.
----------
yep, I confused AH with WH, so that does not explain the problems the
original writer had.
Another way to analyze would be using just the current load:  2x17a = 34a x
30% = 10.2amp

Then 110 A-H/10.2 A = 10.8 hours (theoretically)

But battery discharge curves display the actual battery operating voltage
one can expect to see.  so one would probably reach an unworkable voltage
sooner than 10.8 hours.  The only good analysis would be if you have the
discharge curve for the battery.  These usually assume a <10% load (e.g. <
11 A).  Using the discharge curve you can predict battery life under load.

I used a 30% duty cycle which provides the time weighting factor
(integration factor). You can argue that in FD operations the duty cycle
could be something other than 30%.  I assumed that one does a lot more
calling in FD than normal assumed Tx/Rx ratios that commercial radio
industry uses (10%).

In my professional observations of radio using backup battery power the
useful life never approaches the theoretical expected battery life.  Most of
the time the system becomes unusable in about half the time expected from a
battery bank.  This is on a properly floated battery bank with periodic
equalization (hams normally do not do this).

Theoreticals are only a good starting point as they assume fully charged new
batteries (rarely is either true).

The 120AH solar charging system "theoretically" provides 10A at 12V which
would lead one to believe it would supply the operating load of two radios.
I rarely have seen solar panels output their full rated power.  In the
brightest sun it is normally running about 80% of label ratings.

So there was something else occurring to cause the voltage to sag to 10v in
the original posters operation.






73, Ed - KL7UW, WD2XSH/45
======================================
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