[Elecraft] K2/100 QRP battery considerations

[akern@haverford.edu]

Here's a weird one:

I am giving the K2/100 it's shakedown cruise and I just started running
it with an external 8 AH gel cell battery.  I notice that when I turn the 
power control down below 10 watts--forcing to rig to disable the
KPA100--I draw about 1.7 amps at 11.3 volts according to the K2 display.  
If I turn the power control up to 11 watts, enabling the KPA100 in the
process, and then enter tune mode I draw significantly less current,
despite the higher final output.  In fact, I get up to around 37 watts
with the KPA100 enabled before I draw 1.7 amps again, though now the
voltage drops to 10.7 volts during tune mode.

1.7 amps, 11.3 volts at 5.0 watts out...vs. 1.7 amps, 10.7 volts at 37
watts out.  What am I missing here?  Am I getting something for nothing
at the higher output?  Have I just cheated thermodynamics?

More seriously, which setting is advantageous for power conservation?  I
can't imagine that the higher output is easier on the battery unless the
finals in the base K2 are much less efficient than those in the KPA100.  

Perhaps a more realistic comparison:  given the choice of operating at
9.0 watts with the KPA100 dormant vs. 13.0 watts with the KPA100 active,
which one would preserve the battery more?

When the power control is above 10 watts does other KPA100 circuitry
"wake up" throughout receive that would otherwise be off in QRP <10 watt
mode, thereby negating any power savings due to presumptive higher
efficiency of the KPA100 finals?

K2 is still plugging away.  DSP is amazing.

-Adam, N1KO