[Elecraft] KX1 operating time from 6 AA lithiums

[Charles Vollum]

> Here's how we came up with the 20-hour figure. Average receive current 
> is in the
> vicinity of 35 mA. Average transmit current, running 1.5 watts from an 
> internal
> 8 to 9 V battery, might be around 10 times that (350 mA). Let's say 
> you're
> listening around 80% of the time and transmitting 20% (pretty typical 
> for
> overall QRP activity). Since CW has about a 50% duty cycle, your 
> actual key-down
> duty cycle is around 10%. Simplifying things a bit, the average 
> current drain is
> about 35 mA * 0.9 + 350 * 0.1 = 66 mA.

The data sheet on Energizer L91 (Lithium AA) cells is online at
    
http://data.energizer.com/batteryinfo/application_manuals/pdfs/l91.PDF

This sheet contains a wealth of interesting info, including comparisons 
with E91 alkaline cells.  It also has data on some unusual features of 
the L91 cells including an internal thermal safety switch that limits 
their usefulness in high current (over 2 amp) applications.

Page 12 shows rated mAH as a function of drain at two temperatures, 
with E91 plotted for comparison.  In cold weather, the L91 is a total 
winner.  At higher temperatures, there isn't much advantage to the L91 
at low drains (10ma or less) but great advantage at high drains.  In 
other words, the more transmitting you do, the more you will benefit 
from lithium over alkaline cells.

Of course they are also lighter, have great shelf life, and keep their 
capacity in the cold, so there are lots of different reasons to like 
them.

NiMH batteries can be recharged many times, but they have a high 
self-discharge rate.  Mine always seem to be flat when I need them in 
devices that get sporadic use, but they really save money in devices I 
use daily.

73,

Charles  N7BPT
----
Charles Vollum
http://xsw.com/boojum -- Boojum, a trailerable offshore tug yacht