|
Batterie
s and QRP - Intro
Thanks to the popularity of cell phones, laptops
and other personal electronic devices, the battery industry
has been market-driven to produce higher capacity batteries
in lighter weights than previously possible with yesterday's
chemistries.As a result, a wide variety of battery types are
available to the outdoor QRP operator.
About the Tests 
The goal of these tests was to
determine the discharge curve of various battery chemistries.
A bit of online research at any battery manufacturer's website
will provide you with charts, graphs and statistics of almost
anything you could want to know about any of their batteries:
dimensions, weight, discharge characteristics, capacity and
much more.
One site lists its AA alkaline
cell as having a capacity of 2850 mAh. The fine print tells
you that this number will vary based on the discharge rate (this
is true of all battery types). Off to the side would be a graph
with various curves representing how that battery would discharge
when used in a flashlight, a toy car or a smoke alarm. I never
was able to find the curve labeled "QRP Rig"!
Further complicating matters was
the fact that I not only wanted to compare one cell size to
another (AA vs AAA) but one type to another as well. And I wanted
to compare them at a discharge rate typical of a QRP transceiver.
This would tell me what I could expect from any battery I might
use, the way I intend to use it.
The battery tests here were made
with a West
Mountain Radio "Computerized Battery Analyzer II"
and a Fluke 87 multimeter. The CBA's software allows me to discharge
a battery at a user-selectable rate over time, to a user-selectable
voltage. The Fluke was used to periodically verify conditions
being measured and plotted by the CBA.
The Elecraft KX1 is the rig that I most commonly
use for portable operation, therefore the variables plugged
in for the battery tests were chosen with this rig in mind.
Measurements show that my KX1 draws the most current when the
40-meter band is selected.
Specifically (worst-case scenario):
Rx current (lamp ON) = 62mA
TX current (lamp ON) = 720 mA
Average current = 200 mA
Power output is between 3 to 3.5 watts. To
determine average current, I used the same assumptions made
on page 12.1 of "EMRFD" by Hayward, Campbell &
Larkin. They reason that even when transmitting constantly,
duty cycle is only 50%. And assuming that we receive at least
as much as we transmit, duty cycle is further reduced to 25%.
In reality, receive time is far greater than 50% for me (unless
I obtain a P5 prefix!).
I wanted to further pad the results in such
a direction that any error would result in there being power
remaining in the batteries rather than having a graph tell me
there should be 10 more minutes of "QRP time" while
the rig itself is QRT, so I configured the battery analyzer
to draw 300 mA from each set of batteries tested.
One more factor in determining battery life
is deciding when the battery is dead. Depending on battery,
this is determined by either the battery type or by the lowest
operating voltage of the radio being used. In my case - regardless
of battery type - I consider 8.5 volts to be the lower limit.
Elecraft recommends 8 volts as the lower limit for the KX1 -
but anything less than 8.5 volts in a 3-cell lithium-polymer
battery comes too close to their lower limit without risking
ruining one or more of the cells.
Having said all that, let me say that the purpose
of these tests is not to tell me exactly how long I will be
able to operate my KX1 with a particular type of battery, but
rather to give me (and whoever else is interested) a comparative
indication of one battery type to another under conditions that
simulate how we use them as outdoor/portable QRP operators.
Battery
Chemistries
| 
