[Elecraft] Kx3 lithium batteries life

[Bob]

Some earlier posts have referred to a LED on the charger/clock board
that seems to blink 24/7.  So there may be more load than just the
clock on the internal battery.  I haven't noticed anything glowing or
blinking on mine.

Still your message made me curious, so I measured the drain from the
internal batteries (8 NiMH cells).  With the radio turned off, it was
about 50 microamps.  This was surprising to me as clock chips usually
pull much less than that (a factor of 30 less give or take).

So I pulled out the schematics and found the KX3 uses a Microchip
PIC24F16K101 as the time keeper and charge controller.  The chip spec
puts the real time clock drain at 350 nano amps, and says that the run
mode currents should be 8 uA or so.  Sure enough, there is a Yellow
LED shown on the schematic (D2).  But like I said, I don’t see it
blinking on mine.

Given the 50uA current draw I experienced, that is still only 18 mAH
over an entire year.  If you are talking 2800 mAH for a Energizer
Ultimate Lithium, that is less than 1% of its capacity.  So that can't
explain the early demise of your batteries.  You would expect those
cells to say above 11 volts for almost their entire life, which would
be about 12 hours of KX3 receive, and about 6 hours at with occasional
transmit at 5 watts.

Now you mentioned that you keep the radio powered by a external supply
and really only use the Lithium batteries for time keeping.  If that
were the case then you would need to have an external supply that was
always above about 12.4 volts to keep the Lithium batteries from
partially discharging.

73, Bob, WB4SON