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Portable and Emergency Power Considerations for the KX3

Beginning a discussion of the Elecraft KX3's power consumption and requirements for portable and emergency power use let's start out by quoting Elecraft's specifications as listed in the manual for reference.  For the sake of all of my discussions please assume the unit requires a negative ground and the stock microphone is used.  Also installed in this KX3 are the KXBC3 realtime clock/internal charger, the KXAT3 auto tuner, and the KXFL3 filter.  If you are using this discussion to compare the KX3 to the FT-817ND Please remember to keep it fair. The KX3 can output a max of 10 watts and the FT-817ND 5 watts.

The antenna was connected to a dummy load.  Non-squelched volume level set to 50% on internal speaker.  We chose 3.97Mhz as our test frequency as we most often operate voice in that area.  We chose Data as the operating mode to represent maximum modulation.

Elecraft Quoted Power Requirements
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Normal Supply Voltage: 13.8 VDC
Operating Voltage: 8.0 to 15.0 VDC

Receive Current "As Low As": 150mA
Transmit Current: 2000mA (2 Amps)

Our Observations
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Running from an external 13.4v LiFePO4 battery on 3.97Mhz Data with back light and LED's enabled.
Receive Current: 320mA
Receive Current Squelched: 203mA
10w Transmit Current:  1900mA
5w Transmit Current: 1290mA
2.6w Transmit Current: 850mA
1.0w Transmit Current: 680mA
0.5w Transmit Current: 610mA

Running from an external 13.4v LiFePO4 battery on 3.97Mhz Data with back light and LED's disabled.
Receive Current: 290mA
Receive Current Squelched: 170mA
10w Transmit Current:  1810mA
5w Transmit Current: 1280mA
2.6w Transmit Current: 830mA
1.0w Transmit Current: 660mA
0.5w Transmit Current: 590mA

Running from an external 13.4v LiFePO4 battery on 52.525Mhz FM with back light and LED's disabled.
Receive Current: 318mA
Receive Current Squelched: 258mA
10w Transmit Current:  2489mA
5w Transmit Current: 2250mA
2.6w Transmit Current: 1690mA
1.0w Transmit Current: 1170mA
0.5w Transmit Current: 960mA

Powered Down
Idle Current: 0mA

The test results show the rig as a good choice for those wanting the full features of a modern style rig.  The KX3's current draw comes in less on our 80m tests than our Yaesu FT-817ND however we were surprised to find the FT-817ND as the winner for low current draw on 6m!

Internal Batteries
Since we have the internal charging unit we stock out KX3 with name brand AA NiMH 2650mAh cells.  We find our KX3 draws ~ 340mA while charging the internal cells.  We typically let the KX3 charge the cells and select a charging time based on how depleted the the batteries are at the time.  Since the KX3 doesn't seem to have a way of sensing the batteries charge level we will every so many months remove the cells and run them thru a discharge/charge conditioning cycle on an external charger.

External Power
Like the the FT-817 is much more flexible when it comes to external power sources as can run on anything from 8 to 15VDC as long as it can supply a minimum of 500mA current (Note:  With a 500mA external source you should be able to use the internal charger to charge your internal batteries and listen to your hearts content however to transmit with such a supply you would need to limit your transmit power to 200mW or less.

Our KX3 and 817 follow us around most of the time running off of one of our 13.4v LiFePO4 battery packs which we charge with a pair of 28w of solar panels, by the time one is run down (about two days of monitoring with about 30 min talk time per day) the next has been recharged (even on cloudy days).   We also have a 300w solar panel array residing on our roof which is connected full time to our ham shack and base equipment.  A single 10w portable cell is enough to run the radio in receive mode but does not supply enough current for transmit but it does a nice job of trickle charging the internal batteries batteries.  The 10w panel supplies about 560mA in full sun.  The 300w array on our roof can supply ~18A and is enough to run our entire station, even in full 100w transmit mode (Note: We use a solar charge controller to limit the arrays voltage, without the controller the array can supply as much as 21VDC under low/no load conditions which would most likely fry our favorite radios.)