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Testing 4S Lipo in SOTA EA2/NV-089 Lakarri

Hello SOTA mates,

The homebrew device:
My friend Jorge, EA2LU, who is preparing his portable setup to do more SOTA activation, has receintly purchased a 4S cell 3 Amp LiPo battery (14,8 V nominal, 16,8V max charged) and we decided to do a joint activation to test it.

When the battery is charged to max voltage (16,8V) it is too high for the rated voltage in our portable transceivers. Most standard rigs have an acceptable deviation of 13,8 ±15%, that is max 15,9 volts.

Jorge prepared a voltage reducer composed of 3 diodes P600 connected in series with the battery. He added a bypass switch so that the diodes can be skipped once the voltage drops to a safe value.

Here you see a picture of his device with the sliding switch on top:

Batt testing results:

  • Initial voltage: 16,8 Volts.
  • Final voltage: 15,3 Volts (1,5 volts drop).
  • Activation duration: 1h 35’ running the TRX at 10 watts out (current drain on TX: abt 1,9 Amp)

The diodes were all the time on to reduce the voltage to a safe value for the transceiver.
The reduction with diodes is 2,3 Volts, therefore the max voltage provided with diodes is 14,5 volt, which fits in the acceptance range.

  • Conclusion: After 1h 35 minutes the diodes stayed connected and the voltage was still good (13 volts on the TRX). Therefore much longer activations could be carried out with this battery on summits.

Mt. Lakarri, driving directions:

  • Exit Pamplona towards France (road N-135)
  • Just after Larrasoaña turn right the exit towards Errea (road N-2337)
  • Pass Setoain and keep straight until 42°53’23.6"N 1°30’21.5"W (42.889897, -1.505975) where a dirty road starts at right hand side.
  • Park there and start the climb.

(GPS track is uploaded in the SOTA Mapping web page).


  • Trail is 3,4 kilometer long (one way)
  • Heigh gain: 200m.
    It is an easy trail that follows a wide dirty road for most part of it and in the last hundred meters it is a mountain path. Trail passes a thick pine tree and bleech forest thus making it recommendable for hot summer days.

When reaching the summit a small group of mountain goats were waiting for us, like saying hello from goat to goat:

Trig point at the summit facilitates fixation for the fish pole and antenna. The summit is well covered with box shrub. Jorge extended his EFHW wire as an inverted Vee configuration:


  • Date, Saturday 5th August 2017.
  • EA2LU’s station: KX3 @10 watt. multiband EFHW with traps for auto band change.
  • EA2BD’s station: FT-817 @5 watt. EFHW.

EA2BD’s aerial on the summit:

The summit is not large and soon bends into the forest. Therefore we couldn’t separate both antennas much. Mutual interference was strong.

I decided to activate briefly in 20m SSB only meanwhile Jorge did a longer run in CW, between 7 up to 28 MHz, to test his battery extensively.

Jorge’s CW keyer is comfortably held on the paper log table:

Incoming signals were not very strong and suffered from QSB, but we could log a lot of chasers, as usual on Saturdays.

We both enjoyed the day. It is a pleasure to have a walk on the countryside, test homebrew devices and share our hobby.
73 de Ignacio


Nice activation with the mountain goats! :smile:

Just a quick thought about using LiPos with the KX3 from my limited experience (I usually use 4S LiFePO4, but have recently started playing with 3S LiPos due to lower weight and cost, in spite of the higher fire risk and lower number of cycles).

With recent firmware, the KX3 can output 10 W down to a supply voltage of 10 V (key down). This means that a 3S LiPo is enough (as its voltage will be about 12.5 V when full, then around 11 V for most of the discharge time, finally dropping rapidly down from about 10.8 V near the end). Of course the voltage seen by the KX3 will be lower due to resistance of the cable/connectors, fuse and the ~0.3 V drop across the KX3’s internal reverse polarity protection diode. But you should still be able to use most of a 3S LiPos capacity at 10 W output, have less weight to carry, no hassle with diodes etc. and less heat. In light of that fact, the only advantages I can see in using a 4S LiPo is that you’ll be able to use 15 W output on the KX3, and perhaps get lower transmit IMD – not really relevant at these power levels in my opinion.


Manuel HB9DQM

Hello Manuel,

thanks for your comment, completely agree.

If the purpose was to power up a KX3 at 10 watt, a 3S LiPo would be enpugh.
If the purpose was to power up a KX3 at 15 watt, the current drain would increase to 2,7 Amp and the 4S LiPo would be a better choice.

The 4S would be a good option probably for an FT-857 when running 25 watt (Rx: 1 Amp / Tx: 8 Amp at 25w out). A short activation could be possible using this 3 Ah battery.

Another option could be to use the 4S LiPo on a Field day contest for some hours…

If we carry out some more tests I’ll let you know.
73 de Ignacio

Hola Ignacio,
since last year I hardly ever use the 3S2P LiPo I bought when I started SOTA with the KX3.
I have made a 4S2P battery from a batch of selected good & matching Li-Ion cells out of laptop batteries, and added the automatic voltage reducer you find here :

It’s very easy to make. This is my version :

The battery has about 4800 mAh capacity, and I can easily do 4 activations at 15W (mixed CW and SSB), without need for recharging.
73 - Luc, ON7DQ
PS : I still use the “clothespeg” paddle quite often :wink:


Hi Ignacio,
I have used for years 4S3P Li-Ion reclicated computer (The famous 18650 cells of 2 Amp or more). When my operations are long, or several operations in a day.
I also reduce the voltage with two diodes in series (to be in the correct voltage range for the KX3) and fast fuses of safety.
If the operation is fast and light, then I take 3S2P.
In any case, they have always worked perfectly.

Say hello to Jorge, he looks great !!!

Thanks Luc, nice alternative those Li-Ion cells…
Glad your homebrew paddle is doing fine :wink:

Thanks Dani, nice setup and info. Jorge is doing well and enjoying a lot!

Cheers 73 de Ignacio