A Nifty Internal Battery Mod for the Elecraft KX1

by Leo (DL2COM)

I was very happy to learn more about an interesting internal battery solution that came with my recently purchased Elecraft KX1. The original owner, Rolf DF2FK, built this KX1 himself and has a background in electrical engineering. He wanted an internal solution that is safe and delivers the full power output with this transceiver.

Disclaimer: Working on custom battery solutions can be very dangerous, esp. with Lipo batteries (possible fire, explosion when handled improperly etc.). Please take safety precautions when attempting to build the following solutions and make sure you stick to the safety instructions of the batteries you work on. Everything happens at your own risk – just like always in Ham Radio 🙂

So he developed an idea around a 4S Lipo pack consisting of 4×3.7V Lipo cells each with a capacity of 820 mAh and each with an own battery management system (BMS). Of course all four are connected in series. The individual BMSs help a great deal in keeping the Lipo cells safe such as avoiding deep discharge, excess charge etc.

He added rubber pads for a snug fit and a plastic sheet to isolate the pack from the PCB. He also made a custom balancing cable and taped its connector on the back of the pack. With the help of an adapter the pack can now easily be charged with a common balancing charger for LiPos.

The end-of-charge voltage for this pack would be approximately 16.8V max. but drops quickly below 15V where it stays on a long plateau until the battery is depleted. Rolf wanted to make sure that the KX-1 can actually take this voltage without damage and was given the information from the local dealer that it is in fact capable of handling 16.8V. Looking at the schematics & data sheets for the crucial parts it seems that the KX-1 would even be able to handle more: Voltage regulator 30V, pre-stage transistor 40V and output-stage transistor can even take 60V.

However Rolf suggests staying below 20V in any case as the reverse-polarity-protection diodes are rated for 20V only (Disclaimer: no guarantee, this information hasn’t been verified or tested thoroughly).

I have fully charged this 4S pack and successfully used it. Overall this is a very nice option that weighs only 64g. With it I am getting the following power levels: 80m – 2.89W, 40m – 2.93W, 30m – 1.95W and 20m 3.89W. I have yet to test how long the 820mAh will power the trx but I can say that I have already used it for one hour straight during an activation since its last charge and it is still working fine.

These are the cells that Rolf used.

Other options?

• I believe in one of Thomas’ videos the KX1 runs on 6×1,5V LiIon AA-sized batteries for about 20 minutes or so.
• I have also seen 3.7V AA-sized LiIon cells (14500) with an internal BMS so using four of these (and two dummy batteries) with the internal 6xAA cell holder could also be explored.
• These are 3.2V AA-sized Lifepo4 cells (14500) that Rolf’s friend has been using in his KX-1 for years without issues. He takes five of them in his battery holder and one dummy battery (in this case the 18.25V end-of-charge voltage is even higher compared to the 4-cell Lipo pack).
• I have not seen any AA-sized LiPos yet and 18650 cells are definitely too big for the KX-1.

What else have you seen or heard? Any more options in the QRP house?

Happy experimenting!

vy 73 de Leo DL2COM