For about one year I have been using these battery packs from Xpresstek in my KX2:
These packs contain three Panasonic 18650 cells rated at 3400 mAH. The pack also contains a protection circuit PCB. The pack is rated at 38 WH and weighs 150g - this is quite incredible! It fits the KX2 just as well as the recommended battery. The only issue is that you must add your own DC connector, not for the faint of heart or those with unsteady hands - but the protection circuit probably would avoid disaster, in case of a short.
These batteries appear to be available at a reasonable price:
Just go to the search box and put in LI_111_3400.
I charge these packs with a homebrew 12.50V regulated supply. The voltage is regulated by a linear regulator, with 20T precision pot, screwdriver adjusted. The current is limited to 1.0A maximum by an active current regulator. This has served me safely for hundreds of charges of these and other lithium 18650 cells. This is a very conservative charger for these packs.
Xpresstek has all the specs you need to decide if other chargers are appropriate, and they may be able to recommend a charger as well.
I also have the Elecraft-recommended Tenergy battery, and it is just fine, except that the batteries from Xpresstek last longer in the radio. The Tenergy charger is more aggressive, causing the Tenergy packs to warm slightly during charge, while my homebrew regulator is more gentle, with no obvious warming of the cells. I don’t recommend using the Tenergy charger with the LI_111_3400 battery - play it safe - if you use that charger, add a series resistor to limit the current to about 1.0A.
I realize that this information goes sideways to Elecraft’s strict words about using only their approved internal battery. They have every reason to warn their customers, because using an inferior battery, or using any battery incorrectly, might cause a tragic event.
I know of two other SOTA ops who are also using these same 3400 mAH batteries in KX2’s with happy results. However, you use any lithium battery at your own risk, and using the battery external to the radio probably reduces most of the risks to the radio.
Be aware that the KX2, with its black finish, can get very hot sitting in the sun. I have had mine protect itself once due to solar overheating! Lithium ion batteries can fail at high temperatures, so remember to keep your KX2 or KX3 in the shade - even when off.
I’ve done about 220 activations in the past year, and on almost every activation, I have run my KX2 at the full 10 watts output, for the entire time, using these 3400 mAH batteries. Generally I have no problem doing two or more very long CW activations - about 50 contacts each, + or -, and I still have not dropped below 10W output.
Please note that these batteries do not have enough voltage to allow the KX2 to reach an output of 12W with the new firmware. A higher voltage battery is needed. However, four series lithium ion cells, fully charged, have too high a voltage for the 15V max of the KX2.
Please follow Elecraft’s recommendations about internal batteries, unless you have the required knowledge to use a different battery, charge it properly, etc. I provide this information for those who might want an alternative, and who understand the risks.
Xpresstek ships their batteries to the 48 states with no hassles - please contact them if you have questions.