|Tenergy "Propel" 11.1V, 2600 mAH Li-Ion|
Elecraft's battery for the KX2 is made by Tenergy and costs $60. In addition, Elecraft strongly suggests that you use the $25 Elecraft charger to charge this battery.
For those who are wary of using an off-brand battery in their KX2, Tenergy's version is available with a significant cost savings over the Elecraft-branded battery from that manufacturer.
I've recently received mine, the 2600 mAH battery from Tenergy that seems identical to that offered by Elecraft. It costs $40 (three for the same price as two from Elecraft) and is being charged with the same charger I use for all my batteries, regardless of chemistry.
After several more charge/discharge cycles (to make it a fair comparison to the more oft-used eBay batteries) and using it for my next two NPOTA activations, I'll post the graphs from my WMR battery analyzer.
This will be a name brand vs. generic comparison.