[Elecraft] Internal Batteries for KX1

[Bob via Elecraft]

What do you use for internal batteries in your KX1? 
I just got my new (bought used) KX1 cleaned up, tuned up and running nice.  It has the output power mod (changed 2 resistors) but with 6 internal 1.5V batteries the 1.4Watt output power is just too low for me.  Be for the batteries are dead I'll be running QRPP.  I want around 13V which give me 3Watts, but I really want internal batteries.

The various formulations of 3.6V rechargable Lithium batteries look appealing but I've heard lots of stories about poor reliability.  Any suggestions?